﻿98 Mr. J. A. Phillips on the Chemical Composition and 



Thin sections, when examined under the microscope, exhibit 

 unmistakable evidence of extensive alteration. The felspar does 

 not generally present any distinctive characteristics, although in 

 a few instances the parallel striping peculiar to triclinic varieties 

 was observed. In addition to felspar this rock contains numerous 

 semi-transparent yellowish-brown crystals, probably hornblende ; 

 there is also a green mineral which has a fibrous structure, show- 

 ing colours with polarized light ; this is believed to be a variety 

 of hornblende. 



Many black grains of oxide of iron, and some well-defined 

 hexagonal crystals (which, as phosphoric acid has been shown to 

 be present, are in all probability apatite), are disseminated 

 throughout the mass ; there is likewise an abundance of a green- 

 ish chloritic mineral, which is doubtless a secondary product. 

 Mr. Samuel Allport, F.G.S., an acknowledged authority on the 

 microscopic structure of rocks, to whom I submitted a section, 

 agrees in the opinion that this is probably a diorite that has un- 

 dergone much alteration. 



Those portions of the eruptive dyke which are in close proxi- 

 mity to, or come into direct contact with, the enclosing slaty rock 

 not unfrequently exhibit a quasi-schistose structure. Sections 

 made from specimens of this description were found to have lost 

 all traces of crystalline arrangement, and to consist of an amor- 

 phous felspathic base, in which a greenish chloritic mineral is 

 disposed in parallel bands. Some of the sections which have 

 been prepared from this variety of the rock are traversed by mi- 

 nute veins of crystalline quartz enclosing numerous well-defined 

 crystals of hornblende ; and it was observed that the chloritic 

 bands were frequently bent at their point of intersection with 

 the veins of quartz. 



"Greenstone" Blowing-House Hill, St. Austell. Sp. gr. = 2*89. 

 — Some twenty years since, a quarry was worked for road-mate- 

 rial immediately below the old schoolroom on Blowing-House 

 Hill, St. Austell. The general appearance of the stone obtained 

 from this locality is very similar to that of the rock from the 

 " Sanctuaries," excepting that it has a somewhat duller aspect, 

 and is divided by a series of oblique cross headings which impart 

 to it a tendency to divide into rhomboidal masses. It is also less 

 decidedly crystalline than the rock previously described; and 

 although in no degree fissile, it nevertheless exhibits a grain re- 

 sembling that of some varieties of metamorphosed schist. The 

 following results were obtained by analysis :— 



