222 J. H. Collins — Cornish Serpentinous Bocks. 



Afc this locality I obtained in 1872 a number of interesting crystals 

 of pbarmacosiderite, scorodite, and olivenite, which were noticed in 

 the Mineralogical Magazine, vol. i. pp. 16 and 17. I regard this 

 rock as a slightly serpentinized and otherwise much altered horn- 

 blende schist. 



St. Cleer Downs. — On these downs there are several broad bands 

 of hornblendic rock, mostly lying in the line of strike of the asso- 

 ciated slates, and graduating in places into them. The chief mass 

 extends about one mile from east to west, and nearly half a mile from 

 north to south, dipping to the south-south-west. It is usually more 

 or less decomposed, and in a quarry now filled up near the church, 

 asbestos in fine silky fibres was obtained by Canon Eogers in 1818. 



Mr. J. A. Phillips gives the first two of the following analyses of 

 this rock : ^ — 



Sp. Gr 2-89 2-90 



c is an analysis of selected portions by myself. 

 Both a and h contain free quartz — hence the high percentage of 

 silica. This rock seems to be a partially altered hornblende slate, 

 or else an altei-ed diorite ; but the serpentinous change has not 

 proceeded very far. I have often seen thin films of serpentine in 

 the joints, and by breaking up masses of the more highly altered 

 portions of the rock, and selecting the more serpentinous grains, I 

 have obtained a substance with the composition given in c. The 

 relative proportions of silica, magnesia, and water in this are pretty 

 near what they are in pure serpentine, if we consider that it was 

 impossible to completely separate all the crystals and grains of 

 magnetite, fibres of asbestos, crystals and grains of unaltered or 

 partially altered augite, felspar, hornblende, etc. The approximation 

 is indeed remarkable if we compare the analyses given of the general 

 rock-masses. 



Under the microscope, using a low power, it seems to be composed 

 of a green fibrous granular ground-mass, filled with distinct fibres 

 of asbestos ; the ground-mass is very slightly dichroic, and includes 

 some crystals and grains of magnetite, with some irregular masses 

 having ill-defined and shadowy boundaries, which I think are highly 

 altered augite. Using the y power, the ground-mass is resolved 

 almost completely into a network of fibres, while the magnetite 

 appears as a congeries of rounded grains. In some instances horn- 

 blende crystals are seen, which appear to be changing very slightly 

 1 Quart. Journ. Geol. Soc. 1878, vol. xxxiv. p. 487. 



