THE GEOLOGICAIi AGE OF CENTRAL AND "WEST CORNWALL. 205 



in ancient times many other outcrops of basic rocks were existent 

 between these two groups, but the subsequent intrusions of the 

 granite masses and the enormous amount of denudation which 

 has since taken place, have left comparatively little of the basic 

 rocks. What we have at present, for instance, of the mica- 

 traps, is for the most part included in the great trough, in part 

 elevated, but nowhere more than about 600 feet, between 

 Hensbarrow and Carn Menelez ; but the distance from the 

 great trap vein where it comes out to the sea in Watergate Bay, 

 to the scoriaceous lavas of Pentire Point, is not very great 

 after all. 



Economic Value. As already mentioned — the mica-traps 

 have sometimes been used for building, under the name of 

 "sandstones" and "freestones." Blocks which are slightly 

 decomposed may often be obtained of considerable size, and 

 these are well adapted for building purposes, since they are very 

 readily dressed into shape, and have a warm and pleasant ap- 

 pearance. They are, also, fairly durable, since the decomposition 

 does not seem to progress very fast when once the blocks are 

 removed from their natural beds and built into walls. The more 

 decomposed portions are of course valueless for such purposes, 

 but they are of extreme value to the agriculturist on account of 

 the phosphoric acid and soluble silica which they contain — 

 sometimes amounting to nearly 1 per cent, of the former, and 

 1 \ per cent, of the latter. The extensive excavations which have 

 been made along the course of the veins wherever they are 

 soft and decomposed, and of sufficient size, shews that the 

 farmers of Cornwall have learnt by experience the value of such 

 a material. 



In several localities — and notably at Newquay — small 

 crystals of nickeliferous and cupiferous pyrites exist in patches 

 in the rock, and proposals have frequently been made by mining 

 speculators to work in such localities for "mineral." It is of 

 course not impossible that valuable minerals in paying quantities 

 might be found in, or adjoining, veins of mica-trap, but nothing 

 of the kind has yet been discovered, and as it is not very likely 

 that such deposits will be found hereafter — it may be well to 

 warn the unwary against being misled by the specious state- 

 ments of interested but unscrupulous persons. 



