18 IS.] Transition Rocks of Werner, 421 



Near Oakhampton I found it along with graywacke-slate, in the 

 most unequivocal state ; and on the shores of the Bristol Chan- 

 nel, near Ilfracombe, the rocks are all of the same material.* 

 Here, on the beach, to the west of the town. I spent some 

 liours the evening before I crossed to Swansea ; and found no- 

 thing among the rocks to lead me for a moment to question that 

 they were wholly composed of graywacke. Indeed I even re- 

 marked some of the contortions which are so common in this 

 rock. Next morning, however, when walking down to the 

 boat, under a point where a small battery is built, I found on 

 the trodden surface of the rock an appearance very similar to 

 mica-slate, for which substance it might readily be mistaken 5 

 but this resemblance appears to be owing to the friction of the 

 feet, and the action of the weather, on a variety of graywacke, 

 containing an unusual proportion of mica. 



By casting an eye over the map of Cornwall, it will be ob- 

 served that the above specimens are selected from the most re- 

 mote corners of the peninsula. On examination I think they 

 will be found sufficiently similar to the graywacke of Werner, to 

 be entitled to be classed along with that rock. Dr. Berger, in 

 his paper on the physical structure of Cornwall, gives them no 

 other name, and if authority is to be qualified by experience, the 

 opinion of one who has traced the footsteps of Saussure, and 

 who has studied the geognosy of Werner with the utmost enthu- 

 siasm, cannot fail to be received with respect. 



I have thus endeavoured to show by the selection of speci- 

 mens, and by the opinion of a very scientific observer, that the 

 stratified rock of Cornwall is graywacke. It would be uncandid, 

 however, not to acknowledge that the gejieral texture of this 

 rock was different from ths graywacke of the south of Scotland ; 

 it was more of the slaty variety, and frequently seemed, from its 

 smooth and soft feel, to contain a large proportion of magnesian 

 €arth.+ 



I understand, in a course of lectures now delivering, a very 

 material alteration has been proposed upon the Wernerian sys- 

 tem, in order to introduce this rock in a position distinct, and 

 very distant from graywacke. It appears to me much more 

 simple to suppose that rocks of the same class in different dis- 

 tricts may present peculiar characters, than that the operations 



* The specimens alluded to were examined by the gentlemen present when 

 this paper was read, who considered those from the road leading to Carclaze 

 mine, and from near Oakhampton, as graywacke; and those from the vicinitj 

 of Penzance as greenstone. 



+ Since I read this paper I have had occasion to pass through the transitioQ 

 country of Peeblesshire, &c. On former occasions, I was in the habit of 

 seanihing for characteristic specimens of the graywacke; I now looked fqr 

 such as resembled the killas ofQ «rnwall, which I found in abundance. 



