1813.] Mineralogical Observations on Cornwall. 249 



it is not unlikely that Mr. Playfair may have been deceived by % 

 some appearance similar to that of the patella, which had at 

 first almost deceived myself. 



The clay slate breaks easily into thin slates of a good size, and 

 is often used for roofing, and even for covering the walls of 

 houses. Its most common colour is dark blue, though some- 

 times it is purple. Its lustre is glimmering, from specks of 

 mica, which are best seen "when you examine the cross fracture 

 of a thick piece. It contains numerous thin beds of quartz, 

 and is often traversed by quartz veins running in all directions. 

 All these characters are well known to belong to transition 

 clay slate ; so that they corroborate the conclusions drawn from 

 the limestone, if corroboration be necessary. 



In several places I found beds of limestone and clay slate 

 alternating. In going from Plymouth to the Dock, by the new 

 road over the bridge, we pass by a range of limestone rocks, in 

 which two different beds of clay slate may be observed ; the first 

 in a low part of the rock. The line of junction appeared to me 

 to run from north to south ; but this v. as probably a deception, 

 from the way in which the two rocks had been quarried. The 

 second bed lies farther west; and the line of junction may be 

 traced for some way from west to east, both rocks dipping to the 

 sea at an angle of about 70°. As we walk west the clay slate 

 disappears ; but its place is supplied by a portion of the lime- 

 stone, which assumes a slaty structure. 



Between Plymouth and Grampound, travelling along the road, 

 I observed no other rock but clay slate. At Grampound, clay 

 slate and grey wacke alternate ; and this alternation continues 

 all the way to Truro. The road between Grampound and Truro 

 is mended with grey wacke. About 100 yards east of the fourth 

 milestone from Truro, there is a quarry of greywacke on the 

 south side of the road. The greywacke is remarkably well cha- 

 racterized, and exactly resembles the same rock in the Pentlands 

 near Edinburgh. 



I did not find that the Cornish miners gave the name of killas 

 to greywacke, as Dr. Berger and Mr. Allan seem to have taken 

 for granted they did. The rock does not occur in any of the 

 mines that I visited; nor could I find that it had been distin- 

 guished by any particular name. The term killas is applied in 

 Cornwall'solely to clay slate, both primitive and transition. If 

 mica slate occurs it will also have the same name; but I did not 

 observe any mica slate in any of the mining districts which I 

 visited. 



The town of Truro is built of a stone very much resembling 

 Bath stone. It is a kind of oolite. As this stone is procured 

 from a quarry lyin^ on the south side of Truro, it must, I con- 



