﻿88 Mr. J. A. Phillips on the Chemical Composition and 



preserved fossils which have been discovered, that the majority 

 of them are of Devonian age. 



This extensive region of sedimentary rocks encloses five large 

 and several smaller masses of granite, which, without rising 

 very prominently above the common level, protrude as " islands 

 of granite out of a sea of slate " *. 



Both the slates and granites of this portion of Great Britain 

 are traversed by numerous dykes of granitic and porphyritic 

 rock called elvans, by occasional trap-dykes, by numerous copper 

 and tin lodes, and by various cross-courses and fiucans, the latter 

 being fissures filled with clay. 



This formation is characteristic, not only of the whole of 

 Cornwall, but also of a large portion of Devonshire. jMasses of 

 serpentine occur in various localities in this district, particu- 

 larly in the south-western portion of Cornwall, in the vicinity of 

 the Lizard Point. 



Typical killas is a clay-slate, often of a grey, bluish-grey, or 

 greenish-grey colour, and frequently acquires a brownish-yellow 

 or buff tint, from weathering : its colour, however, varies very 

 considerably in different localities, besides which it is not unfre- 

 quently stained by dendritic markings of oxide of iron and 

 other minerals. It is usually exceedingly fissile, the planes of 

 fracture being very commonly much contorted, and presenting 

 smooth unctuous surfaces, not unlike those of faults or fissures 

 called slickensides, and apparently produced by friction : killas is 

 often more compact near its junction with the granite than else- 

 where, and in such localities, without any well-defined line of 

 demarcation, frequently encloses numerous subordinate layers of 

 granular or semi-crystalline rocks. 



Professor John Phillips remarks, with regard to these depo- 

 sits, " The general impression concerning the schistose rocks of 

 Cornwall is that their mineral composition is a mixture of quartz, 

 felspar, and mica, and so* probably is that of most clays and 

 shales "f. He further observes, "It is hazardous to reason on 

 phenomena so remarkable as those of Cornwall without refe- 

 rence to other districts. Nothing but prejudice or indolence 

 will permit geologists acquainted with other districts to neglect 

 the singular and curious facts connected with the Devonshire 

 and Cornish chain. We may freely admit that they in some 

 cases point to agencies not yet familiar to our philosophy ; that 

 a full examination of the whole series of granites, porphyries, 



* A "Treatise on Ore Deposits/' by Bernhard Von Cotta, Professor of 

 Geology in the xtoyal School of Mines, Freiberg. Translated from the 

 second edition by Frederick Prime, jun., revised by the author, p. 403. 



t Manual of Geology, Practical and Theoretical, by John Phillips, M. A., 

 F.R,S.,F.G.S.,&c.,p. 117. 



