﻿Microscopic Constitution of certain Cornish Rocks. 101 



Serpentine, Lizard. Sp. gr. = 2-59. — The specimen selected 

 for analysis was broken from a slab prepared for decorative pur- 

 poses, and is of a very dark green colour, in places verging on 

 black ; it is thickly spotted with red, and has a coarsely granular 

 structure. This serpentine is massive, without any indication of 

 foliation, and encloses imperfectly defined crystals presenting 

 brilliant surfaces when broken through. 



Its chemical composition was found to be as follows : — 



I. II. 



Water* 1552 15*52 



Silica ...... 38-86 38-58 



Alumina 2'95 3-06 



Ferric oxide 1-86 1*95 



Ferrous oxide 5'04 5* 10 



Oxide of manganese . . trace trace 



Oxide of nickel (NiO) . . '28 -30 



Chromic oxide (Cr 2 O 3 ) '08 -08 



Lime trace trace 



Magnesia ...... 34-61 34-32 



Potassa -33 -30 



Soda '77 '76 



100-30 99-97 



The microscope shows this rock to consist of a cryptocrystal- 

 line base, spotted by oxide of iron &c, and enclosing indistinct 

 green or yellowish-brown crystalline forms — pseudomorphs after 

 pyroxene ? These crystals give irregular colours with polarized 

 light. 



Orthoclase, Roche. Sp. gr. = 2*55. — On the right-hand side 

 of the road leading from St. Austell to the village of Roche, and 

 at a distance of a mile from the latter place, a deposit of yellow- 

 ish-white crystalline felspar is quarried in the bottom of a shallow 

 ravine. 



The area over which this mineral extends has not been accu- 

 rately determined ; but it has been worked, on the course of the 

 hollow, by means of shallow pits, for a length of about one hun- 

 dred yards, and to a width of some sixty feet. It is enclosed in 

 granite containing considerable quantities of schorl, and is tra- 



which in other portions of the same deposit remains unchanged, with the 

 exception of having become hardened into a kind of stone. 



This asbestus, which is found for a distance of several yards on either 

 side of a band of schistose serpentine crossing the valley nearly at right 

 angles, has evidently been produced in situ, and encloses pepitas of water- 

 worn gold, together with fragments of wood, which are still readily cut 

 with a knife. I visited this locality, and carefully examined the conglo- 

 merate in question, during the fall of the year 1866. 



* Of which 2*06 was lost in the water-bath. 



