Ch. II.] DEFECTIVE NOMENCLATURE OF PRIMARY ROCKS. 1 1 



form of the surface ; and thus it is that, whilst some streams 

 are short and rapid, others, which at first run in parallel and 

 adjacent valleys, are conducted by a long circuitous route, 

 exhibiting rapid falls, or gliding slowly along, according to 

 the nature of their channel. 



The longitudinal valleys are the most extended, and often 

 present a more gentle outline ; whilst the transverse commonly 

 form narrow and abrupt hollows, or gorges, in which the 

 rock is exposed to view, affording to geologists opportunities 

 for examination as favourable as any in Alpine regions. 



Such is the external appearance of the primary rocks ; and 

 we now proceed to describe their mineral characters, dividing 

 these rocks into two artificial classes, the granitic and schistose, 

 in order to facilitate their description. 



Under the head of granitic rocks it is proposed to comprise 

 all those various compounds which are usually associated with 

 common granite, being continuous therewith, and forming an 

 integrant part of the same unstratified mass. 



On account of this intimate association, it has become a 

 pretty general practice to designate all these varieties of rock 

 by the name of granite. They are certainly all geologically 

 identical when they occur in the same mass ; but, as will be 

 seen hereafter, they are not confined to this position, and 

 therefore their nomenclature ought not to be founded on a 

 geological basis. This practice is also objectionable, inasmuch 

 as it prevents the student from taking practical lessons in the 

 book of nature until he has completed the course of theo- 

 retical instructions ; and then it too frequently happens, that 

 he can only make his observations according to the prescribed 

 rules, and under a particular bias, prejudicial to impartial 

 observations. 



If a rock be designated according to its geological relations, 

 its name must be liable to change, because the principles of 

 the science are not as yet permanently established ; besides, 

 on this plan, in the examination of countries previously unex- 

 plored, too much depends on the correct judgment of the 



