Public domain
[p. 476]
The great crustal movements which brought the Proterozoic era to a close converted a large area within the limits of the North American continent into land. This is shown by the geographic distribution of the basal strata of the Cambrian,[1] the oldest system of the Paleozoic era. Where accessible, the base of the system is, in most places, unconformable on underlying formations. The distribution of the successive parts of the system gives some idea of the relations of sea and land throughout the period, for most of the strata are of marine origin, as their fossils show.
The Subdivisions of the Cambrian and their Distribution
The Cambrian system has been divided into three series, known as the Lower, Middle, and Upper Cambrian, respectively. Other names are sometimes assigned to these divisions. Thus Georgian (Vt.), Acadian, and Potsdam (Saratogan) (N.Y.), names of localities where the corresponding divisions of Cambrian were first differentiated in North America, are synonyms (in America) for Lower, Middle, and Upper Cambrian respectively.
The Lower Cambrian. The Lower Cambrian formations are known in North America only near the eastern and western borders of the continent (Fig. 357). In the east, they are found in the Appalachian belt and at some points farther east; in the west, they are found in various states between the 110th and the 120th meridians. In both the east and the west, the strata contain marine fossils. The strata of the east were accumulated in straits, sounds, etc., rather than on the shores of the open sea.
[p. 477]
[p. 478]
The great tract between the Appalachian Mountains on the one hand, and western Montana and Utah on the other is believed to have been land during the early part of the period. From this land, sediments were probably being carried to the sea on either hand. These sediments, subsequently cemented into rocks, made some of the shales and sandstones of the Lower Cambrian.
The unconformity between the Lower Cambrian and its base represents what is sometimes called a “lost” interval. This designation for such an interval is not altogether appropriate, for the unconformity records a time of exposure and erosion, followed by submergence and deposition.
The Middle Cambrian. The strata of the Middle (Acadian) series of the Cambrian system are found with those of the Lower Cambrian, and in addition they are known in Texas, Oklahoma, Arizona, some parts of Montana, and perhaps elsewhere. Since the Middle Cambrian beds contain marine fossils, their distribution indicates that the continent was being invaded by the sea from the south before the close of the Middle Cambrian epoch. Like the preceding series, the Middle Cambrian beds are absent from much of the interior, if present identifications are correct. Where the Middle Cambrian rests on the Lower, the two are generally conformable.[2] Where the Middle overlaps the Lower, it is unconformable on older formations.
The Upper Cambrian. In the Later Cambrian (Potsdam or Saratogan) epoch, the sea covered much more of the continent, for the Potsdam series covers not only the eastern and western borders of the continent, but much of the interior as well. The Upper Cambrian is conformable on the Middle Cambrian in the east and west, but in the interior it rests unconformably on pre-Cambrian formations.Fig. 358 shows something of the distribution of the Cambrian system as a whole (see explanation beneath the Fig.)
Great submergence during the Cambrian. The distribution of the several series of the system shows that the great physical event of [p. 479] the Cambrian period in North America was the progressive submergence of the continent. Theoretically, this submergence may have been brought about by a rise of the sea or by a lowering of the land, or by both together. Both the lowering of the land and the rise of the sea may be due to gradation, to diastrophism, or to the two combined.
Gradation a possible cause of submergence. Gradation is perpetual and inevitable where land and sea exist. The waves attack the land along its borders, and the agents of land degradation lower its surface. The former is a direct cause of encroachment of sea upon land, and the latter is an indirect cause, since all sediments transported from land to sea displace an equal volume of water, and raise the surface of the sea correspondingly. Small as this rise is for any brief period, its effect is to cause the sea to advance on the land; and the lowering of the land by degradation at the same time, increases the area of the advance. If continued long enough, shore-cutting about the borders of the lands, down-cutting over the whole surface, and the accompanying rise of the sea-level, must inevitably cause the water to cover the continents, and to spread deposits over all but the last remnants of them, provided there is no deformation of the body of the earth in the meantime.
It has been computed that if the earth, in its present condition, were to remain without deformation long enough for the continents to be base-leveled, the deposition of the sediments thus derived in the sea would raise the sea-level about 650 feet. This would submerge a large part of the base-leveled land. The evidence of gradation in the Cambrian period is clear and firm. Most of the sediments which make up the Cambrian system of rocks were eroded from the land and deposited in the sea. This lowered the land and raised the sea. Gradation was, therefore, a factor in the submergence of the continent, and there is evidence that great progress was made toward the base-leveling of America and other continents, before the close of the Cambrian period. Base-leveling u_, and hence in itself favors the view that no great deformation affected the continent while it was going on. In harmony with this view, there is an absence of direct evidence of profound deformation during the [p. 480] Cambrian period. There is therefore a presumption against much diastrophism during the period.
If gradation were the sole agency involved in the submergence of the lands, the advance of the sea should have been steady, though not necessarily equal in rate at all times and places. Without going into details, it seems certain that there were changes in the areas of deposition other than those which can be accounted for by gradation, but none of these changes imply warpings of pronounced type like those recorded in the rocks of the Proterozoic and Archeozoic eras.
Deformation as a cause of submergence. The deformations which cause submergence of land (and emergence of sea-bottom) may be either superficial, involving the rocks down to depths of a few miles at most, or deep-seated, involving the rock to much greater depths. One or two phases of deformation may be mentioned.
