Fig. 666.—Fern (Pecopteris ligata) from upper shale, Scarborough.
These “Coal Measures” occupy an area of five hundred square miles in Great Britain alone; and as we have already said, the period which elapsed while these deposits were being laid down represents hundreds of thousands of years. The deposit would increase at the rate of about three feet in a thousand years. And this is only one period of the many changes to which our world has been subjected since it first began its revolution in space around the sun.
Fig. 667.—Section across the carboniferous rocks of Derbyshire and Lancashire (After Ramsay).
1. Carboniferous Limestone. 2. Yoredale Shales. 3. Millstone Grit.
4. Coral Measures. 5. Permian Limestone. 6. New Red Sandstone.
Coal is usually found in “basins” or depressions—a sort of trough owing to the upheaval of surrounding strata which became in time denuded (or washed away) with any coal that was there. So it is in places where it is concealed by overlying beds that protect it, that we now find the coal saved from disturbance. When we search and come upon red sandstone and grey-wacke, we may be almost certain that we are near coal, particularly if the surrounding rocks form a “basin.”
Fig. 668.—Labyrinthodon.
We have now briefly sketched the Carboniferous system, for in our recreations we can do little more. We shall find in many places in England tree trunks in the sandy shore, and ample evidence that a forest has been at one time submerged in that spot. So inland the land sank down again and again in successive periods—water, mud, soil, vegetable growth succeeded, to be again submerged and form a new coal seam for the use of man, who was destined to appear after the lapse of ages.
The Permian Period.
