﻿Microscopic Constitution of certain Cornish Rocks. 99 



I. II. 



Water* 1-00 1*00 



Silica 47-68 47-33 



Titanic acid trace trace 



Phosphoric acid .... trace trace 



Alumina 17*13 16-86 



Ferric oxide 11-73 11-77 



Ferrous oxide 10-71 10-71 



Manganoso-manganic oxide. -42 *40 



Lime 6-28 6*29 



Magnesia trace trace 



Potassa 2-94 2'84 



Soda 2-53 2-56 



100-42 99-76 



It will be observed that the chemical composition of this rock 

 is almost identical with that of the diorite extending from Quoit 

 farm to Duporth, with the exception of its containing an appre- 

 ciable amount of oxide of manganese, and that the percentage of 

 soda is only about one half of that found in the former case. 



Under the microscope it is seen to consist of a compact fel- 

 spathic matrix, enclosing a few indistinct crystals of felspar, 

 together with large quantities of the greenish chloritic mineral 

 referred to in the description of a diorite from St. Mewan. This 

 last is arranged in approximately parallel bands and patches, 

 many of the latter having the appearance of partially decom- 

 posed crystals of hornblende ; the central portion of these retain 

 to a certain extent their original structure and colour, whilst the 

 edges have become transformed into chlorite, or some nearly 

 allied mineral. 



This rock, which also contains many black grains of oxide of 

 iron, and a few hexagonal crystals, which are perhaps apatite, 

 may be either a metamorphosed slate, or a diorite which has be- 

 come much changed by the effects of a gradual re-arrangement 

 of its constituents. I however incline to the opinion that it is 

 an altered clay-slate. 



Serpentinous rock, Menheniot. Sp. gr. = 2*77. — A cutting of 

 the Cornwall railway intersects, at the Menheniot station, a wide 

 band of a dark green rock which is there extensively quarried 

 for road- making. Its total width may be taken at about fifty 

 yards ; but it is more or less intermixed with the enclosing clay- 

 slate, from which it is separated by no well-defined walls or lines 

 of demarcation, since the two gradually merge into each other ; 

 and beds of unaltered slate are found interstratified with the 

 serpentinous greenstone. Its general structure is decidedly 



* Of which *33 was lost in the water-bath. 

 H2 