1. Lateral spread or continental creep. The continents stand about 15,000 feet above the ocean bottom. Their weight causes a pressure of 15,000 to 20,000 pounds to the square inch on their bases. This pressure tends to cause the continents to spread by creep into the ocean basins, on the same principle that a great body of ice, such as an ice-sheet, spreads. Spreading is opposed by the hydrostatic pressure of the oceans against the sides of the continental platforms. This is some 5,000 pounds per square inch at the bottom, so that there remains an unbalanced pressure of 10,000 to 15,000 pounds per square inch, tending to cause creep. Is this enough to overcome the strength of the rock, which opposes creep? Even the lesser of these figures is equal to the crushing strength of some of the weaker rocks, and is a notable percentage of the crushing strength of even the strongest. Under less pressure than this, the rock in mines is often observed to creep. It is not improbable, therefore, that such a pressure, constantly exerted for a prolonged period, might cause some spreading of the great continental platforms, and hence (1) some lowering of their surfaces, (2) some submergence about their borders, and (3) at the same time some rise of the sea-level. Many phenomena which cannot be cited here seem to lend support to this hypothesis of lateral creep,[3] but its efficiency is not determined.
[p. 481]
2. Adjustments between continental and oceanic segments, a possible cause of submergence. It has been shown [4] recently that there is a plane about 100 miles below the surface of the earth where the pressure downward beneath the overlying continental areas, is equal to the downward pressure beneath the overlying oceanic areas, in spite of the fact that the surfaces of the continents are, on the average, three miles above the ocean bottoms. The reason is that the rock beneath the oceans is denser than that beneath the continents. At all horizons above this isostatic plane at the depth of about 100 miles, a column of average continental rocks measured downward from the surface of the land, weighs more than a similar column of sub-oceanic rocks. This tends to aid the spreading movement noted above, and so tends to bring about submergence of the land. A similar state of things probably prevailed in Cambrian times. It appears however that notably elevated regions, like the Cordilleran tract, stand higher than they would if they were in isostatic balance.[5] This lack of balance may give rise to special movements to secure a more perfect equilibrium, and hence to regional or local deformations. During any great period of deformation, like that at the close of the Proterozoic, it is probable that some portions of the crust were pushed up above a position of equilibrium, and later tended to settle back. Other portions may have been depressed below the position of equilibrium, and later tended to rise. Such movements would be unsymmetrically distributed, and might result in slow and quiet, but unequal warping.
3. Other adjustments, as possible causes of submergence. Various other changes and adjustments of the surface of the different parts of the lithosphere, such as the unloading of the continents by erosion and the loading of the ocean basins by deposition, the outpourings of lava, and unequal additions or losses of heat in different places, may also have helped to cause submergence or emergence of the lands.
Basis for the Subdivision of the Cambrian
We have now to inquire the means by which the Cambrian [p. 482] strata may be recognized, and further the means by which the Lower Cambrian may be distinguished from the Middle, and the Middle from the Upper.
Superposition. Where a formation is conformable on another of known age, as the Middle Cambrian on the Lower, the presumption is strong that the upper was formed after the lower, without interruption. In this case, the approximate age of the upper is known. But where one formation is unconformable on another of known age, the stratigraphic relations between them do not determine the age of the upper, beyond the general fact that it is younger than the lower.
Fossils. The Cambrian is the oldest system of rocks known to contain abundant fossils. Most of them represent the shells or other hard parts of marine animals buried in the sands and muds at the time they were deposited. The fossils in the strata of any division of the Cambrian constitute the known fauna of that stage, though it is not to be supposed that fossils of all the species that lived have been preserved.
The Lower Cambrian series contains certain fossils which are distinctive. Among them are species of a genus of trilobites known as Olenellus (Fig. 359, a). Along with the representatives of this genus, many other species of various types of life are found. To the aggregate, the name Olenellus fauna has been given, and Olenellus Cambrian is synonymous with Lower Cambrian and with Georgian. It is not to be understood that representatives of the genus Olenellus are found in the Lower Cambrian everywhere,[6] or that other genera of trilobites are absent.
[p. 483]
Where formations representing the whole of the period are present, the fossils in the higher beds are not the same as those below. At no single plane is there, as a rule, a very striking change in species, but in successively higher beds some of the species found below disappear, and new species come in, as if to take their places. These variations show that the inhabitants of the sea changed as time went on. At about that stage in the Cambrian system where the genus Olenellus drops out, the genus Paradoxides (Fig. 359, b) appears on both sides of the North Atlantic basin. The other species associated with Paradoxides are somewhat different from those associated with the genus Olenellus. The Paradoxides and their associates constitute the Paradoxides fauna, a fauna which includes also many species of other genera of trilobites, and many species of other classes of animals not related to trilobites. By general agreement, the Middle Cambrian, on both sides of the North Atlantic, is defined by the Paradoxides fauna, so that Paradoxides Cambrian is synonymous with Middle Cambrian and with Acadian (p. 476). In the western part of North America, and on the opposite side of the North Pacific as well, the Middle Cambrian does not contain Paradoxides, but Olenoides, and its fauna is known as the Olenoides fauna, but it is none the less distinct from fauna of the Lower Cambrian.
In like manner the Paradoxides and Olenoides faunas are succeeded by another, known as the Dikellocephalus fauna (359, c), and this fauna characterizes the Cambrian strata above the Middle Cambrian. Geologists have agreed to define the Upper Cambrian as the series of strata carrying the Dikellocephalus fauna.
