NOTES OX GEOLOGICAL RESULTS. 



281 



40 feet wide in places and a length of 100 feet or more. The 

 sea erosion has undermined the floor and carried it away after 

 reducing it to sand by constant wear and concussions. It 

 follows that if habitable the cave has lost the greater part of 

 its value. 



The outcome of these excursions is that : — 



1. Caves may exist of which Ave now have no knowledge, 

 and to find these the cliffs must be diligently searched. 



2. The lower caves are without promise and are useless 

 as regards the search for paleolithic man, but are 

 worth working by the geological section as they offer 

 proof of rock varieties not to be met on the surface. 



3. That the only caves now known to be likely to yield 

 evidences of man are : — 



(a) The large cave at Les Tielles. 



(b) The cave over the arch at Les Tielles. 



(c) The Dog's Cave just described. 



In any case the Society has here work which cannot be 

 undertaken without some form of good resulting, but this 

 will take several years to accomplish. In the nature of things 

 the results of the search for caves have been less geological 

 than my title would have led you to expect ; that fault I will 

 redeem in my remarks on the rocks we have studied. 



Rocks. 



In the excursion to the Corbiere there were rock features 

 somewhat unexpected and of such interest that our search for 

 caves became, as far as the geological section was concerned, 

 a secondary interest. 



The Corbiere point had never been worked right down to 

 the bottom by the Society, hence the detail was new. The 

 usual gneiss of the locality was much cut up by greenstone 

 dykes, but on reaching the base of the point a wide dyke of 

 felsite, which weathered with a whitish surface instead of the 

 usual brown, was found. This dyke was intrusive, and of later 

 date than any other intrusive rock seen. The question at 

 once arose, is the rock a true felsite or an aplite ? As I may 

 presume that my listeners, or at all events some of them are 

 not students of geology, I claim your indulgence while I 

 speak of the nature of these rocks and of the problems 

 involved, and I shall have failed in my object if I do not 

 make it evident that the matter was interesting. 



First point. — Aplites exist in these islands, but except 

 for one place in Sark and one on the west coast of Guernsey, 

 I have not been able to trace the rock. 



c 



