of the Cornish RocJcs. 107 



primitive sort of argillaceous slate, from that which is of more 

 recent origin ; and considering the true grey-wacke as apper- 

 taining to the latter ? 



Beuig desirous of ascertaining the synonyms of the rocks 

 which constitute the principal strata of Cornwall, I transmitted 

 to Mr. Werner in the year 1793 a well chosen collection of 

 specimens of some of their varieties ; and in the autumn of 

 the same year, I had the satisfaction of hearing his decision 

 upon them. No doubt was expressed of the nature of our 

 metalliferous rock, the killas. It was pronounced to be a ge- 

 nuine thoii-schiefer, or argillaceous slate, in no respect differing 

 from that which occurs in Saxony. 



As the belts of iron-stone which encircle the granitical hills 

 in Cornwall had very early engaged my attention, I felt par- 

 ticularly anxious to submit some specimens of this interesting 

 rock-formation to the examination of such an experienced ob- 

 server. Those taken from the neighbourhood of Penzance 

 were, after a rigid scrutiny, pronounced to be hornblende- 

 rock intimately mixed with the constituent mass of argil- 

 laceous slate, and in part genuine hornblende-slate. Some 

 specimens of iron-stone from Penpillic and Corrogat in Tre- 

 wardreth parish, which I procured from the late Mr. Rash- 

 leigh of Menabilly, were characterized by Werner as follows : 

 " This rock is an intimate mixture of hornblende and dense or 

 compact felspar, in which a little quartz may be occasionally 

 perceived. Some specimens bear a very near affinity to the 

 horn-schiefer of the Swedes." 



To the judgement of the same accurate observer I submitted 

 various specimens of our Cornish elvan; and the result of their 

 examination was expressed in the following words: " The 

 globules in the Chyanoweth elvan consist of quartz and mica 

 intimately mixed. This, together with the elvan from Pe- 

 denavonder in Sithney, perfectly resembles the substance of 

 the granitical veins near Eibenstock and Johangeorgenstadt, 

 and is in fact a fine grained variety of granite. The elvan 

 from Portkellis-Bal, and Huel Whidden in Breage, is the 

 same, with this difference only, that the felspar is dissolved into 

 porcelain earth. The elvan from Polgooth is an argillaceous 

 porphyry, which in the lowest levels of the course or channel, 

 approximates to the same fine grained granite which composes 

 the veins at Eibenstock, The Pentuan stone is common por- 

 phyry." 



I shall now lay before the Society some thoughts and re- 

 flections that have naturally arisen during the consideration of 

 this subject. 



O 2 There 