It is not to be understood that every species of the Paradoxides fauna is unlike every species of the Olenellus fauna below and the Dikellocephalus fauna above. This is not the case; but so many [p. 484] of the species of the three faunas are different, that with a considerable number of species to judge from, their separation is possible by those familiar with Cambrian fossils.
In the discrimination of any of these faunas, an analogy with living animals is suggested. The present faunas of North and South America are reasonably distinct; but it does not follow that there are no species in common. Given a single animal, even an expert might not be able to say that it was from the one continent or the other, though with certain species even this might be done; but if a large number of animals from either continent are available, it is possible to determine to which continent, that is to which geographic fauna, they belonged. So with the several Cambrian faunas. They have some species in common, and such species do not distinguish the groups of strata which contain them from one another. But certain species are found only in the Lower, certain other species only in the Middle, and still others only in the Upper part of the system, and these species serve to distinguish the principal divisions.
Sequence of faunas based on stratigraphy. It is not to be understood that rocks which contain such faunas are classed together simply because they contain certain fossils. This is not the reason, or at least not the principal reason for grouping them together. The order of sequence of faunas is first determined by the superposition of the strata. The Lower Cambrian fauna could not be known to be older than the Middle Cambrian fauna, if the beds containing the former did not underlie beds containing the latter. In other words, the primary basis for correlation by means of fossils is stratigraphy.
Sedimentation in the Cambrian Period
Sedimentation in the Cambrian period appears to have followed the general laws that govern deposition in periods of comparative freedom from great deforming movements, and hence of progressive base-leveling. Most of the known Cambrian sediments were deposited in the sea, and their area may be regarded as a rough measure of the area of the sea at that time. Sedimentation was probably faster in the early stages of the period when the land [p. 485] area was largest and highest, and slower in the later stages after the land surface had been worn down by erosion and narrowed by the encroachment of the sea. Sedimentation was probably greater near the land, and less far from the shores in deep water.
Sources and kinds of sediments. As in other geologic periods, the land-derived sediments came from all formations then exposed to erosion. The sediments along the immediate borders of the land were doubtless different from those deposited farther from it. Even along shore there were considerable variations, both because of variations in the sources of the sediments, and because of differences in wave, river, and current action.
The Cambrian formations include all common phases of sedimentary rocks. There are conglomerates, presumably accumulated near the shores of the time; there are sandstones, the sand of which was deposited in shallow water where the waves were sufficiently vigorous to keep the mud from settling; shales representing the deposits made in stiller or deeper water; and beds of limestone representing, for the most part, the accumulations of shells, etc., where terrigenous sediments were not carried in quantity.
Geographic variations in the sediments. The distribution of these various sorts of sedimentary rocks shows that various kinds of detrital beds were accumulating in different places at the same time, and at the same place at different times. Not only this, but they were accumulated at very different rates. Thus the full section of the Middle Cambrian (all that was deposited during the whole of the Middle Cambrian epoch) seems to be present at many points, yet the thickness of the Middle Cambrian strata is far from uniform. Equal thicknesses of rock do not necessarily accumulate in equal periods of time.
The fact that in the northern interior of the United States the Upper Cambrian formation is generally of sandstone, and that this sandstone is wide-spread, indicates that the water was so shallow during its deposition that the waves were competent to roll sand long distances. Furthermore, the structure of the strata, with their cross-bedding (Fig. 266), ripple-marks, etc., shows that the whole of tfee thick series from bottom to top was deposited in shallow [p. 486] water, and therefore on a surface which was gradually depressed, relative to sea-level, as the sediments were accumulated. The greater proportion of limestone (chiefly dolomite) in the Upper Cambrian of the southern and southeastern interior, points to clearer seas, but perhaps not to deep ones. The adjacent lands were perhaps too low to yield abundant sediment. Limestone is also an important part of the Upper Cambrian of the Rocky Mountains[7] though clastic rocks predominate farther west. Where the Upper Cambrian is limestone, it is not usually sharply differentiated from the overlying Ordovician.
Distribution and Outcrops of the Cambrian System
The Cambrian formations were once as wide-spread as the Cambrian seas themselves, but they are not now present over all the area they once covered. The exposed edges of the strata have suffered erosion, so that the border of the system as it now appears about the areas of pre-Cambrian rock, is not its original border, and does not represent the shore-line of the Cambrian sea when its waters were most wide-spread.Fig. 360 represents the conditions which often exist. Each of the Cambrian formations, represented by A, B, and C, formerly extended farther to the left.
[p. 487]
The areas where the Cambrian formations are exposed are not to be confounded with the areas where they actually exist. The Cambrian formations are exposed, for example, in Wisconsin, in Missouri’, and in Texas; but the strata of Texas are doubtless continuous, beneath younger formations, with those exposed in Missouri, and those of Missouri with those of Wisconsin, and these in turn with those of the Black Hills on the west, and with those of New York on the east (Fig. 361).
