jK. H. Rastall — Mount Sorrel Granite. 501 



IX. — On Basic Patches in the Mount Sorkel Granite. 

 By E. H. Eastall, B.A., F.G.S. 



DURING a visit to Mount Sorrel at tlie end of last year I collected 

 a number of specimens of the dark-coloured patches which 

 are so common in the granite. I have had about a dozen of these 

 sliced, and an examination of them has yielded some results which 

 seem worthy of a brief description. 



These dark patclies vary a good deal in character, and may be 

 divided into three fairly well-defined types, as follows : — 



(1) Small black or grey, generally angular patches, without 

 porphyritic felspars. 



(2) Somewhat larger and usually ovoid patches of a brown colour, 

 generally enclosing felspars of porphyritic habit. 



(3) Kather large black bodies, distinctly banded and often 

 penetrated by parallel veins of granite, in the manner usually 

 described as lit-par-Ut injection. These have an obvious outward 

 resemblance to blocks of banded or bedded rock. 



Corresponding to this macroscopic classification ai'e distinct 

 difi'erences in the microscopic structure, and the special character 

 of each type may be shortly described. 



Type 1. The patches of this class consist essentially of felspar 

 and hornblende with only a little interstitial quartz. The felspar 

 is usually a plagioclase, in rather narrow lath-shaped sections, of 

 the habit usual in basic intrusive rocks. The hornblende also 

 occurs in small prisms, and is often chloritised ; in places it has 

 a distinctly ophitic character. Near the centre of the patch these 

 two minerals, with a very little quartz, make up the whole 

 mass, but towards the outside the crystals become more widely 

 separated and are enclosed in poecilitic fashion in large plates of 

 orthoclase or perthite. These are often continuous with the felspars 

 of the normal granite. It is very noticeable that in such patches 

 the minerals are often much decomposed, forming ' saussurite ' 

 chlorite, epidote, and other secondary products, while the normal 

 granite surrounding them is very fresh. 



Type 2. Brown patches with large pink felspars. The structure 

 here is very similar to that just described, but more quartz is present ; 

 the quartz is interstitial, and in parts has a sort of pseudogranophyric 

 appearance. The large felspars are often much rounded and also 

 sliow internal zones of corrosion. 



In both the black and the brown patches there are often to be 

 seen large crystals of sphene of a very peculiar habit. The sphene 

 is moulded on crystals of felspar, etc., in an interstitial manner, and 

 in a slice numerous disconnected patches extinguish simultaneously 

 over a large area. Sphene of this kind must almost of necessity 

 be of secondary origin. 



Another noteworthy point is the almost total absence of biotite 

 in the dark patches, although it is more abundant than hornblende 

 in the normal rock. 



