THEIR RELATIVE AGES. 145 



debifnunenised carbon. They are all coals, and belong to the 

 same family those in the younger formations still retaining 

 much of their vegetable structure and full of volatile matter, 

 while those in the older formations have seemingly lost all 

 traces of structure, and have been all but deprived of their 

 volatile constituents. But even where no structure is ob- 

 vious to the naked eye, it can generally be rendered apparent 

 by submitting thin transparent slices to the microscope. 

 By this means the vegetable origin of the most compact and 

 glistening coal is often revealed as clearly as the tissues in 

 living plants, and thus the observer is enabled to determine 

 not only the organic nature of the mass, but the botanical 

 peculiarities of the order concerned in its formation. 



Since coal is thus merely altered and mineralised vegetable 

 matter, and since vegetation must have nourished more or 

 less during every period of the earth's history, there must 

 be coals of some kind or other occurring in every geological 

 formation. It may appear more abundantly and more 

 availably in one formation than in another ; still, believing 

 in the uniformity of nature's operations, we must be pre- 

 pared to admit its presence in every stratified system, and 

 not to regard it, as was at one time done, as a product 

 peculiar to the Carboniferous era. Arranging the forma- 

 tions in chronological order, we have their coals, or rather 

 the coal family, associated with them in something like 

 the following conditions : 



Quaternary, Peats. 



Tertiary, Lignites. 



Cretaceous, Lignites and Coals. 



Oolitic, Coals. 



New Red Sandstone, Coals. 



Carboniferous, Coals and Anthracites. 



Old Red Sandstone, Coals and Anthracites. 



Silurian, Anthracites. 



Cambrian, Anthracites and Graphites. 



Laurentian, Graphites. 



E 



