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[p. 436]
The sketches of the early stages of the earth’s history presented in the last chapter are no more than inferential, yet they afford a helpful introduction to the study of that part of the earth’s history which is recorded in the rocks. Figs. 337-339 represent diagrammatic radial sections illustrative of the different conceptions of the constitution of the earth’s interior. The following summary should make the figures clear:
According to the conception of the history of the earth based on the Laplacian or gaseo-molten hypothesis, there should be pre-sedimentary igneous or meta-igneous rock at all points below the prevailing sedimentary rocks of the surface (Fig. 342) . The plane of demarkation between these two sorts of rock should, as a rule, be distinct.
According to the suggested modification of the gaseo-molten hypothesis (p. 430), the above distinction would fail; for between the original crust and the sedimentary rocks above, there should be a zone composed of mingled igneous and sedimentary rocks, or their metamorphic equivalents (Fig. 338). This intermediate zone might not be sharply differentiated either from the original crust below or the sedimentary group above.
According to the planetesimal theory, (1) the core of the earth (Fig. 339) is made up of nebular or planetesimal matter. After aggregation, the planetesimal matter was probably recrystallized under the influence of the heat and pressure which the aggregation involved, the resulting rock being essentially igneous in its nature. Outside the central core there should be (2) a thick zone made up largely of planetesimal matter, but partly of igneous rocks erupted from below, and partly of sedimentary rocks. The planetesimal matter is assumed to predominate in the lower and major, part of this zone. Igneous rock, eruptive and irruptive, is assumed to have a somewhat irregular distribution through it, while sedimentary rock increases in importance above, but remains throughout a subordinate constituent. This zone records the growth of the earth from the initiation of volcanic and atmospheric processes to the close of the period of notable growth by accretion. The central core and this thick zone about it represent the Formative Eon (p. 435). (3) The next zone, relatively thin, is assumed to be made up largely of extrusive igneous rocks, with subordinate amounts of sediment and matter gathered in from space. This zone represents the Extrusive Eon (p. 435). (4) On the outside lies the superficial zone in which sedimentary rocks predominate, though associated with not a little rock of igneous origin. The first two zones outside the core are assumed to be universal, while the outermost one fails to completely cover the globe.
[p. 437]
[p. 438]
Conjectures concerning the oldest accessible rocks. With these theoretical sections in mind, it is pertinent to inquire what might be the nature of the oldest rock formations accessible, on the various hypotheses which the diagrams represent. The deepest excavations yet made in the earth are but little more than a mile in depth, and while, by reason of deformation and erosion, rocks once at greater depths have been exposed, the maximum thickness of rocks open to observation is but a few miles. Definite knowledge of rock formations and structures is therefore limited to some such thickness.
(1). According to the gaseo-molten hypothesis in its simple form, we might hope to reach the original crust; for it is not rational to suppose that the original crust, the principal source whence sedimentary rocks were derived, is everywhere covered so deeply by the material derived from it as to be inaccessible. (2). According to the modified form of the gaseo-molten theory, the oldest accessible rock should be the zone of mingled extrusive and sedimentary rocks between the original crust and the doininantly sedimentary formations above. (3). On the planetesimal hypothesis, the oldest rocks to which we might hope to gain access would be those referred to the Extrusive Eon (p. 435), during which more or less sedimentary rock was mingled with the volcanic. On [p. 439] this hypothesis, as on the preceding, no sharp line of demarkation would be expected between dominantly sedimentary rocks above, and dominantly non-sedimentary rocks below.
The oldest rocks known. The rock-formations now most widely exposed at the surface are sedimentary, and were formed during the great Gradational Eon (p. 435). In not a few places, however, diverse formations which are predominantly extrusive (igneous or meta-igneous) are found, either beneath the prevailing sedimentary rocks, or projecting up through them in such relations as to show their greater age. In many cases these lower and older rocks were thoroughly metamorphosed, and in essentially their present condition, before the deposition of the overlying beds. These dominantly igneous and meta-igneous formations which antedate the oldest known series of dominantly sedimentary rocks are the oldest formations known, and the era during which they were formed is the first era of which there is definite record in the accessible formations of the earth.