Position of outcrops. The map (Fig. 358) showing the areas where the Cambrian system is now exposed, reveals several points of significance. (1) . Many of the outcrops occur in association with outcrops of the Archean and Proterozoic systems (Fig. 341). In some places, the exposed Cambrian lies along the border of the exposed parts of these older systems on one side only, while in others it completely surrounds them. This distribution is not peculiar to the Cambrian, but is characteristic of most formations as compared with those of greater age. (2). The exposed areas of Cambrian in the Appalachian Mountains [8] occur in parallel or subparallel belts (Fig. 358). This is the result of (a) the folding to which the Cambrian and later strata of this region have been subject, and (b) the erosion which the folds have suffered.Fig. 362 will help to explain the repetition of outcrops. In this diagram, A represents pre-Cambrian strata, Ç represents the Cambrian, and O, S, D, and C the Ordovician, Silurian, Devonian, and Carboniferous systems, respectively. After the strata were folded, erosion cut the folds down. A fold which involves Cambrian beds, if truncated below the level of the bottom of these beds at their highest point, exposes two belts of Cambrian strata, one on either side of a pre-Cambrian axis, as represented in the left-hand part of the figure. If the truncation is at a level below the top and above the bottom of the Cambrian (right-hand side of Fig. 362), the strata of that system are exposed in a single belt along the crest of the fold. (3) . In some places, Cambrian outcrops are surrounded by older formations. In such cases the Cambrian outcrops presumably represent remnants which have escaped erosion. They might occupy depressions in the surface of pre-Cambrian formations, or they might constitute hills (Fig. 363).
[p. 488]
Width of outcrops. The most extensive continuous outcrops of the Cambrian (Fig. 358) are in Wisconsin; yet there the Upper Cambrian only is present, with a thickness of less than 1,000 feet, while in the Appalachian Mountains, where the system has an aggregate thickness of several thousand feet, it appears at the surface in narrow belts. That is, the outcrops are narrow in the east where the system is thick, and wide in the interior where it is thin. The explanation of this apparent anomaly is found primarily in the attitude of the strata. In Wisconsin they are nearly horizontal, while in the mountain regions, both east and west, they are tilted, often at high angles. Where strata are vertical, the width of their outcrop on a horizontal surface is about the same as the thickness of the beds (Fig. 364) ; where they are nearly horizontal, as in the left-hand side of Fig. 364, the width of outcrop on a horizontal surface is much greater.
[p. 489]
It is not to be inferred, however, that horizontal strata necessarily have a wide outcrop. The width of the outcrop is also influenced by topography, as shown in Fig. 365. Here the horizontal stratum between B and C has about the same thickness as •€ of Fig. 364, but its outcrop is narrow. In general, the width of outcrop, so far as determined by topography, depends on the angle between the bedding-planes and the surface where the formation outcrops. The width of the outcrop decreases as this angle increases.
Changes in the Cambrian sediments since their deposition. The sediments of the Cambrian system have undergone more or less change since their deposition. In most regions, the gravels, sands, and muds have been compacted and cemented into conglomerates, sandstones, and shales respectively. In some places, the cementation of the sandstone has gone so far as to convert it into quartzite.
[p. 490]
Over great areas in the interior (Missouri, Wisconsin, Texas, etc.) the strata still remain in horizontal or nearly horizontal position, while in other regions they have been tilted, folded, and faulted. Where close folding has taken place, the rocks have been more or less metamorphosed. In extreme cases the sandstones have been converted into quartz schists, the shales into slates and schists, and the limestones into marble.Fig. 361 shows the general position of the Cambrian strata (Ç) over much of the interior, and Figs. 367 and 368 illustrate their position and relations where they have been folded and faulted.
Close of the Cambrian. No physical changes of great importance seem to have marked the close of the Cambrian period in America. Nowhere in our continent, so far as now known, were mountains made at this time, and nowhere were great areas of sea-bottom converted into land, though local unconformities[9] between this system and the next record local changes in the sites of deposition.
In Other Continents
Europe.[10] In Europe, as in North America, wide-spread deformation before the beginning of the Cambrian converted large areas [p. 491] of the present continent into land, and there is evidence that these lands, like those of America, were subjected to protracted erosion before the deposition of the Cambrian system, for it is generally unconformable on older strata.
The Cambrian formations of Europe, like those of America, are mainly clastic. A considerable portion of the material involved in them is coarse, and the strata are often ripple-marked, and affected by cross-bedding and by sun-cracks, — features which show that a large part of the Cambrian sediments were laid down in shallow water, and some of them where they were not continuously covered by water.
In Wales (Cambria), the country from which the system received its name, the system has a thickness of 12,000 feet or more. This great thickness is equalled or exceeded in Brittany. In Scandinavia, on the other hand, where Lower, Middle, and Upper Cambrian all are present, the aggregate thickness is sometimes no more than 400 feet. In western Russia also it is thin. These differences probably mean that sediments were being deposited in some places many times as rapidly as in others. They probably also mean that the sediments were sometimes spread over flats under shallow water and sometimes on shelving bottoms where the layers were inclined. The Middle Cambrian is much more wide-spread than the Lower or Upper, showing that changes in the relation of sea and land were in progress during the Cambriar period, shifting the areas of erosion and sedimentation.
The Cambrian strata of western Europe have been much folded since their deposition. In central and eastern Europe, on the other hand, they are essentially horizontal. Beds of clay which are still plastic, and beds of sand which are still uncemented, are here found in the system.
No geographic change of great importance seems to have marked the close of the Cambrian. In this respect, as in some others, the Cambrian histories of Europe and North America correspond.
Other countries. Cambrian rocks occur in various parts of Siberia, China, India, Australia, and Tasmania, and in the northwestern part of Argentina, but their distribution outside of North America and Europe is but poorly known.
