Ch. IV.] SCHISTOSE GROUP OF CORNWALL. 41 



manner. The slates hi question, and indeed all the primary 

 slates of Cornwall, form distinct suites or genera : some of the 

 members or beds of each genus are schistose, and others 

 compact ; in the latter the basis is very frequently pure, and 

 the colouring mineral is aggregated into either particles, 

 scales, crystals, or such a body that its nature can be detected ; 

 and from these compact or crystalline beds, there is a gradual 

 and almost imperceptible passage into the perfect slate, so 

 that it appears to be a justifiable conclusion, that both the 

 compact and schistose beds have the same composition. 



Thus nature, as it were, proffers her own analysis to assist 

 us in our researches, by giving us an insight into the con- 

 stitution of rocks, in which the elementary minerals are so 

 blended as to present only a homogeneous mass. 



If this view of the subject be correct, we learn that 

 cornubianite and proteolite are both composed of felspar, 

 quartz, and mica, shorl, or an analogous mineral ; that is, of 

 precisely the same ingredients as granite. Mr. Hawkins, 

 many years ago, appears to have entertained a similar opinion, 

 for he suggested " that the common killas is an intimate 

 mixture of quartz with mica, talc, chlorite, and perhaps, in 

 some instances, with felspar." There can, however, be little 

 doubt of the presence of the last-mentioned mineral ; for, as 

 in the case of granite, it generally constitutes by far the 

 greater part of the mass. 



After what has been now said, the nature of the remaining 

 schistose rocks of the porphyritic series will be easily under- 

 stood, since they have all a basis of compact felspar, united 

 with hornblende, actynolite, or chlorite. 



All these rocks appear to pass gradually into each other; 

 and these transitions are not only effected by the variations 

 in the composition of the compact felspar basis, but the 

 accessary minerals themselves appear to be subject to similar 

 changes, mutually passing into each other, giving rise to 

 intermediate substances, which, if they be definite compounds, 

 have not been hitherto described : it is more probable, how- 



