158 ON THU ORIGIN OF COAL. 



quite as sensitive of such a change as a Fauna. 

 The balance of evidence, therefore, is much in 

 favour of the water having been of one kind, and 

 on the whole, probably salt and not fresh. 



The materials composing the various beds, 

 known by the term Coal measures, are the main 

 characters that will enable us to judge of the 

 circumstances under which they were deposited. 

 These are to be regarded as true measures of the 

 intensity of the currents of water, which brought 

 them to the places where they are now found, 

 and are, therefore, of great value in ascertaining 

 the physical condition of the globe at that period. 

 They maybe conveniently divided into arenaceous 

 and argillaceous beds.* The first, consisting of 

 rough pebbly gritstone, gritstone, fine sandstone, 

 and sandy shale. The last, of shale, bind, soap- 

 stone, fire clay, and indurated silt. Black bass is 

 also an argillaceous deposit, mixed with a 

 considerable proportion of bituminous matter. 

 Probably these deposits may not always occur in 

 the exact order here pointed out, or all of them 

 together ; still, in the rich part of a Coal-field 

 they graduate one into another with great 



* Beds of limestone are met with in the upper Coal-field, 

 but they are very rare. 