This lowest and oldest group of rocks is very complex, embracing lava flows, volcanic tuffs, igneous intrusions of various types, and sedimentary rocks, all more or less metamorphosed and deformed. Distinct fossils have not been found in them, but the presence locally of (1) carbonaceous slates similar to younger slates which derived their carbon from organic sources, and (2) occasional formations of limestone and chert which, as a class, are usually the products of organisms, are thought to imply the existence of life, and to warrant placing the era when these rocks were formed in the zoic group (p. 435). The era during which or during the later part of which, this oldest system of accessible rocks was made, is the Archeozoic era.
Under the planetesimal hypothesis, the oldest known rocks may be confidently referred to the Archeozoic era, for, according to this hypothesis, rocks of organic origin and rocks containing organic products were not only mingled with all series that are accessible, but with a deep series below, since life is supposed to have originated long before the earth acquired its present size. The oldest formations known may also be archeozoic under the modified phase of the molten hypothesis which has been presented (p. 430) ; [p. 440] but under the simpler form of the hypothesis of a molten earth, the original crust cannot be called archeozoic, since it antedated life. The term Archean (Archean System, Archean Complex) is often applied to the formations here referred to the Archeozoic era. This term is applied to the oldest group of accessible rocks, whatever their origin, and whether contemporaneous with life or antedating it.
Delimitations. The lower limit of the Archean system is assumed to be inaccessible. Its upper limit has been differently fixed by different authors. The term Archean (very old) was originally introduced to displace the older terms Azoic (without life) and Eozoie (dawn-life), whose etymological meanings made them no longer appropriate for the rocks to which they were applied. As first defined the Archean included all rocks below the Cambrian (p. 476); but progressive study has shown that there are several great systems of sedimentary or meta-sedimentary rocks (with much igneous rock), unconformable with one another, between the Cambrian system above and the Archeozoic system below. The systems of pre-Cambrian rocks which are dominantly sedimentary, should be separated from the dominantly igneous or meta-igneous complex below, and the term Archean is now generally restricted to the latter. The upper limit of the Archean is therefore the base of the oldest dominantly sedimentary system.
General Characteristics of the Archean[1]
As now understood, the Archean is made up of two great classes of formations: viz., (1) a great schist series, and (2) a great granitoid series.
[p. 441]
(1) The schist series is made up chiefly of the metamorphosed products of lava flows and volcanic tuffs. In composition these schists vary greatly; but the dominant types are hornblende schists, greenstone schists, mica schists, etc. Associated with the metamorphosed surface lavas and pyroclastic formations, there are some massive igneous rocks and occasional beds of metamorphosed conglomerate, sandstone, shale and limestone, and beds of iron ore, all of which imply the contemporaneous activity of water. In a few places, as at Vermilion, Minn., the iron ore is in workable quantities.
(2) Among the most conspicuous features of the Archean rocks, in their present eroded condition, are the great masses of granite and gneiss that protrude through the schists. Until recently, these granites and gneisses were commonly regarded as the oldest known rocks, and were styled “primitive” or ''fundamental"; but it is now known that many of them, at least, are intrusions into the schist series, and therefore younger than the latter. The gneisses are regarded as metamorphosed granites.
In the formation of both the surface flows and the intrusions, the ascending lavas must have occupied numerous fissures or conduits connected with the interior, and hence there came to be numerous dikes and other intrusions, traversing the older parts of the Archean. It is also to be borne in mind that all younger intrusions and extrusions of lava must have passed through the Archean, leaving dikes and other bodies of igneous rock in it. These later intrusions are of course not strictly a part of the Archean system, but they are not always separable, and their presence adds to the complexity of the Archean as a whole.
Diastrophism and metamorphism. The most satisfactory explanation of the prevalent foliated structure of the Archean (Fig. 340) seems to be that which refers it to the movements of the outer part of the earth, in Archeozoic and later time. Intrusions of igneous rock probably helped to metamorphose the rocks by furnishing heat and by developing pressure in the zone where they were intruded. Not only this, but the transfer of so much material from below developed lateral pressure by causing the outer parts to settle down to take the place of the lava transferred upward. [p. 442] The result would be lateral thrust in the outer parts of the earth, and this thrust would be relieved by the bending and crumpling of the rocks, and by shear.