[p. 492]
Glacial formations. (1) In the vicinity of Varanger fiord, in northern Norway, Lat. 70° 8’ N., there is a bed of bowlder-bearing rock (the Gaisa beds)[11] resting on a smoothed and striated pavement of distinctive glacial type. The Gaisa beds rest upon the eroded surface of a crystalline terrane, and have been thought to belong to the oldest part of the Cambrian system, or to antedate it. (2) Recent exploration in China[12] has shown the existence, on the Yangtse River, in latitude 30°, of a thick formation (170 feet) of bowlderbearing rock of glacial origin, containing many striated bowlders of diverse sorts of rock (Fig. 369). The glacial formation here lies at the base of the Paleozoic, and beneath the series that carries the Cambrian trilobites.
Glacial formations of about the same age have been found in Australia, and perhaps in South Africa.[13] The most probable interpretation, with present knowledge, is that these bowlderbearing formations of Norway, China, and Australia (Fig. 370) belong either to the transition period that accompanied and followed [p. 493] the deformation that closed the Proterozoic, or to the opening stages of the Paleozoic, previous to the demonstrated Cambrian. The profound climatic significance of these glacial formations is obvious. The testimony of Cambrian fossils, on the other hand, implies nearly uniform climatic conditions throughout all regions where fossils have been found, and the wide spread of the sea during the later part of the period would seem to point to oceanic, rather than continental climates at that time.
Duration of the Cambrian Period
There is no way of making a reliable estimate of the duration of the Cambrian period. The destruction and the removal to the sea of such large volumes of rock as are represented by the sediments of the Cambrian required a very long period of time; but since there is no standard rate at which any sort of sediment is known to [p. 494] accumulate, this long period cannot be reduced to years. It has been estimated that limestone sometimes forms at some such rate as one foot per century. In some parts of the West there are 6,000 feet of limestone, besides thick bodies of fragmental rock. At the above rate of accumulation, the 6,000 feet of limestone would call for a period of 600,000 years, and if time be allowed for the other formations of the same region, the period would be greatly lengthened. It should be remembered, however, that while one foot per century may be a rate at which limestone sometimes accumulates, it does not follow that it is the rate at which the Cambrian limestones were formed. The data on which this estimated rate is based are believed to give too high, rather than too low a rate, and a less rapid accumulation would mean a correspondingly longer period of time.
Many estimates of geological time, based on various data, have been attempted.[14] These estimates, so far as applied to the Cambrian, generally assign to that period a duration of 1,000,000 to 3,000,000 years. It should be distinctly borne in mind, however, that the chief value of these figures is to give emphasis to the fact that the period was one of great duration.
Perhaps no single event in the history of the earth possesses greater interest than the first appearance of life; but the date of its advent is not known. There is good evidence that life existed before the close of the Archeozoic era, and under the accretion hypothesis, it is not improbable that its beginning antedated, by a long period, even the oldest accessible Archean formations. If so, it is quite beyond hope that the earliest forms of life will ever be known from its relics. The evidences of plants and animals in the Proterozoic era are so indirect, obscure, or meager that they give but a very inadequate conception of the life before the Cambrian period. The information which the imperfect fossils give, indicates that they do not tell even the main part of the story of Proterozoic [p. 495] life, for the life represented by the fossils could not have lived without other life not represented.
The Cambrian is the oldest system in which there is a reasonably adequate record of life, and even here the record is far from complete. The animal kingdom is fairly well represented among the fossils, but plant remains are barely identifiable.
The scantiness of plant fossils. The existence of plants in the Cambrian period would perhaps be doubted were it not known that all animals depend on them, directly or indirectly, for food. It must be supposed, therefore, that plants abounded. The inadaptability of the lower plants to fossilization is doubtless the chief "explanation of their poor representation among the Cambrian fossils. Reasons of a physical nature have been given previously for thinking that the surface of the land was clothed with some kind of vegetation. Yet there are no identifiable traces of landplants, and but obscure impressions of sea-plants. The lesson taught is the extreme imperfection of the fossil record.
The Animal Fossils
Every great division of the animal kingdom, except the vertebrate, had its representatives in Cambrian times. The Arthropoda (p. 945) were represented by crustaceans; the Mollusca, by gastropods, pteropods, and pelecypods; the Molluscoidea, by brachiopods; the Vermes , by annelids; the Echinodermata, by cystoids; the Coelenterata, by graptolites, medusas, and corals; the Porifera, by sponges, and the Protozoa, by rhizopods. All the representatives of these groups among the Cambrian fossils appear to be marine. Of land animals there are no traces, but this does not prove that land animals did not exist. Though no vertebrate remains have yet been found, it would be rash to assume that none of them were in existence.
Arthropoda. Of the Arthropoda, crustaceans (represented now by crayfish, crabs, etc.) only have been found in the Cambrian strata. Their representatives were trilobites and entomostracans. The trilobites were easily the most distinguished forms of life in the Cambrian seas. They were not only the highest in organization, but they were the most characteristic of the period. Their [p. 496] successive genera best distinguish its successive stages, and their distribution is a chief means of correlating the formations of different continents, and of different provinces of the same continent, as previously set forth (p. 481). They have long been extinct. Figs. 359 and 372 show their three longitudinal lobes (whence their name), and their three transverse divisions, head, thorax, and caudal shield. The trilobites were well advanced in the scale of development, possessing nearly all the anatomical systems and physiological functions of modern crustaceans. Perhaps their compound eyes, formed of many eyelets, are the best index of their development. In this and succeeding periods, the number of eyelets in the trilobites’ eyes ranged from a score to several thousands. Some of the Cambrian trilobites, however, had no eyes, while, others possessed abortive rudiments, implying that their ancestors had possessed eyes. The acquisition and abortion of so important an organ seems to indicate variation in the conditions of life. This may mean no more than migration to deep dark waters, or the habit of burrowing in the mud, where eyes became useless. The eyes were often slightly raised on crescentic lobes, with the convex face outwards. In later epochs, these crescents became more and more curved, extending the sweep of vision fore and aft, to the trilobite’s obvious advantage.
