Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 47 



in all their modifications, traverse the serpentine in irregular 

 strings, veins, and layers. 



The talc-schist of Cornwall is not a compound of quartz 

 and talc, as described in systems of geology, but of the same 

 substance as forms the basis of the serpentine, intimately 

 blended and laminated with talc ; which mineral may be seen 

 in distinct scales, in the quartz-veins and nodules which 

 occur in this slate. It appears to be produced by the transi- 

 tion of the diallage into talc, in the same manner as horn- 

 blende passes into mica. It is worthy of remark, that the 

 ollareous serpentine and talc-schist of Pollaphant, is accom- 

 panied by a bed of crystalline limestone; which is glossy 

 and variegated by the intermixture of talc, and abounds in 

 tabular spar and hornstone. This single instance is curious, 

 as we find this association extensively developed in other 

 countries. 



In the foregoing enumeration of the Cornish primary 

 strata, it may have been observed, that no mention has been 

 made of the quartz-rock of Macculloch. This omission has 

 been intentional, because nearly all the varieties of this rock 

 do occur in Cornwall, and some of them occupy extended 

 tracts, the boundaries of which are distinctly marked by 

 barren downs : but the quartz is generally combined with 

 the component minerals, or the substance itself of the adjoin- 

 ing slate ; so that it appears to be a more natural arrange- 

 ment, to consider these rocks as quartzose varieties of the 

 slates into which they graduate. 



It has already been stated, that the rocks of the schistose 

 group are composed of compact felspar, united in various pro- 

 portions with mica, shorl, hornblende, or some other accessary 

 mineral ; and that some species of these rocks are produced 

 by the relative quantity of the component parts of the compact 

 felspar base : thus, when silica predominates, the species be- 

 comes quartzose ; and if it prevails, to the exclusion of all 

 other substances, then it gives rise to masses, layers, or veins 

 of quartz, which may be regarded as subordinate to the rock 