That the rocks should have undergone extraordinary metamorphism under these conditions is natural. It has been demonstrated that massive igneous rocks, by crushing and shearing, have been transformed into rocks with a foliated or schistose structure, and it is in the rocks of this era especially that metamorphism of this type is found. It is now believed that the larger part of exist ing gneisses, as well as a considerable part of existing schists, acquired their foliated structure in this way; but it is to be understood that some of the schists and perhaps some of the gnei arose from clastic formations by other processes of transformation. It is not to be understood that the metamorphism of the Archean rocks was completed during the Archean era. All metamorphosing processes of subsequent times have affected them.
[p. 443]
It would indeed be difficult to obtain an exaggerated idea of the complexity of the rock which has caused this system to be called the "Archean Complex/’ the “Basement Complex,” the " Fundamental Complex/’ etc. The rocks of no later era are so generally and so notably deformed, or so generally and so highly metamorphic. Because of these complications, the interpretation of these rocks is difficult, and such views of their classification and correlation as are now entertained are to be held subject to emendation as knowledge advances.
General distribution. The Archean is the one accessible rock system which, theoretically, completely envelopes the globe. No later system does this, for wherever the Archean comes to the surface, later formations are absent.
In speaking of the distribution of a formation, its distribution at the surface is generally meant, and in speaking of its surface distribution, the mantle rock (glacial drift, etc.) which overlies and conceals it is usually ignored unless it is so thick as to make the underlying formation indeterminable. When the surface distribution of the formation is given, therefore, it is not to be understood that the formation is literally at the surface everywhere within the area specified, but rather that it is exposed here and there within that area, and that between the points of exposure it is the uppermost formation beneath the mantle rock. In this sense, the Archean rocks are estimated to appear at the surface over about one-fifth of the area of the land; but since great areas in some continents have only been reconnoitered geologically, this figure is only a rough estimate.
In North America[2] by far the largest area of Archean rock [p. 444] lies in Canada (Fig. 341), but it is to be noted that formations of the Proterozoic and later eras occupy numerous small tracts within the area shown on the map, though the Archean underlies them at no great depth. Lying rudely parallel to the great Canadian area on the southeast is an interrupted series of probable Archean tracts, extending from Newfoundland to Alabama. Similarly, on the southwest, there is a belt of detached areas stretching from Mexico to Alaska. In few places within these belts have the ancient rocks been studied in great detail. Lesser areas of Archean rock appear in Michigan and Wisconsin, in Minnesota, and in the Adirondack region of New York. In some of these regions, Archeozoic rocks have not been carefully separated from Proterozoic.
Detailed work has covered but a small part of this great tract of Archean rock. The vicinity of Lake Superior in Canada, Michigan, Wisconsin, and Minnesota, the area north of Lake Huron, and the Ottawa region in Ontario, are the areas where the system is best known.
Summary. By way of summary it may be said that the Archean system consists in some places of rocks which are mainly massive (igneous intrusions); in other places, of rocks which are mainly gneissic (chiefly meta-igneous) ; and in still others, of rocks (largely meta-igneous and subordinately meta-sedimentary) in which a schistose structure predominates. Furthermore, the rocks of each of these structural types have a wide range in composition, from acid on the one hand to basic on the other. Rocks of all these classes are often intimately associated, and any one may predominate over the others. In some places the rocks of the several structural types graduate into one another so completely as to leave no line of separation, while in others their definition is sharp. Thus massive rock sometimes appears in distinct dikes in the gneisses and schists, while schists are frequently in dike-like sheets in rocks which are more massive. Furthermore, the relations of these several sorts of rock have been enormously complicated by the distortion to which they have been subject. The structure and relations of the several sorts of rock in the system indicate that it was (1) by successive intrusions, large and small, of rocks of different chemical composition into (2) still older rocks which were originally (a) chiefly extrusive-igneous and of varying chemical composition, but (6) subordinate^ sedimentary; and (3) by successive dynamic movements resulting in various degrees of metamorphism and deformation of the various parts, that the intricate structure and composition of the Archean complex was attained.
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Though the variations in the rocks of the Archean system are great, there is, nevertheless, a certain homogeneity in the heterogeneity of the whole. No one considerable part of the system is very different from any other considerable part, and no definite and orderly relationship between the different parts has been made out over any considerable area. There appears to be no traceable succession of beds, and no definite stratigraphic sequence, such as can be made out in great series of meta-sedimentary rocks, however much folded and metamorphosed.
