THE GEOLOGICAL AGE OF CENTRAL AND WEST CORNWALL. 



201 



triclinic felspar. Very often, too, minute acicular crystals of 

 apatite * are visible, and these probably account for the traces 

 of phosphoric acid, which substance is always present in greater 

 or less proportion. We think, too, we have recognised augite in 

 3ome specimens, and manv specimens contain magnetite. Opacite 

 and Ferrite are always present. Generally one may see little 

 veins or patches of calcite. Altogether the resemblance to the 

 mica-traps of the Kendal district, as described by Messrs. Bonney 

 and Houghton, seems to be very close indeed. 



Chemical Composition. Considering the very varying 

 proportions of the constituent minerals and the extent of 

 ground over which these mica-traps are found, this may be 

 regarded as remarkably constant. In the accompanying tables 

 we give many analyses illustrative of this point, each being in 

 most cases the mean of two concordant analyses. 



Tahle 1. Unweathered specimens.! 



7, 



* My attention was first called io the crystals of apatite by Professor Bonney, 

 to whom I had sent a specimen of the rock. J. H. C. 



fa is the analysis of a specimen from Trelissick Creek— analysed by Mr. J, 

 A. Phillips, {loc. cil.) ; for all the other analyses we are ourselves responsible, 

 b is also from Trelissick ; c is from Lamb-Creek ; d is from near the Nare Point. 



