THEORY OF THE ACCUMULATION OF COAL. 373 



Theory of the Accu?nulation of Coal. 



There is no question connected with the Carboniferous period con- 

 cerning which there has been more discussion than the mode in which 

 coal has been accumulated. There are some things, however, about 

 which there is little difference of opinion. These we will state first, 

 and thus narrow the field of discussion. 



Presence of Water. — That coal has been accumulated in the pres- 

 ence of water, or at least of abundant moisture, is evident : a. From the 

 preservation of the organic matter. By aerial decay vegetable matter 

 is either entirely consumed, or else crumbles into dust. Only in the 

 presence of water is it preserved and accumulated in larger quantities. 

 b. The interstratified sand and clays and limestones have, of course, 

 been deposited like all strata in water, c. The coal itself is not un- 

 frequently distinctly and finely stratified, d. The plants found in con- 

 nection with the coal-seams are mostly such as grow in moist ground. 



Thus far, then, theorists agree, but from this point opinions diverge, 

 and until recently have very widely diverged. Some have thought 

 that coal has accumulated by the growth of plants " in situ" as in 

 peat-bogs and peat-swamps of the present day. Others have sup- 

 posed that it has accumulated by driftage of vegetable matter by rivers, 

 like the rafts now found at the mouths of great rivers of the present 

 day. According to the one view, a coal-seam is an ancient peat-swamp ; 

 according to the other, it is an immense buried raft. The one is called 

 the " Peat-bog theory" the other, the Estuary or raft theory. 



Kecently, however, scientific opinions have converged toward a 

 common belief. We will not, therefore, discuss these two rival theo- 

 ries, but simply bring out what is most certain in the present views on 

 this subject : 



1. Coal has been accumulated by growth of vegetation in sitti, as 

 in peat-swamps of the present day. This fact is now demonstrable. 

 The reasons for believing it are the following : a. The purity of coal. 

 The coals of the American coal-fields are, with few exceptions, abso- 

 lutely pure, i. e., the amount of ash is not greater than would result 

 from the ash of the plants of which it is composed. The same is true 

 of coals of most extensive coal-fields everywhere. Now, it has already 

 been shown (p. 142) that in extensive peat-swamps, like the Great Dis- 

 mal Swamp, absolutely pure vegetable accumulations unmixed with 

 sediment occur ; but in buried rafts or drifted vegetable matter of any 

 kind there must be a large admixture of mud. b. The preservation of 

 the most complex and delicate parts of the plant in their natural rela- 

 tions to each other. Large fronds are spread out and pressed as in a 

 botanist's herbarium. Delicate leaves are preserved with all their finest 

 venation perfectly visible. This is exactly what we would expect if 



