420 



Remarks on the 



sidered nearly of the highest antiquity, were there common 

 productions. I therefore expected to meet with a perfect epi-- 

 tome of the Wernerian system, containing the usual series of 

 primitive rock, descending from granite, through gneiss, mica- 

 slate, and clay-slate, with all the et carter as of serpentines, traps, 

 and porphyries; but in this I was mistaken. 



On my approach to Exeter through vSomerset, 1 first observed 

 the transition strata between Bridgewater and Taunton; and 

 from thence traced them, more or less distinctly, till I crossed 

 the river Teign, which bounds Dartmore on the east. Thus far 

 great part of the country is very flat, some of it extremely hilly 

 as a road, but none of it mountainous. The transition strata are 

 by no means continuous, aad in many places appear only in 

 small projections above the surface. 



On the right bank of the Teign the road winds up the side of 

 a steep hill; and where the rock is cut there is a considerable 

 display of strata, having all the external appearance 6i gray- 

 wacke. On examining it, I found some of the strata coarser 

 than others ; but, in general, the grain was extremely fine, the 

 texture solid and compact, the colour very dark grey: it was 

 very tough under the hammer, it broke with a smooth and 

 somewhat conchoidal fracture, and did not split into the thin 

 laminae of the graywacke-slate. This appearance puzzled me at 

 first; the rock presented all the external characters of gray- 

 wacke, and yet internally it was different. I had not proceeded 

 many paces, however, when I came upon granite, the proximity 

 of which, as before mentioned, is always marked by a very ma- 

 terial alteration in the consistence of the adjoining rock. This 

 alteration, I observe, was not unnoticed by Dr. Berger, in his 

 interesting paper* on the physical structure of Devon and 

 Cornwall. In mentioning graywacke, \vhich he distinguishes 

 from graywacke-slate only by its compactness, he says, *^ It is 

 found higher up than the graywacke-slate, it may be supposed 

 to have been precipitated more slowly, and under less powerful 

 pressure; whereby the mass has been allowed to contract, and 

 to assume a kind of crystallization. It rests immediately on 

 granite." The conclusions he draws are different from mine ; 

 but from the above quotation it appears that the circumstance I 

 observed at Teign Bridge is usual in similar situations all over 

 Cornwall. 



Near St. Austle, on the road leading to Carclaze mine, I 

 found graywacke, in my opinion extremely well characterised; 

 also on the road to Cambourn, not far from Dolcoath ; likewise 

 on the shore near Penzance. Here it is also fine-grained, and 

 tough under the hammer, and at no great distance from granite. 



* Geological Transactions, voL i. p. 112. 



