248 Mineralogical Observations on Cornwall. [Oct. 



* I visited the manganese mine at Upton Pyne, three miles 

 north from Exeter. The manganese occurs at the depth of 

 120 feet. The surface, and for a considerable depth, consists 

 of a red coloured gravel : then comes a coarse and very hard 

 sandstone, composed of fragments of flinty slate, quartz, clay 

 slate, &c. I conceive it to be a transition sandstone, and inti- 

 mately connected with greywacke. The manganese, as far as 

 I could determine, constituted a bed ; but particular circum- 

 stances prevented me from being able to ascertain the point in 

 a satisfactory manner. 



Whether the transition rocks continue all the way from 

 Exeter to Plymouth, I do not know. The hill immediately to 

 the south of Exeter is thick strewed with flint. To the south 

 of that occur beds of limestone : but from Ivy Bridge to Ply- 

 mouth the rocks are clay slate. This clay slate I conceive to 

 be transition, because it is continued without interruption to 

 Plymouth, where the clay slate is indisputably transition. 



The rocks round Plymouth are so well exposed, and so well 

 characterized, that there can be no hesitation about the class to 

 which they belong. I saw only two kinds of rock, namely, 

 limestone and clay slate. The limestone rocks form low round- 

 backed hills, which surround the bay ; and they are every where 

 wrought for lime, which is carried to considerable distances, and 

 employed as manure. The colours of this limestone are various, 

 blue, red, white, and grey, often mixed irregularly, but some- 

 times disposed as if in regular beds. Its lustre is glimmering, 

 and seems to proceed from small crystals of calcareous spar 

 scattered through it. The fracture is splintery, with a tendency 

 to the small conchoidal. This description characterizes the 

 most common transition limestone. Hence there can be no 

 hesitation in considering the Plymouth limestone, and all the 

 rocks that alternate with it, as transition. 



In most places, as in the rocks between Plymouth and the 

 Dock, and at the north side of Mount Edgecumbe, it seems to 

 dip towards the sea at an angle of about 70°; but on the north 

 face of Mount Wise, under the telegraph, it has the aspect of 

 horizontal strata, varying in thickness from that of a leaf of 

 paper to a foot or more. These apparent strata are distinguished 

 by the alternation of different colours, chiefly red and white. 



I was at some pains in examining this limestone, to see whe- 

 ther I could detect any animal remains in it, because Mr. Play- 

 fair informs us that he found a shell in it; but all my labour was 

 to no purpose. I found, indeed, the patella vulgata mixed with 

 fragments of it ; but the shell was recent, and obviously had 

 come from the shore. Indeed, I am not aware that any shells 

 have ever been observed in this kind of transition limestone, the 

 petrifactions which it contains being of a different kind. Hence 





