NOTES ON THE IGNEOUS ROCKS OF SOUTH-EAST CORNWALL, 



AND THEIR STRATIGRAPHICAL VALUES. 



By R. N. worth, F.G.S., Cor. Mem. 



Various points affecting the correlation of the rocks of 

 Central and "West Cornwall, have been discussed before this 

 Society of late, particularly by Mr. J. H. Collins, F.Gr.S., and 

 Mr. A. Somervail. I have not the requisite knowledge of the 

 geological detail of the district in question, to authorise my com- 

 menting upon their views and work; nor is it my intention in this 

 paper to do more than throw out a few suggestions derived from 

 the conditions of South-East Cornwall, which I think may aid 

 in the solution of the "still vexed" problems of our strati- 

 graphical relations. I have no belief myself that this solution 

 will be authoritatively attained until the whole county has been 

 officially resurveyed geologically on the six-inch scale ; but when 

 that is done I believe we shall find that instead of the whole of 

 the sedimentary rocks of Cornwall being Devonian, — the small 

 areas officially admitted as Silurian and Carboniferous only 

 excepted, — we shall find that Devonian rocks are almost wholly 

 confined to the east of the county ; that Silurian occupy a very 

 much wider area than is at present recognised ; that Mr. 

 Collins's suggested Cambrian has a very substantial basis ; and 

 that the Archaean assignment to the Lizard district of Professor 

 Bonney is maintained. Further, I have a very strong impression 

 that by far the larger part of our mining area from Tavistock 

 westward is pre-Devonian, These points are, however, to a large 

 extent at present merely matters of faith, in agreement with the 

 confessedly imperfect evidence now before me. 



Two great difficulties hamper the definite issue of this con- 

 troversy. The first is the great confusion which rules among our 

 Cornish strata, in consequence of the frequent disturbances to 

 which they have been subjected. The second — the great simi- 

 larities among our slaty rocks in their general characters, and 

 the absence of fossils over wide areas. Any means that wiR 

 enable us to differentiate our sedimentary rocks, and assign them 

 a definite place in the scheme of stratigraphy, which neither De 

 la Beche, Sedgwick, nor Murchison can be held to have closed, 

 should then be welcome. 



