1813.] Miner alogkal Observations on Cornwall. 247 



1 703. In the year 1 703 he was chosen President of the Royal« 

 Society, and continued to fill that honourable situation till the 

 time of his death. In 1705 he was knighted by Queen Anne, 

 at Cambridge. 



(To be continued.) 



Article II. 



Some Mineraloa'ical Observations on Cornwall. By Thomas 

 Thomson, M.D. F.R.S. 

 (With a Plate of St. Michael's Mount.) 

 Cornwall has been lately visited by so many mineralogists, 

 and such ample details on its structure have been furnished by 

 the publications of M. Deluc, Dr. Berger, and Mr. Allan, that 

 perhaps any thing farther on the subject may seem superfluous. 

 I travelled through it with such rapidity, and made my observ- 

 ations in such a hasty and imperfect manner, that I ought to 

 apologize for presuming to lay them before the public : but as 

 «ome of the conclusions which I thought myselt warranted to 

 draw are different from those drawn by preceding observers, and 

 as the knowledge of the structure of Cornwall constitutes a very 

 material point in the geology of Great Britain, I thought it 

 better to present my notes to the public, even at the hazard ot 

 falling into mistakes. They will at least serve to call the atten- 

 tion of future observers to some very material points, and may 

 in some measure facilitate their examination ol this interesting 

 but difficult province. . 



The chalk country, as far as I could observe, terminates ten 

 miles west from Dorchester : but the country between that place 

 and Exeter, though hilly, is so completely covered with grass 

 or corn that it is very difficult to make out its structure. I he 

 onlv rocks which I perceived were a floetz limestone, contain- 

 ing petrifactions and a coarse sandstone, which seemed to con- 

 tinue almost to Exeter. The soil round Exeter is red, very 

 deep, and reckoned one of the richest m England. It has the 

 aspect of sand, but the tenacity of clay, and seems to be a mix- 

 ture of both. . . . 



On the road that leads north from Exeter, transition rocks 

 may be seen. The road has been sunk in them in some places 

 to the depth of about six feet, so that they stand as a wall on 

 the west side. The J consist of alternate beds ot greywacke ami 

 grevwacke slate. These beds are nearly vertical. The slate is 

 Shivery, and possesses the usual characters of greywacke slate. 

 The beds of greywacke are few, and the rock itself not very 

 well characterized. I Iere and there I thought I observed thm 

 b of flinty slate in these rocks. 



