223 



by a predominant degree of compactness not 

 often found in the secondary ; but that is rarely 

 sufficient to form a distinction, except in the 

 hands of a geologist long versant in the minute 

 characters of rocks. This instance affords a 

 striking example of the necessity of attending 

 to geological characters in the classification and 

 nomenclature of rocks, and of the utility of an 

 arrangement founded on geological principles; 

 since it had always been confounded with the 

 secondary sandstone, till it was first pointed out 

 in that work on the Western Isles of Scotland, 

 which is here referred to, on several occasions, as 

 the authority for many of the present statements. 



The distinctions between the argillaceous 

 schists of the primary class and the shales of the 

 secondary, are, in many instances, sufficiently 

 marked. But cases also occur in which they are 

 undistinguishable when separated from their 

 connections ; a circumstance which, in this in- 

 stance also, renders the examination of those 

 connections necessary. 



In the limestones, the same difficulties some- 

 times occur. It has been asserted that the pri- 

 mary calcareous rocks are always to be distin- 



