XIV.] LIVING MATTER AND ITS EFFECTS. 243 



of the ground ] and, when it is remembered that, in the 

 South Wales coal-field, as many as eighty distinct beds of 

 coal may be recognized, it will be seen that the coal- 

 measures offer striking evidence of oscillations of the level 

 of the land. Between each elevation and depression, there 

 must have been time enough for the formation of a thick 

 vegetable soil, and, in some cases, this must have taken vast 

 jieriods of time ; thus, in South Staffordshire, there is, or 

 rather was, a famous bed of coal measuring as much as 

 thirty feet in thickness. Remembering, then, the slow growth 

 of a forest, the great thickness of some of our coal seams, 

 and the number of separate beds in the coal-measures, it 

 M'ill be readily conceded that these strata represent a lapse 

 of time which is probably to be counted by hundreds of 

 thousands of years. 



Before it was understood that each bed of coal gre\Y 

 where it now stands, it was supposed, by many geologists, 

 that coal had been formed by the alteration of wood 

 drifted out to sea. Great rafts of timber and other ac- 

 cumulations of vegetable matter are unquestionably carried 

 down by such a river as the Mississippi; and this matter, 

 becoming buried in the silt of the estuary, might undergo 

 changes resulting in the formation of coal. But, though 

 small deposits of coal may have been formed in this way, 

 no accumulation of drift-wood would be competent to pro- 

 duce beds of pure coal of uniform thickness and great 

 extension, such as those in any of our coal-fields. More- 

 over, there are the stigmarice to show that the plants grew 

 where their remains are found. 



There is, however, a kind of imperfect coal which shows 

 by its texture that it has been formed from wood. So 

 ligneous indeed is its texture that the coal is commonly 

 called lignite. In this country, such coal is found only in 



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