occur on the surface of all the large beds of coal.  None of these appearances could have been produced by subaqueous action. (6) Though the roots of the Sigillaria bear more resemblance to the rhizomes of certain aquatic plants; yet, structurally, they are absolutely identical with the roots of Cycads, which the stems also resemble.  Further, the Sigillariae grew on the same soils which supported Conifers, Lepidodendra, Cordaites, and Ferns—­plants which could not have grown in water.  Again, with the exception perhaps of some Pinnulariae and Asterophyllites, there is a remarkable absence from the coal measures of any form of properly aquatic vegetation. (7) The occurrence of marine, or brackish-water animals, in the roofs of coal-beds, or even in the coal itself, affords no evidence of subaqueous accumulation, since the same thing occurs in the case of modern submarine forests.  For these and other reasons, some of which are more fully stated in the papers already referred to, while I admit that the areas of coal accumulation were frequently submerged, I must maintain that the true coal is a subaerial accumulation by vegetable growth on soils, wet and swampy it is true, but not submerged.”

I am almost disposed to doubt whether it is necessary to make the concession of “wet and swampy;” otherwise, there is nothing that I know of to be said against this excellent conspectus of the reasons for believing in the subaerial origin of coal.

But the coal accumulated upon the area covered by one of the great forests of the carboniferous epoch would, in course of time, have been wasted away by the small, but constant, wear and tear of rain and streams, had the land which supported it remained at the same level, or been gradually raised to a greater elevation.  And, no doubt, as much coal as now exists has been destroyed, after its formation, in this way.  What are now known as coal districts owe their importance to the fact that they were areas of slow depression, during a greater or less portion of the carboniferous epoch; and that, in virtue of this circumstance, Mother Earth was enabled to cover up her vegetable treasures, and preserve them from destruction.

Wherever a coal-field now exists, there must formerly have been free access for a great river, or for a shallow sea, bearing sediment in the shape of sand and mud.  When the coal-forest area became slowly depressed, the waters must have spread over it, and have deposited their burden upon the surface of the bed of coal, in the form of layers, which are now converted into shale, or sandstone.  Then followed a period of rest, in which the superincumbent shallow waters became completely filled up, and finally replaced, by fine mud, which settled down into a new under-clay, and furnished the soil for a fresh forest growth.  This flourished, and heaped up its spores and wood into coal, until the stage of slow depression recommenced.  And, in some localities, as I have mentioned, the process was repeated until the first of the alternating beds had sunk to near three miles below its original level at the surface of the earth.