122 Dr. Berger on the physical Structure 



About a mile from St. Mary Tavy, near the place where the 

 roads join which lead from Plymouth to Oakhampton, and from 

 Tavistock to Two Bridges, there is a bed of greenstone of some feet 

 in thickness, in the grauwacke slate ; it decomposes into a green 

 earth. With regard to the grauwacke itself, the direction and in- 

 clination of its strata continue the same, it only contains more 

 quartz as we approach St. Mary Tavy, and becomes at the same 

 time less slaty. 



St. Mary Tavy is six hundred and forty-eight feet above the level 

 of the sea. A copper mine is worked here at a great depth, and 

 amongst the rubbish I found grauwacke and schistose limestone,* 

 heaped one above the other ; which shews that the epochs of forma- 

 tion of these two rocks are nearly coeval, since we find beds of the 

 one included in those of the other. 



From St. Mary Tavy to Launceston by Brentor and Lifton, we 

 cross successively the Lyd and the Tamar, continuing in the grau- 

 wacke slate formation to within a mile of Launceston. The strata 

 of the grauwacke slate are very distinctly seen at the ferries of these 

 two rivers. It is succeeded by a schistose limestone having a very fine 

 paste of a dark blue colour and dull lustre, dividing into large flags, 

 which are put to the same use as slate, and which I should have 

 taken for such, if 1 had not found that it effervesced with acid, f 

 Besides, it is here only in subordinate beds, and I do not believe that 

 it extends very far. 



I saw at the house of the Rev. William Gregor of Creed, two 

 rolled pieces, one of which appeared to me to be idocrase (vesu- 



* Similar to that at the mouth of the Plym. 



+ It is in fact, what some German geologists call transition, fhonschiefer, and which 

 they say alternates with transition limestone. Brochant Traite de Mineralogic, tome 

 II. p. 587. 



