252 Mineralogical Observations on Cornwall. [Ogt. 



J doubt that the same rock extends all the way south to Mullyan. 

 The clay slate round Helston is of a dark blue colour, almost 

 black, with hardly any lustre ; nor could I detect any mica in 

 it ; but it is traversed in every direction by quartz veins. It has 

 no resemblance to primitive clay slate ; and though I found no 

 greywacke, nor any other rock but the slate, I conceive, from 

 its appearance and connection with the neighbouring country, 

 there can be no hesitation in referring it to transition slate. 



From Helston to Merazion is a distance of eight miles ; and 

 the whole country, as far as I had an opportunity of seeing it, 

 is composed of the same transition slate. Merazion stands upon 

 the sea shore upon the east shoulder of Mount's Bay. The rocks 

 below it are exposed for about 740 yards along the coast, and 

 in some places arc about 20 feet above the level of the sea. 

 Within that space I reckoned 14 alternate beds of clay slate, and 

 a blue stone, to which Dr. Berger and Mr. Allan have given the 

 name of grey wacke. It is very hard ; and the materials of which 

 it is composed cannot be distinguished by the naked eye. Though 

 no person could recognise it as greywacke merely from seeing 

 spccimer.s of it, yet, from its situation and connection, I cannot 

 avoid thinking that these gentlemen have named it right. The 

 clay slate is by far the predominant rock; but the beds of this 

 greywacke rock are sometimes also pretty thick. The thickest 

 which I observed was 73 yards, and the thinnest five feet thick. 



These clay slate and greywacke rocks lie opposite to St. 

 Michael s Mount, which is about 500 yards south from the Dry 

 Beach. They extend both farther east and farther west than the 

 mount. Now it deserves notice that those beds which are on 

 the east side of the middle of the mount dip east at an angle of 

 about 70°, and those on the west side dip west at about the same 

 angle. Hence if they were continued without interruption till 

 they met, they would form a sloping hill considerably higher 

 than the mount, and the mount itself would form the southern 

 extremity of it. The farthest west bed of greywacke is the 

 highest. Tin ore was formerly found in it, and a mine existed 

 for some time from which it was extracted ; but it has been for 

 a considerable period abandoned. 



The clay slate rocks cannot be traced without interruption 

 from the sea shore to St. Michael's Mount, the middle part of 

 the way for about 100 yards being covered by sea sand ; but that 

 they continue in reality under this sand cannot be doubted. 



St. Michael's Mount itself is a small conical hill, according 

 to Dr. Berger's measurement 231 feet high. As it is the most 

 remarkable spot in Cornwall, in a mineralogical point of view, 

 I have given an outline of its shape in the annexed plate. 

 When we reach it from the north, the first rocks we meet with 

 are clay slate, possessing exactly the same characters as the clay 

 late at Merazion, and undoubtedly a continuation of the sam>: 