The Archean in other countries. The general characters and relations of the Archean in North America seem to be duplicated in other continents. Corresponding systems of rocks, made up primarily of meta-igneous rocks, but subordinately of meta-sedimentary rocks inextricably involved with them, are known in all continents. The general characteristics and relations of the Archean, as developed in North America, therefore appear to be essentially world-wide.
Bearing of the Archean on the theory of the origin of the earth. With the essential facts concerning the constitution and structure of the system in mind, it is in order to inquire to what hypothesis of the earth’s origin they best adjust themselves. The constitution of the system makes it clear that it does not represent the original crust of the earth or its downward extension. It cannot be affirmed, however, that no part of what is now classed as Archean is referable to the original crust; that is, it cannot be affirmed that no part of the igneous or meta-igneous rock of the Archean is referable to an azoic or prezoic period, strong as the evidence against such reference may seem. On the other hand, all the facts now known concerning the Archean adjust themselves to the planetesimal hypothesis, or to the modified form of the gaseomolten hypothesis. They cannot, however, be said to establish either, or to preclude other hypotheses of the origin of the earth.
[p. 447]
The question of the origin of the Archean must, therefore, still be regarded as an open one.
Earlier views concerning the Archean. In explanation of the Archean system, many different hypotheses have been suggested at one time and another, most of them starting with the Laplacian hypothesis as a beginning. One of them is that the Archean rocks are wholly of metamorphosed sediments; a second, that they are igneous rocks produced by the fusion of sediments; and a third, that they are igneous rocks intruded beneath the oldest known sedimentary rocks after the deposition of the latter. These hypotheses have a historic interest, but since they are not now generally held by geologists, their consideration will be omitted.[3]
Life during the Archeozoic era. The presence of carbonaceous material, of bedded iron ores that were once carbonates, of cherts, and of limestones, implies the presence of life during the era occupied in the formation of the Archean rocks. Since no fossils have been found, nothing is positively known of the character of the life, and little, except by inference, of its abundance.
Duration of the Archeozoic era. Of the duration of the Archeozoic era nothing can be said beyond the general statement that it was very great, a conclusion which is independent of any particular conception of the earth’s origin. If the planetesimal hypothesis is the true one, there is no readily assignable lower limit to the Archeozoic system, and the duration of the Archeozoic era may exceed that of all subsequent time.
Climate. Of the climate of the era nothing is known except that it seems to have been such as to permit the existence of life, and the ordinary phases of sedimentation.
1
Bull. 86, U. S. Geol. Surv., pp. 478-484, and 16th Ann. Rept., U. S. Geol. Surv., Pt. II, pp. 747-756. The interpretation of the Archean as set forth In these publications needs some modification in the light of recent discoveries. The sedimentary 'character of parts of the system is set forth in the following: Mono. XLV, U. S. Geol. Surv., pp. 129, 172-212; 21st Ann. Rept., U. S. Geol. Surv., pp. 317 and 402-404; Mono. XXVIII, U. S. Geol. s.nv… pp. 186 188; Am. Geol., Vol. XXVIII, 1901, pp. 14-19; Jour. Geol., Vol. X, p. 71: Rent. of Bureau of Mines of Ontario, 1902, pp. 155-162; Geol. Surv. of Canada, Vol. Ill, Pt. I, F, 1887-99; Am. Jour. Sci., Vol. 50 (1895), pp. 58-69. Trans. Roy. Soc. Canada, 1899 and 1901. In the references to Canadian localities. some of the rocks classed as Archean (U. S.) are described under the name of Lower Huronian. ↩︎
The literature on the Archean (as well as Algonkian [Proterozoic] ) of North America was summarized by Van Hise in Bull. 86, U. S. Geol. Surv., in 1892. This publication gives full bibliography to its date. A later (1896) and briefer summary of the same subjects by the same author was published, Pt. II, 16th Ann. Rept., U. S. Geol. Surv., pp. 744-843. The pre-Cambrian literature since 1892 has been summarized from time to time by the same author, and by Leith, in the Jour, of Geol., as follows: Vol. I, pp. 304 and 532; II, pp. 109 and 444; III, pp. 227 and 709; IV, pp. 362 and 744; VI, pp. 527, 739, and 840; VII, pp. 190, 406, 702, and 790; VIII, p. 512; IX, pp. 79 and 441; and XII, pp. 63 and 161. ↩︎
See the authors’ Earth History, Vol. II. ↩︎