362 J. H. Collins — Cornish Serpentinous Mocks. 



parish of St. Keverne. It comes in as a wedge between the dark 

 hornblendic "greenstones" of the Nare Head and the Lower Silurian 

 slates and quartzites of the same locality. The excavations upon this 

 band are now mostly filled in, and it is greatly decomposed where 

 it reaches the sea in Gerrans Bay, and appears only as a soft ferru- 

 ginous mass, so that it is almost impossible to determine whether 

 it is simply an altered bed or occurs as an intrusive mass. For 

 reasons which I need not now specify, I incline to the latter opinion, 

 yet it is worthy of remark, as pointed out by Canon Eogers, that 

 the strike of the band lies in a straight line with that of the serpen- 

 tine of Porthalla, in the parish of St Keverne, which is distant some 

 15 miles to the south-west, and this is at least suggestive of a 

 common origin. However this may be, the serpentines of these 

 two localities are extremely different ; the Veryan rock more 

 resembling the ordinary Lizard serpentine than does that of 

 Porthalla. It has no cleavage, but breaks with a splintery fracture 

 like ordinary serpentine. In the mass it is slightly magnetic. 

 Under the microscope, using a 2-inch power, it is seen to consist of 

 a transparent green base, which incloses and surrounds many large 

 much-fractured crystalline masses, not definite crystals, which may 

 be altered olivine. No calcite is visible, but in some places numer- 

 ous fibres of asbestos may be seen. Throughout the serpentinous 

 ground-mass are sprinkled numerous dark patches of ferruginous 

 matter, as in the serpentine of Coverack. The base of course is not 

 dichroic, nor are the crystalline patches ; but these latter are 

 brilliantly coloured by the use of polarized light, especially the 

 asbestos fibres when both polarizer and analyzer are used. The base 

 changes as one Nicol is rotated from pale yellowish green to black. 

 It is possible that this may be altered picrite or some similar rock ; 

 but if so, the metamorphosis is very complete, as it contains only one 

 per cent, of alumina. The following are my analyses of this rock : 



1. 2. 



Silica 36-55 36-05 



Alumina 1-00 -95 



Ferric Oxide 8-75 8-00 



Ferrous Oxide 2-25 1-00 



Chromic Oxide trace trace 



Lime trace trace 



Magnesia 32-79 33'79 



Alkalies 5-12 4-89 



Water (by ignition) 14-59 12-39 



101-05 > 97-07 



I have observed thin layers of serpentinous matter in the joints and 

 fissures of the hornblendic " greenstone " of the Nare Head, where 

 it comes in contact with this serpentine, but I have not suffi- 

 ciently examined this latter to be able to say whether it exhibits 

 evidence of serpentinous change in itself, or whether these thin films 

 have been merely deposited from water percolating through the rock. 

 Duporth. — This serpentinous rock has been analyzed and described 

 by Mr. J. A. Phillips 1 and by myself. 2 It seems to form the eastern 

 termination of a dyke of hornblendic greenstone which extends from 



1 Phil. Mag. Feb. 1871. 2 Min. Mag. Nov. 1877. 



