36 THE GEOLOGICAL AGE OP CENTRAL AND WEST CORNWALL. 



Succeeding this is a consideraLle thickness of ^' spotted killas " — 

 a true " knoten-schiefer." Mostly the particular mineral forming 

 the " spots " or "knots " cannot be determined, but occasionally 

 they have the appearance of very imperfectly formed garnets, 

 and chiastolite crystals have been recognised in the schists of 

 Oarn Brea Mine, St. Ives, St. Erth, Lelant and many other 

 localities. 



These highly metamorphosed strata have no doubt been 

 produced by the closely contiguous granite masses, and as might 

 be expected, strata of different ages — and therefore presumably 

 of different original chemical or mechanical natures, have under 

 the influence of this common external agency, given rise to 

 different kinds of metamorphism. Thus — as a rule, spotted 

 schists are developed from the Pre-silurian, and tourmaline- 

 schists from the Lower Silurian rocks of "West Cornwall, where 

 they come in contact with the granite masses. 



Litrusive RocJcs. 



Each of the formations just described affords abundant 

 evidence that it has been upheaved, contorted, and extensively 

 denuded away before the deposition of its successor — indeed it 

 has no doubt furnished a considerable part at least of the 

 material from which that successor has been formed. With the 

 possible exception of the Fowey beds, each too has been sub- 

 jected to the intrusion of igneous rocks before the extrusion of 

 the granite. Thus, the Ladock beds and the Lower Silurians 

 were traversed by the remarkable dyke of mica-trap first 

 noticed by Mr. A. K. Barnett, '^' then analysed by Mr. J. A. 

 Phillips, f and afterwards analysed, figured and described by 

 myself. % 



The Lower Silurians are traversed by many undoubtedly 

 intrusive dykes — among which I especially mention the trappean 

 mass of the Nare Head, with its associated gabbro and serpen- 

 tine, the diorite of St. Mewan, near St. Austell, and the 

 remarkable series of partially interbedded diorites which form 



* Rtport Miners' Assoc, of Corn, and Dev. 1873. 



t Quart. Journ. Geol. Sac 1875, p. 335. 



t On the Trelissick Elvan, &c. Trans, Eoy. Geol. Soc. Corn, ix, 221, 