[p. 497]
The upper surface of the body was ornamented variously with granules, spines, and other markings, the significance of which is little understood. These ornamentations varied as time went on, increasing, in general, until after the climax of the trilobites had been passed. Trilobites possessed a row of slender articulated limbs on either side, and delicate filaments which served the function of respiratory organs. The nature of the limbs indicates that the trilobites both walked and swam. They possessed antennae which doubtless served as organs of touch, and they moulted the shell at successive stages of growth, like modern crabs. Omitting further details, it is to be observed that, at this early day, a highly complex, well-differentiated organization had been acquired, possessing nearly all the organs and functions of arthropods of the present day. Molluscoidea. This branch was well represented by brachiopods (lamp-shells,Fig. 373) . In geological importance, the brachiopods of the period were second to trilobites only; but unlike the trilobites, brachiopods still live, and are conspicuous representatives of stability and persistence. Though the species and most of the genera have changed, the class as a whole has been but slightly modified since the Cambrian period. Then, as now, the valves of one division of the group were hinged, while those of another were not. One division formed shells of calcium phosphate, and another of calcium carbonate.
[p. 498]
Mollusca. Cephalopods (chambered shells), the highest class of mollusks, are found in the uppermost beds of the Cambrian. As they were even then highly developed, there is little doubt that the class had passed through a long history before the latter part of the period.[15] Pelecypods (bivalves, 6,Fig. 374) lived even in the early part of the period, though their remains are not abundant. Gastropods (univalves, c, d, e,Fig. 374) were rather plentiful throughout the period. The early forms are chiefly of the low conical type, while more amply coiled and spiral forms became common later. The close resemblance of some of them to modern gastropods is worthy of note.
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Vermes and echinodermata. Sea worms left evidence of their abundance by borings, tracks, etc. (Fig. 375). A few cystoids, the forerunners of the beautiful crinoids (stone lilies) were present, but fossil crinoids have not been found.
Coelenterata. The coelenterates were represented by hydrozoa (graptolites and medusae) and anthozoa. The eccentric freaks of fossilization are nowhere better illustrated than here. Relics of graptolites, among the most delicate of animal forms, and of medusae, among the softest of animals, were preserved, while some much less easily destroyed forms left scant record of themselves. The graptolites, now extinct, were slender, plume-like organisms, consisting of a series of* indurated cells, in which the individual zooids lived, attached to a common slender axis which united the colony. The whole colony appears to have floated free in the sea (Fig. 376, e). The secret of their preservation probably lies in the fact that, being floating forms, they often settled in quiet and rather deep waters off-shore, where fine silts accumulated, and where the [p. 500] conditions were favorable for burial without destruction. The most singular case of fossilization is the preservation of traces of jelly-fish, or at least of what are so identified. These are illustrated in Fig. 376, c and d. Their impressions are found in the Lower Cambrian. Obscure fossils of corals are present (Fig. 376, a and b), the forms of which resemble sponges so much that they were long regarded as such. Corals seem to have been more abundant in some other parts of the world than in North America.[16]
Lower types. Sponges were present in some abundance throughout the period. It is probable that many of the low, simple forms classed as protozoans existed, but only a few identifiable forms have been found.
Implied life. The existence of so much animal life implies much vegetable life to supply the necessary food, as heretofore noted. Furthermore, various characteristics of the fossils suggest the presence of animals not known from fossils. A large percentage of the known Cambrian animals were provided with shells, tests, plates, or other forms of hard coverings. In the main, these appear to have been protective devices, and imply enemies or combative rivals against which protection was needed. Perhaps the [p. 501] most significant feature of the protective devices lies in the fact that they are usually of the same type as those possessed by the similar animals of later times. The shells of the gastropods, pelecypods, and brachiopods differ from thoss of to-day in minor features only. The coverings of the trilobites were much like those of their living relatives, and much the same may be said of nearly every form. If there had been a radical change in the character of their enemies or rivals, we might expect some notable change in the defensive devices. It is a natural inference, therefore, that the conflicts of life in the Cambrian seas were similar to those of the present time. The inference may be pushed a step further, and the deduction drawn that the conflicts which led to the evolution of the defensive devices were much like those throughout the period of their retention.
Stage of evolution represented. What stage of advancement in the development of life had been already attained by the beginning of the Cambrian period? Do the fossils of the system indicate that the life of the period was primitive, or do they imply that it had advanced far beyond primitive forms? The answer to these questions is to be sought (1) in an estimate of the degree of development of the various organic structures and functions, and (2) by the amount of divergence of the animal types.
(1) For comparison it may be assumed that the primitive forms of life were as simple as the simplest existing forms. There are multitudes of plants and animals that consist of a single cell, and if these are taken as representing the nearest existing approach to primitive forms, how far had the Cambrian life advanced beyond them?
