438 SPECIAL GEOLOGY. 



MODE or DEPOSIT OF COAL. Although the vegetable 

 origin of coal is admitted, a considerable difference of opinion 

 prevails as to the circumstances under which it has become 

 imbedded in its present position. The idea usually enter- 

 tained is, that groves and forests of the luxuriant vegetation 

 of an ultra-tropical climate were swept away by floods and 

 inundations into lakes, bays, estuaries, or the mouths of 

 rivers ; while the instances are few in which the coal plants 

 grew and were submerged on the spot ; it was considered, 

 that drift was the rule, and submergence the exception. An 

 opinion the reverse of this has recently been expressed, that 

 while the greater part of the vegetable elements of coal 

 have grown and been imbedded on the spot, the cases where 

 the plants have been drifted are chiefly the accidental results 

 of the overflows and inundations by which the submersion 

 was effected. The following objections may be urged against 

 the theory that coal was formed by drift : 



1. The purity of coal, and its freedom from extraneous 

 substances. Had it been drifted, it must have acquired some 

 portion of foreign substances in its transit, such as pebbles, 

 gravel, &c.; but, since we find extensive seams of coal unmixed 

 with any other matters, its freedom from these is considered 

 to be incompatible with the idea of its having been drifted 

 from a distance. 



2. The generally uniform thickness of each coal-seam is 

 considered to offer another difficulty. The lower main seam 

 of the great northern coal-field, according to Mr. Bowman, 

 extends over at least 200 square miles, while a thin seam 

 is pointed out as reaching in a straight line from Whaley 

 Bridge to Blackburn, a space of thirty-five miles. Had th$ 

 coal been washed away by floods or torrents, such currents, 

 either from the different specific gravity of portions of the 

 same mass, of the .roots and stems, for example, as contrasted 

 with the branches and foliage, or from the mechanical ob- 

 structions occurring in such a transit, would have deposited 

 them in an unequal manner ; whereas, no such effects are 

 observable in the coal-seams, which are invariably free from 

 inequalities of this kind. 



3. The exceeding minuteness of many of the coal-seams, 

 which thin-out into mere filaments, and extend, in this con- 

 dition, over extensive areas of solid rock, militates against 



