A FEW WORDS ABOUT COAL * 293 



fields are undoubtedly due to the vegetation of former 

 eras, it is far from being the case that the primeval 

 forests became converted in a general way into coal. 

 Conditions of a peculiar, and to some extent excep- 

 tional, character were requisite for the formation of 

 coal-fields. If we consider the evidence given by the. 

 coal-fields themselves, we shall see what these con- 

 ditions were. 



The beds or seams of coal form but a small portion 

 of the thickness of the great geological group of strata 

 to which they for the most part appertain. This group 

 is called the carboniferous, and not uncommonly c The 

 Coal ;' but even where coal is most abundant, it forms 

 only a minute part of the whole mass. Thus it has 

 been estimated, Sir Charles Lyell tells us, that in South 

 Wales the thickness of the carboniferous strata amounts 

 in all to between 11,000 and 12,000 feet (or more than 

 two miles);* 'but the various coal-seams do not,' 

 according to Professor Phillips, c exceed in the aggre- 

 gate 120 feet,' or little more than one-hundredth part of 

 the whole. In North Lancashire the carboniferous strata 

 occupy a depth of more than three and a half miles, 

 with the same relative disproportion between the 

 thickness of the coal-seams and that of the complete 

 series of strata. Again, in Nova Scotia the coal- 

 bearing strata attain a thickness of more than three 



* It is, perhaps, hardly necessary to remark that this depth has not 

 been measured anywhere in a vertical direction. The thickness of the 

 several layers can be measured where they either crop out, or show at 

 the surface, or else come within the range of mining operations ; and 

 thus the total depth of the series can be estimated. 



