THE GEOLOGICAL AGE OF CENTEAL AND WEST CORNWALL. 19 



Conclusion. 



Existence of Four Unconformalble Formations. 



Why not Discovered Before ? 



Small Mechanical Effects of Granitic Intrusions. 



Chemical Effects Proportionate to the Masses of the Intrusive 

 Eocks. 



Dislocations of Strata, Cumulative Effects. 



Total Thickness of Sedimentary Eocks. 



Evidences of Eapid Accumulations — great lapse of time not- 

 withstanding. 



Introduction. 



The Greologieal Age of central and west Cornwall lias long 

 been regarded as a more or less open question. The maps of 

 the Geological Survey, almost unaltered since the time of Sir 

 Henry de la Beehe, are coloured as Devonian over nearly the 

 whole of the county, the only notable exception being the small 

 but well-known patch of Lower Silurian rocks on the south coast, 

 extending from Chapel Point to Pendower, an area of about 12 

 square miles, which includes the parishes of Gorran, Caerhayes, 

 and Yeryan. 



De la Beche himself seems to have known that these rocks 

 occupied a much greater area than is shewn on the survey maps, 

 as he speaks in his "Peport" of their extension to Mesack 

 Point, some miles up the J^almouth Harbour,^" and hints at their 

 being continued into the district to the south of the Helford 

 River,! which I have lately shown to be the case in a paper read 

 to the Poyal Greologieal Society of Cornwall. | He also supposes 

 that the calcareous rocks of the Van cliff, further to the east, are 

 of the same age as the Q-orran Beds, but these, I believe, are of 

 intermediate age.§ 



Before proceeding let me just mention the chief difficulties 

 with which the geological observer has to contend in Cornwall, 



* Report on the Qeology of Cornwall, S^c, p 86. 



t IhiA, p 95. 



J On the Geological Structure of the northern part of the Meneage Peninsula 

 Trans. Roy. Geol. Soc. Corn., X, p 47. 



§ Bep. Roy. Geol. Soc. of Corn., <Sfc., p. 83. See, however, the statement 

 made by Mr. C. W. Peach, that some of the fossils he had collected at the Black 

 Head, close to the Van cliff, had been recognised by Professor Edward Forbes as 

 GraptoUtes. — Trans. Roy. Geol. ioc. Corn., ITT, p. 121. My own opinion is, 

 that the Van cliff is Upper Silurian, and the Blackhead partly Lower Silurian 

 and partly eruptive. 



