Coal layers often hold both gas and oil.  The coal layers alternate with shale, stone, and conglomerate.  In some parts of North America and Europe the coal-bearing strata are 18,000 feet in thickness.  Coal is the water-preserved and pressure-modified remains of the rank vegetation growing in the bogs and on the swamp shores of this faraway age. 
In many regions the glacial deposit of local ice formations may be found even among some of the upper and later coal deposits.  Carboniferous was the era of coal. 
200 millions years ago the really active stages of the Carboniferous period began. For twenty million years prior to this time the earlier coal deposits were being laid down. 
See also: UB 59:5; UB 60:2.7; UB 60:3.9; UB 61:2.3.