THE GKOLOGIOAL AGE OF CENTRAL AND WEST CORNWALL. 23 



with much assistance from Mr. Clark, our able road surveyor at 

 Truro, who knows nearly every quarry and exposure of strata in 

 the district, I have, I think, succeeded in determining pretty 

 accurately the southern boundary of these Ladock beds, as shewn 

 on the accompanying map. Their northward extension has not 

 yet been accurately made out. 



It is quite possible that outliers of these Ladock beds may 

 exist towards Chacewater and lUogan^ but if so they are very 

 thin, — and certainly I have not been able to discover any such. 

 To the south of the Helford Eiver, however, an area of several 

 square miles consists of rocks which I believe to have been once 

 continuous with the Ladock beds. At any rate they have the 

 same east and west strike, and both can be shewn to rest 

 unconformably upon Lower Silurian rocks. See fig. 2 -pi. a, and 

 fig. 5 pi. B. The area in question is composed chiefly of schistose 

 rocks, but it includes a remarkable bed of very coarse conglome- 

 rate which stretches westwards and a little inland, from the 

 Nare Point, towards Trelowarren, as stated by De la Beche. It 

 perhaps appears again at Poljew Cove, on the west coast near 

 MuUion. 



Some of the included masses in this conglomerate weigh 

 several tons, and are themselves portions of a still older con- 

 glomerate. The materials are in larger masses at the eastern 

 extremity of the bed near the Nare Point, and become finer as 

 they are followed westward. 



This conglomerate has been described by Sedgwick, Boase, 

 and others, as containing fragments of hornblende slate, serpen- 

 tine, and diallage rock, indicating the existence of these rocks 

 in the neighbourhood before the formation of the conglomerate. 

 Sir H. De la Beche, however, — while insisting on the inclusions 

 of hornblende rock, denies the occurrence of diallage or serpen- 

 tine.'^' For myself I can only say that I have hitherto failed to 

 find the smallest portion of either of these rocks, and I greatly 

 doubt their occurrence. 



The conglomerate has generally a siliceous cementing material, 

 and besides the masses of an older conglomerate already referred 

 to, it includes very large masses of a grey quartzite much 



* See Mem. Geol. Survey, I, p, 69, and Bep. on the Geol. of Corn. S(c., p. 30. 



