A FEW WORDS ABOUT COAL. 295 



of vegetable matter, the roots of the trees being- 

 imbedded in the soil which forms the deposit next 

 below the coal. The vegetable layers may probably 

 have been two or three times as thick as the resulting 

 coal-seam, and were reduced by pressure to their 

 present thickness ; but such layers cannot at any time 

 have reached to the branches of the forest-trees. 

 Then the process of deposition began. This can only 

 have happened when some subsidence of the soil had 

 caused it to be submerged to a greater or less depth. 

 We can infer from the depth of the strata overlying 

 the coal-seams that this state of submergence continued 

 in many cases for a long period of time ; and it is 

 equally clear that the formation of the vegetable layers 

 themselves must have been a process occupying a 

 considerable time, since tall trees grew before the next 

 submergence took place. So soon as submergence was 

 complete, the tall trees perished and began to decay. 

 The stout trunks above the vegetable layer were broken 

 off and swept away by the sea. The forest itself, pro- 

 perly so called, was for the most part thus destroyed. 

 It was the decaying refuse of the forest, intermixed 

 with the lowlier growths, which formed the coal-seam 

 as it now exists. Amongst these were the lower parts 

 of the trunks of the ancient forest-trees. These 

 became converted, like the rest of the vegetable 

 matter, into coal. 



But it may be asked how those portions of the trunks 

 which still remain above the level of the vegetable 

 layer are to be accounted for. Are we to suppose that 



