THE HISTORY OF LIFE 189 



ordinary soft coal at a second, and may consist of an 

 iron-stone at a third part. Coal usually rests upon a bed 

 of fire-clay or shale, through which the roots of the trees, 

 of which coal is partly formed, branch in all directions. 

 Very little is known of the conditions under which coal 

 was deposited. The evidence tends to the view that 

 the vegetation which formed coal grew in marshes near 

 the sea. The nearest analogy to these conditions is 

 probably furnished by the cypress swamps or the man- 

 grove swamps existing in Florida, Bermuda, and other 

 places, That coal is of vegetable origin may be shown 

 by inspecting a thin section of coal by means of the 

 microscope. In some coal there are seen a considerable 

 number of the spore cases which held the spores the 

 fruit of the vegetation, which has been compressed and 

 hardened in time, to form what we call coal. From the 

 " fact that a succession of coal-seams, each representing 

 a former surface of terrestrial vegetation, can be seen 

 in a single coal-field extending through a vertical 

 thickness of 10,000 feet or more, it is clear that 

 the strata of such a field must have been laid down 

 during prolonged and extensive subsidence." l Most 

 probably the vegetation grew under a moist and warm 

 climate. 



Plant life in these strata, presents special interest, 

 inasmuch as it forms the oldest terrestrial vegetation 

 which has been abundantly preserved. It presents a 

 singular monotony all over the world from the Equator 

 to the Arctic circle. Plant life consisted of the lower 

 classes as club- mosses, ferns, &c., which under a moist 



i "Text-Book of Geology," Sir Archibald Geikie, F.R.S., 3rd 

 edition, 1893, p. 808. 