We are not left entirely to the presumption that the earliest forms of animal life were much simpler than those of the Cambrian, for the stages of development of the young of certain of the Cambrian animals reveal something of their ancestral history. It is a well-established law of embryology that animals, in the early stages of their development, pass through a succession of changes in which their structure resembles that which their ancestors had in their maturity; in other words, the individual history of any animal is an epitome of the history of its ancestors. Now the [p. 502] Cambrian trilobites are known to have passed through a series of remarkable changes after the individuals had developed far enough to be fossilized, and it is inferred they passed through other stages previously. There is, therefore, specific ground for believing that they had a long line of ancestors.
On the anatomical and physiological side, it is clear that nearly or quite all the fundamental organs had been developed. There were skeletal systems of several forms, muscular systems, nervous systems of high development, as implied by eyes and other senseorgans, devices for capturing and ingesting food, organs of digestion, secretion, excretion, and respiration; in short, practically all the great anatomical and physiological systems now possessed by animals. The Cambrian animals had acquired the various habits of life possessed by existing animals of their kind, as well as the various modes of preserving their lives.
(2) The studies of recent decades have convinced investigators that later forms of life were derived from earlier ones by processes of evolution, the exact method of which is not altogether understood; but the fact of derivation is not now regarded as an open question. As the various forms developed and diverged from a common ancestral stock, many of the intermediate forms disappeared, and thus the diverging forms became somewhat widely separated. By continued divergence, with the loss of intermediate types, a more and more discontinuous series of forms was developed, and those branches which lived on became more and more distinct. The process was not unlike the evolution of a tree-top, in which the dying out of most of the interior branches leaves a few great limbs which bear the more numerous and more recent branches, while these in turn bear the uppermost and outermost twigs which represent the living phase. In some such way, it is thought that the existing divergence of organisms into kingdoms, branches, classes, orders, families, genera, species, and varieties came to be established.
If it is assumed that the whole system of living things has been derived from a common primitive form or from a few primitive forms, a comparison of the primitive state with the degree to which divergence had gone in the Cambrian times will give some im
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pression of the amount of evolution already accomplished. If to this be added a comparison between the Cambrian life and that of the present time, an estimate of the relative amount of evolution before and since the Cambrian period may be made.
It is to be noted that not only were all the animal sub-kingdoms, save perhaps the vertebrate, present, but that, in many of them, the forms had come to have so nearly the aspect of living forms that the classes and some orders are readily recognized. The initiation and divergence of the structures and types that preceded the Cambrian stage mean much more in the way of evolution, than all the evolution of later times. Formulated in numerical terms, we may perhaps say that 60 to 90 per cent of the evolution represented by the life of today, had been accomplished in pre-Cambrian times.
Mental development. The wars of the Cambrian animals, implied by their weapons of offense and defense, can scarcely have been unaccompanied by some notable measure of mental development, however the nature of such development may be interpreted. That the trilobites sought their food or pursued their prey by sight, and were guided by touch, is implied by their eyes and antennae, and it is difficult to conceive of functional senses without the mental processes that usually attend pursuit and capture.
Ecological adaptations. The distribution of the Cambrian fossils indicates that then, as since, there was an adaptation of life to its physical environment. There seem to have been zones of shore life, off-shore life, and deep-sea life, although the evidence of the last is scant. These variations must be taken into account in the comparison and correlation of faunas, for considerable geographic differences may occur among faunas which were strictly contemporaneous.
Zoölogical provinces. The assemblages of the life of the period leem to have varied in a broader way, giving rise to zoölogical provinces. It is probable that the leading factors in the developnent of these provinces were barriers that isolated, or partially solated, certain portions of the sea from other portions. The separation must have reached such a degree as to cause the life of each area to develop along its own lines, in more or 3ss independence of the evolution of other regions.
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The early faunas of the Cambrian were somewhat provincial in nature, but toward the close of the period, as the seas spread over the continents, there was a marked tendency toward cosmopolitanism. The impression must not be gained that life was everywhere the same at a given stage. There was probably less variation geographically, during most geologic periods, than there is to-day, though it is not certain that this was true of all past epochs. It is improbable that there was ever uniformity over the whole globe.
The Succession of Faunas
Under the doctrine of evolution, it is presumed that the life of every past stage has grown out of that which immediately preceded it, and that it has merged into that which immediately followed it. It is usually assumed that if no exceptional influences affected the process, there was a continuous series of slow changes without sharp lines of demarkation. If this conception were realized in fact, it would be less appropriate to speak of a succession of faunas than of one continuous ever-changing fauna. It is not yet demonstrated, however, that evolution proceeded solely by very slight changes coming in from generation to generation. It may have proceeded by distinct and abrupt changes.[17] This doctrine, as now held, does not maintain that a whole fauna would be likely to change into a different fauna abruptly, but merely that new species might arise in it abruptly. Irrespective of this or any other specific hypothesis, it is to be noted that the geological record, as now known, does not show complete gradations from one species into another. In some cases there is a close approximation to a graded series from one species to another, but the steps of the gradation are not sufficiently close and definite to decide between evolution by an infinite number of small changes, and a smaller number of greater changes.
