576 HISTORY OF GEOLOGY. 



stratum of a known locality as a type, were good, as 

 far as an identity with that type had been traced ; 

 but when this had been incompletely done, they 

 were liable to great ambiguity. If the Alps or the 

 Jura contain several formations of limestone, such 

 terms as we have noticed, borrowed from those 

 mountains, cease to be necessarily definite, and may 

 give rise to much confusion. 



Descriptive names, although they might be sup- 

 posed to be the best, have, in fact, rarely been for- 

 tunate. The reason of this is obvious ; the mark 

 which has been selected for description may easily 

 fail to be essential ; and the obvious connexions of 

 natural facts may overleap the arbitrary definition. 

 As we have already stated in the history of botany, 

 the establishment of descriptive marks of real classes 

 presupposes the important but difficult step, of the 

 discovery of such marks. Hence those descriptive 

 names only have been really useful in geology 

 which have been used without any scrupulous re- 

 gard to the appropriateness of the description. The 

 Green Sand may be white, brown, or red; the 

 Mountain Limestone may occur only in valleys; 

 the Oolite may have no roe-like structure ; and yet 

 these may be excellent geological names, if they be 

 applied to formations geologically identical with 

 those which the phrases originally designated. The 

 signification may assist the memory, but must not 

 be allowed to subjugate the faculty of natural clas- 

 sification. 



