34 THE GEOLOGICAL AGE OF CENTRAL AND WEST CORNWALL. 



cases the strike and mineral character of the rock is often found 

 to change in a very marked degree on crossing the most insig- 

 nificant stream.* 



The actual unconformability of the two sets of rocks may 

 however be seen in many places, as, on the coast a little south 

 of Pentewan, and also at Chapel Point, near Mevagissey. 



A very fine unconformability is visible on the south side of 

 the Helford River — near the Nare Point, where the conglom- 

 erate may be seen dipping very gently to the south and resting 

 upon nearly vertical dark slates, having a strike to the N.E. as 

 shewn in the figure (Pig. 2, Plate A). 



I have now to draw attention to a series of rocks of a still 

 older date than the Lower Silurians just described, the existence 

 of which has hitherto not been even suspected. 



On the Survey Maps these are marked and coloured as 

 Devonians although their dip and strike are usually marked 

 correctly ; as a matter of fact they underlie the Lower Silurians, 

 and their strike is at right angles, or nearly so, to that of the 

 latter series. 



At dead low water, on the coast to the S.W. of Pentewan, 

 several outcrops of very hard siliceous rock, having a strike to 

 the north-west may be seen. These I fancy may have been 

 brought up by a fault or series of faults, the locality being one 

 of great disturbance. However this may be, similar rocks appear 

 from beneath the Lower Silurians in the East Wheal Rose 

 Valley, and come out on the coast at Penhale Point, between 

 Perranporth and Holywell Bay, where they are vertical — but 

 with a N.W. strike. 



A great part of the country between Devoran and Ponsanooth 

 is occupied by rocks having a similar strike and a steep N E. dip, 

 and the Lower Silurian rocks may be seen resting upon them in 

 the river just below Devoran (Fig. 6, Plate B), at Feock, in 

 the roadside not far from the church ; and at the southern end of 

 Ponsanooth tunnel between Perranwell and Penryn. The beds 



* The reason for the occurrenoe of valleys iu such situations is not far to 

 seek. Springs of water frequently break out iu such places, and even v?hen this 

 is not the case, a chemico-galvauic action often renders the beds near a junction 

 of two even slightly dissimilar rocks softer — and less able to resist the atnjos- 

 pheric denudation. 



