COAL. 613 



strata, and associated with different members of 

 that series. The whole of its connections of this 

 nature, have not been ascertained ; but to detail 

 even that which is known, would here be out. of 

 place ; while it is not of a nature to admit of an 

 useful or intelligible abridgment within the pres- 

 cribed limits. It forms, in its application to the 

 uses of life, one of the most important branches 

 of geological science. 



The lateral extent of these deposits, com- 

 monly known by the name of coal fields, is often 

 very considerable, and it is observed, that, in 

 many cases, they occupy basin-like cavities, often 

 incurvated upwards on all sides. In others, they 

 are inclined at various angles, in such a manner 

 that their probable extent cannot be conjectured. 

 It is usual for more than one bed of coal to be 

 found in a deposit, and sometimes they reach to 

 ten, twenty, or more. At Liege, it is said that 

 there are sixty beds. The alternating substances 

 are sandstone, limestone, shale, clay, and sand. 

 The thickness of the coal strata varies from less 

 than an inch to many feet. Where thjy are 

 traversed by trap veins, or in contact with masses 



