165 



ably extended to rocks obviously compounded, 

 such as bituminous limestone, shale, and chlorite 

 schist. In here enumerating, for the purpose of 

 the following catalogue, the minerals which 

 enter, singly or jointly, into the composition 

 of rocks, these most obviously compounded sub- 

 stances have been omitted; but the list is ex- 

 tended to those which present, at least, the 

 appearance of simplicity. 



\ 



In the catalogue of varieties which accom- 

 panies each principal rock in the Synopses, it 

 has been attempted, as already indicated, to se- 

 parate those of which the characters are most 

 constant, from those which seem the most purely 

 accidental, and which derive their peculiarities 

 from containing imbedded minerals that are 

 foreign to the prevailing composition of the 

 species. If this distinction cannot be rendered 

 very precise or certain, it is nevertheless con- 

 venient ; and, in the far greater number of in- 

 stances, well founded. In the catalogue of the 

 component minerals which here follows, the dif- 

 ferent substances are, in the same way, separated 

 into two divisions ; the one containing those that 



