4-8 DESCRIPTION OF THE PRIMARY [Ch. IV. 



in which it occurs. This mode of classification appears to be 

 more simple, and more in accordance with nature, when the 

 quartz is blended with the substance of the adjacent strata ; 

 but when the quartz is pure, or combined with crystalline 

 minerals, then it becomes a true quartz-rock ; often graduat- 

 ing into hyalomicte, pegmatite, and analogous rocks, and 

 must then be regarded as the equivalents of those granitic 

 beds which are interstratified with the primary schistose rocks. 



The same genus does not always contain a quartzose species 

 of the same extent in every locality ; this, like all the cir- 

 cumstances relating to the primary rocks, cannot as yet be 

 reduced to any fixed laws : but it may be remarked, that the 

 quartzose species very often form the connecting link, through 

 which two adjoining genera pass into each other. 



And it may also be here observed, that these quartzose rocks 

 very often assume a brecciated or fragmentary appearance, 

 evidently arising from some portions of the quartz being free 

 from the colouring or slaty matter with which the greater 

 part is intimately combined. The variegated and agate-like 

 figures which these rocks assume, cannot be easily described ; 

 sometimes the pure, and sometimes the coloured quartz, pre- 

 dominates ; and not unfrequently the latter resembles frag- 

 ments of slate, the nearest sides of which, though sometimes 

 many inches asunder, so correspond, as if they had been at 

 one time united : but a careful scrutiny of this rock will con- 

 vince every one, that this fragmentary appearance is only a 

 curious coincidence ; for if one part of it seems to support 

 such an opinion, a thousand others will show that the rock 

 could not have been formed by the aggregation or cement- 

 ation of fragments. These curious fragmentary appearances 

 are not peculiar to Cornwall : we shall, therefore, have occa- 

 sion to revert to this subject, when treating of the structure 

 of the primary rocks. 



In order to complete the sketch of the schistose group of 

 Cornwall, it remains to state, that granitic rocks often occur 

 therein as large beds or dykes, called by the miners elvan- 



