THE GEOLOGICAL AGE OP CENTRAL AND WEST CORNWALL. 3o 



are particularly well developed in this latter locality, and it may 

 be convenient to derive their provisional name from here. 



The black rocks of the Lelant cliffs, and the mica-schist of 

 the Old Lizard Head have the same strike ; the former dip to 

 the S.W., and the latter to the N.E. They will probably prove 

 to be the out-cropping edges of a trough lying still further to 

 the west than the beds which are nearly vertical at Ponsanooth. 



The greater part of the north coast between St. Agnes and St. 

 Ives appears to be composed of these pre-silurian rocks, the 

 Lower Silurians having thinned out northwards. 



In these rocks up to the present time neither sandstones nor 

 limestones have been observed, nor have any fossils been dis- 

 covered. That they are Pre-silurian is certain, they are probably 

 Cambrian — but it may be better to speak of them at present as 

 the Ponsanooth beds. They are all extremely ancient in appear- 

 ance, generally very siliceous and rough to the touch, except 

 where they are black and carbonaceous, or where scales of mica 

 or talc have been developed between their laminae. 



These rocks may be traced from Ponsanooth to Nansilgans, a 

 distance of about 5 miles in a straight line, in which the dip is 

 everywhere to the N.E. at an average angle of 45° for the first 

 2 miles, and of 20° for the last three miles. The total thickness 

 can hardly be less than 12,000 feet. 



As is the case with the Lower Silurians these Pre-silurians 

 include a number of very highly altered beds which have been 

 hitherto regarded as greenstones, and some of them are so 

 marked on the Survey Maps. Among the most important of 

 these are the beds situated near Tresa^ean Mine, in G-wennap, 

 which are largely worked as road-stone. In hand-specimens 

 some of this rock certainly does resemble an aphanite, but the 

 distinctly banded structure which is visible in the quarry 

 suggests stratification, and there is no appearance of intrusion 

 to be seen. Under the microscope the sedimentai'y origin of 

 these rocks is usually plain enough, but crystals of hornblende, 

 magnetite, garnet and axinite, are frequently developed between 

 their lamiuse. 



A band of what appears to be true gneiss, nearly half-a-mile 

 in width, has also been developed in these Pre-silurian rocks 

 jiear Penryn, close to the junction of the killas and granite, 



