1 2 GRANITIC ROCKS OF CORNWALL. [Ch. II. 



observer : but when a rock acquires a name from the nature 

 of its composition, whether purely mineralogical or blended 

 with organic remains, then this part of the science becomes 

 perfectly descriptive, like the other branches of natural history; 

 and much curious and accurate information may then be ex- 

 pected from travellers who are not accomplished geologists. 



This subject of nomenclature will be entered on more 

 fully hereafter, when the general reader will be better pre- 

 pared to appreciate the importance of mineralogical distinc- 

 tions among the primary rocks ; and it is hoped that the 

 geologist will admit that they are not frivolous, when it is 

 shown that by their means some arrangements in the structure 

 of the earth will be developed which otherwise could not have 

 been detected. 



In most of the lately published geological accounts of coun- 

 tries we look in vain for details concerning primary rocks : 

 we sometimes, indeed, learn that such and such a district 

 consists of granite, but cannot collect any information con- 

 cerning its composition, or the manner in which its varieties 

 are associated together. We are therefore left quite in the 

 dark as to the nature of this rock ; for the various kinds 

 which may come under the designation of granite are not only 

 exceedingly numerous, but have very frequently no resem- 

 blance whatever to the common variety so universally known 

 by the name of granite. 



This ought not to be. And Dr. Macculloch*, although 

 he has so strongly advocated this method, admits, in his 

 directions for conducting geological observations, that " a 

 correct description of its mineral characters is necessary, as 

 the Varieties of this rock are highly interesting, especially 

 under its passages into trap." Now, this trap is only a variety 

 of that kind of granite which abounds with hornblende, the 

 characteristic mineral of this rock in many parts of Scotland : 

 and if the different forms of granite are deserving of remark 

 when hornblende is present, surely they ought also to be 



* System of Geology, vol. ii. p. 473. 



