J. H. Collins — Cornish Serpentinous Hocks. 223 



into asbestos. The following is an analysis of some of the larger 

 asbestos fibres carefully separated : — 



Silica 45-22 



Alumina and a little Ferric oxide 30-09 



Lime 800 



Magnesia 10-19 



Potash 1-12 



Water 5-20 



99-82 



Tregerla. — This rock, which occurs in the neighbouring parish 

 (Menheniot), and under very similar conditions, much resembles the 

 preceding. 



Catacleiise. — This is a somewhat altered dolerite which has been 

 extensively used in North Cornwall for the ornamentation of ancient 

 pulpits, fonts, etc. ; and the interesting tomb of Prior Vivian iu 

 Bodmin church, which has been so well described and illustrated 

 by the Eev. W. Jago in the "Journal of the Eoyal Institution of 

 Cornwall," is made of it. It is only slightly serpentinous, yet the 

 change is distinct, especially near the joints, and in the less compact 

 masses. It is often mistaken for Polyfant stone, ^ but it is much 

 harder. The following are analyses of the rock, — 1, of an ordinary 

 mass ; 2, of a fragment specially selected as being more highly 

 altered than usual : — 



100-37 101-6 



Here in 1 serpentinous change is hardly if at all recognizable, while 

 in 2 serpentinous matter exists to a very considerable extent. 



Altered eruptive rocks exhibiting serpentinous change occur in 

 a great many other localities in North Cornwall, and especially at 

 Cant Hill near St. Minver. My acquaintance with the rocks of this 

 district dates from the year 1871, and I can quite confirm the 

 observations made by Mr. F. Eutley in his paper.^ He gives the 

 following examples from this locality : — 



No. 1. East end, at top of hill — " bands and small knots of felsitic 

 matter (microcrystalline), separated by bands of translucent greenish- 

 yellow or brownish-yellow serpentine," also " a thin vein of greenish- 

 yellow to brownish -yellow serpentine which is traversed by opaque 

 rust-coloured strings," the bands and the vein being continuous. 

 We seem here as in many other instances to have evidence at once 



1 See Geol. Mag. Dec. III. Vol. III. 1886, p. 365. 



2 Eruptive Eocks from the Neighbourhood of St. Minver," by F. Rutley, F.G.S., 

 Quart. Journ. Geol. Soc. 1886, vol. xhi. 



