of the Bocks of the Coal Formation. 141 



coal partaking so much of its character as to be capable of under- 

 going combustion, which in receding from it they become gradually- 

 more and more earthy, until eventually all trace of organic matter 

 is lost. 



The succession of the rocks in each group, taking them in the 

 most general point of view, is as follows, in descending order : — 



Fire-clay or Thill. 



Sandstone. 



Blue shale. 



Bituminous shale. 



Coarse coal (sometimes Cannel.) 



Coal. 



Coarse coal. 



Fire-clay. 

 This succession, as already observed, is not absolutely invariable, 

 for even sandstone is occasionally found in contact with the coal, 

 and other strata (as clay-ironstone for instance) are sometimes as- 

 sociated with the foregoing, whilst in other instances one or more 

 members of the group are entirely wanting. But these are ex- 

 ceptional cases, the arrangement in the majority of instances being 

 that which I have given above. The rocks employed for analysis 

 were as far as possible selected from one group, and embrace all its 

 characteristic members, so that from the analysis of these few 

 substances we have a tolerably correct idea of the composition of 

 the rocks of the whole formation. 



The actual order of superposition of the specimens analysed is as 

 follows : — The order is ascending, and figures are attached to the 

 strata analysed. 



1. Fireclay (or Thill), 

 Coarse coal, . 



2. Good coal, 



3. Coarse coal, 



4. Bituminous shale, . 



5. Blue shale, or slate-clay, . 



6. Micaceous sandstone, 

 Blue shale (again). 

 Sandstone (again), 

 Blue shale, inclosing no- "I 



dules of ironstone, . J 



Bituminous shale and coal, 

 Ironstone with shells, 



7. Muscle-bind, 



Before entering upon the details; it may be desirable to glance at 

 the methods of analysis. 



Carbon and Hydrogen were deterYnined by combustion with chro- 

 mate of lead, copper turnings being made use of for the deoxidation 



