37 



menclature of rocks, arises from the adoption of 

 a double principle, namely that of the geological 

 relations, and that of the mineralogical nature : 

 and that inconvenience is increased in the works 

 of many authors, even of the highest reputation, 

 by capriciously admitting one of these principles 

 in some instances, and rejecting it in other and 

 similar cases. Thus the same rock is called slate 

 clay in one position, and clay slate in another ; 

 while, in another case, the term greenstone is 

 applied to two or more rocks, in the same manner 

 identical in mineral composition, but differing in 

 the important circumstances of being either stra- 

 tified or unstratified, and of occurring either be- 

 low the most antient, or above the most recent 

 rocks. Whatever system be adopted, it is most 

 necessary that consistency should be preserved. 

 Else it is obvious, that this practice may be made 

 to serve the purpose of any hypothesis ; determin- 

 ing, or not, the position of a rock from its n'ature, 

 just as it may suit the views of the geologist, and 

 thus producing the grievous error of reasoning in 

 a circle. 



In the present state of things there is no re- 