If we turn from species to faunas, it is obvious that a more general point of view must be taken. Observation shows that in some cases one fauna graduates into the succeeding one, while in [p. 505] other cases the change appears to be abrupt. If the progress of life the world over could be studied as a unit, it would probably appear that there was a nearly perfect gradation of the life of one stage into that of the next. This gradation probably took place more rapidly at some times than at others, and it is quite certain that some forms changed much more rapidly than others. But when we limit our study to the succession of faunas on any one continent, or to any one province, it is evident that the progress of evolution in the region studied was interrupted by physical changes which affected the depth, temperature, or clarity of the water, and the nature of the bottom, and that these changes brought about variations in the character and distribution of life. Out of these local influences superposed on the general progress of life, there came to be rather definite times of notable change, between which the faunas retained a rather constant character, though always undergoing some modification. Where the faunal change in a conformable series is abrupt, and there is no evidence of a hiatus in the record, the explanation is usually to be sought in migration, as when a new fauna came in from some other region, overwhelming the old fauna. The whole process is closely analogous to the well-known succession of human races brought about by the migrations of man.
In the study of faunal progress, therefore, there is occasion to recognize (1) rather abrupt changes brought about by overwhelming invasions; (2) less abrupt changes brought about by the more gradual ingress of outside species, and the gradual commingling of the immigrants with the resident species; (3) very gradual changes due to the slow evolution of resident species when not much affected by immigration or by physical changes; and (4) more rapid evolution due to profound changes in the physical conditions or to other agencies less well understood.
The abrupt appearance of the Cambrian fauna. The explanation of the apparent suddenness of the appearance of the Cambrian fauna is one of the open questions of geology. In a general way, it may be said that older formations have been subjected to metamorphism, and that this tended to destroy their fossils; but this suggestion is not altogether adequate, for some of the older formations are not greatly changed, and seem quite suitable for the preservation [p. 506] of fossils. Furthermore, fossils are sometimes retained in later formations that have been much disturbed and altered. It is also true that some later formations which seem well suited to receiving and retaining organic impressions are devoid of them. Geologists are inclined to refer the scantiness of pre-Cambrian fossils, and hence the apparent abruptness of the introduction of the Cambrian fauna, to unfavorable conditions for fossilization in pre-Cambrian time, combined with subsequent changes in the rock. This makes the abruptness a matter of record, rather than a matter of fact.
Map Work._ The following folios of the U. S. Geological Survey furnish good maps _for the study of the stratigraphy and the stratigraphic relations of the Cambrian system in different parts of the United States. The texts of the folios give some account of the physical history of the several regions: Alabama, Gadsden; Arizona, Clifton; Colorado, Anthracite-Crested Butte; Georgia, Rome; Maine, Penobscot Bay; Massachusetts, Holyoke; Michigan, Menominee; Montana, Little Belt, Fort Benton; New Jersey, Franklin Furnace; North Carolina, Mount Mitchell, Nantahala, and Pisgah; Oklahoma, Tishomingo; Tennessee, Maynardville, and Morristown; Utah, Tintic; Virginia-West Virginia, Bristol, Harper’s Ferry, Tazewell; Wyoming, Absaroka (Crandall Sheet), Bald Mountain-Dayton, Cloud Peak-Fort McKinney, and Sundance.
A summary of the literature on the North American Cambrian prior to 1892 is given by Walcott in Bull. 81, U. S. Geol. Surv. ↩︎
Ulrich and Schuchert think that the Appalachian synclinorium of Early Cambrian times was largely drained at the close of that epoch. Bull. 52 (Paleontology 6) N. Y. Mus. Rept. of the State Paleontologist, 1901, p. 636. ↩︎
Chamberlin and Salisbury, Earth History, Vol. II. ↩︎
Hayford, Washington Acad, of Sci., Vol. VIII, 1906, pp. 25-40. ↩︎
Putnam and Gilbert, Bull. Phil. Soc. Washington, Vol. XII, pp. 31-75, and Gilbert, Jour. Geol., Vol. Ill, p. 351. ↩︎
Walcott, lour, of Geol., Vol. XVII, 1909. ↩︎
Dawson, Bull. Geol. Soc. Am., Vol. XII, pp. 64-68. ↩︎
See folios of the U. S. Geol. Surv. ↩︎
Cushing. Bull. Geol. Soc. Am., Vol. XIX, p. 155. ↩︎
The best summary, in English, of the Cambrian of Europe, is found in Geikie’s Text-book of Geology, 4th ed., Vol. II. ↩︎
Reusch, Norges geologiske Undersoegelse: Det nordlige Norges Geologi, 1891. Also Strahan, Quar. Jour. Geol. Soc, Vol. LIII, 1897, pp. 137-146. ↩︎
Willis, Researches in China, Vol. II. ↩︎
David, Report of International Geological Congress at Mexico, 1907; and Howchin Quar. Jour. Geol. Soc, Vol. LXIV, p. 234, 1908. ↩︎
For a general discussion of this matter, see Williams’ Geological Biology, Chap. II. ↩︎
Ulrich would refer the beds containing the Cephalopods to the Ozarkian, a system which he would make to include the upper part of the Cambrian and the lower part of the Ordovician, as usually classified. ↩︎
Howchin, Quar. Jour. Geol. Soc. Vol. LXIV, p. 237. ↩︎
DeVries Die Mutationstheorie, 1903. See also Bateson’s Material! for the Study of Variation, 1894; and W. B. Scott, On Variations and Mutations, Am. Jour. Sci., 1894, p. 355. ↩︎