364 J. H. Collins — Cornish Serpent inous Rocks. 



From these analyses it appears that the change is not great at 

 Gewans as compared with St. Mewan, a distance of a mile and a 

 half, and that it consists chiefly in a lessening of the proportions of 

 iron oxides and a slight increase in the silica. Within the next mile, 

 however, a very notable change takes place, the silica being reduced 

 by upwards of 12 per cent., and the alkalies entirely carried off, 

 while the proportion of water increases from less than one to over 

 eight per cent., and the magnesia from less than one to nearly sixteen 

 per cent., or in Mr. Phillips' specimen over 22 per cent. Chemically 

 and mineralogically the rock now consists essentially of something 

 over 30 per cent, of serpentine as a base or ground-mass, containing 

 in crystals and grains nearly 50 per cent, of kaolin, and nearly 20 

 per cent, of peroxide and magnetic oxide of iron, while in e it is a 

 fairly pure serpentine. 



I must refer to the paper above mentioned for illustrative draw- 

 ings of the microscopic structure of the rock. Prof. Bonney con- 

 siders that from the high proportion of alumina present in this 

 rock, it ought not to be classed with the serpentines ; in his 

 Presidential Address to the Geological Society he says of it, " The 

 serpentinous rock of Duporth .... appears to be a member of the 

 picrite group. Many of the picrites certainly hang on very closely 

 to the dolerite group, and can be seen to graduate into representatives 

 of it; " but the analyses given above will show that this is an accidental 

 rather than a characteristic component, arising from the distinctly 

 imbedded crystals, and that, in the portions of the rock where these 

 crystals are more sparsely distributed, it approaches in composition 

 an ordinary serpentine rock (which is usually a very different thing 

 to the mineral serpentine). 



I have said above why I think this rock should not be classed as 

 picrite. It does indeed seem to have been originally a dolerite, 

 but there is no proof that any considerable proportion of olivine was 

 ever present. The first alteration of the augite seems to have been 

 its conversion into hornblende, and this has subsequently been altered 

 to serpentine. 



As to the cause of this remarkable change of a hornblendic into 

 a serpentinous rock, I may remark that the district is one of very 

 extensive disturbance, traversed in all directions by mineral lodes 

 and elvan courses ; and it is not more than two miles from the St. 

 Austell granite. I have elsewhere l given my reasons in detail for 

 believing that the extensive kaolinization of the felspar of that 

 granite has been produced by mineral solutions acting through 

 fissures from below. I have little doubt that this production 

 of serpentine is due to a similar action, and probably fluoride of 

 magnesia was the principal cause of this alteration, both of the 

 hornblendic base and of the felspathic crystals. 



Clicker Tor, near Lislceard. — This remarkable intrusive serpen- 



my specimens have undergone more extensive change than yours, as you will see from 

 the fragment which I inclose." He also observes, " My recent analysis of some 

 remarkably fresh rock from the Sanctuaries gave T75 per cent, of magnesia." 

 1 The Hensbarrow Granite District, 1876. 



