452 REMNANTS OF EXTINCT VOLCANOES. 



Good specimens of conglomerates may be obtained from 

 the heaps of stones by the side of the road, or, from some of 

 the projections on the face of the road, which it would be a 

 charity to remove, for it is yet a conglomerate of unequal 

 resistance. 



If we travel about one a and half miles south to the cliffs 

 between Porthley and Porthlewey we shall find similar conglom- 

 erates of about 2,000 feet in thickness, described by Sir H. De 

 la Beche as Trappian Conglomerates and by Mr. J. H. Collins as 

 Siliceous Conglomerates ; but the Microscope reveals undoubted 

 evidence of their true volcanic origin. 



I was not rewarded by finding a grand volcanic vent, from 

 which may have flowed incalculable masses of volcanic tuff, but 

 I did find conglomerates overlaying the upturned edges of the 

 stratified rocks, but not anything overlaying the conglomerates. 

 I am not satisfied that I have discovered all the volcanic evidence 

 the locality affords, but believe it to be well worthy a fuller 

 investigation, and that we may safely make Tolcarn our starting 

 point to work out the volcanic evidences of West Cornwall. 



At Tolcarn the conglomerates thin out just above the stream 

 a little east of the mill, where they are cut through and the 

 upturned edges of the old formation exposed. A little above 

 the stream, by the side of the road, great quantities of the 

 conglomerates have been removed for road metalling, and have 

 left the slate rock bare. 



Nare Head, Very an. 



As we travel south-west the evidence becomes clearer and 

 more abundant. A little in from the coast line we find a 

 neck of unbroken gabbro filling an old crater, or vent, from 

 which may have flowed some thousands of feet of volcanic tuff, 

 or it may possibly have been a parasitic vent of a main crater 

 situated far beyond the Glull Eock and the Whelps. The lowest 

 layer of the conglomerate is composed of fragments of various 

 rocks through which the vented matter must have passed in its 

 upward course, such as clay, calcareous slates, quartzites, felspar 

 and olivine. 



