23 



admit of a satisfactory reply. Some of them have 

 indeed been already anticipated in the preceding 

 observations, 



It is said that the simple and compound rocks 

 are thus united in one class, and that, as the for- 

 mer have already been described in the systems 

 of mineralogy, any further description is super- 

 fluous. 



To this it may be answered, as it was already 

 remarked, that this very association is conve- 

 nient ; since the general relations of the simple 

 and compound rocks are the same, and since 

 they pass into each other by imperceptible gra- 

 dations. Nor can it be considered any inconve- 

 nience to describe a simple rock twice, if, indeed 

 it were esteemed proper that it should be enume- 

 rated in a treatise on mineralogy : it is even ne- 

 cessary to describe these simple substances in 

 every catalogue of rocks ; since that study does 

 not comprise all the circumstances which apper- 

 tain to their geological history. 



It is again objected, that, according to a geo- 



