﻿516 DR. F. H. RAICH AND MR. R. H. RASTAlL ON [Nov. I9IO, 



two cleavages approximate^ at right angles. The extinction is 

 straight with reference to cleavages. A qualitative analysis by 

 Mr. Campbell Smith shows that the mineral consists of titanium 

 dioxide, no other element being found. The three minerals which 

 consist of titanium dioxide are brookite, anatase, and rutile. The 

 high density and the presence of a well-defined cleavage exclude 

 brookite. Anatase is ruled out by the high density and the 

 absence of a basal cleavage; consequently the mineral must be 

 rutile. 



A considerable quantity of the spinel was isolated. It consists 

 of fairly well-formed crystals with predominant octahedral faces. 

 Its density is approximately 3-6. 1 Its lustre is vitreous. In thin 

 sections it occurs in grains, which are mostly bounded by straight 

 edges. It has a well-marked but rather imperfect octahedral 

 cleavage. The refractive index is high ; in ordinary light there is 

 no trace of colour, and between crossed nicols the mineral is 

 perfectly isotropic. It is most probably the magnesian spinel 

 corresponding to the formula MgO . A1 2 3 . 



The forsterite in thin sections appears in grains which bear a 

 superficial resemblance to the spinel, from which it is distin- 

 guishable by its more rounded contours and by its strong birefrin- 

 gence. Alteration to serpentine has only just commenced along 

 the imperfect cleavage-cracks. 



In addition to the above-mentioned minerals, there is very 

 sparingly present a colourless mica which is biaxial (2E small) 

 and optically negative. Under the microscope it shows one good 

 cleavage, parallel to which it extinguishes, and the birefringence 

 is seen to be strong. A few flakes of this mica, examined by 

 Mr. Campbell Smith, were found to have a density of less than 3*7, 

 and a refractive index of about 1*58. It is probably a pblogopite. 



This association of minerals, namely, spinel, forsterite, and 

 phlogopite, recalls the rock described by Mr. C. T. Clough & 

 Dr. W. Pollard from Gleuelg." 



V. Discussion of IIesults. 



The chemical analyses quoted above show that the marble of 

 Port Shepstone is a dolomite : whether originally deposited as 

 such, or resulting from the alteration of a limestone, is not clear ; 

 at the time of its marmorization by the granite, however, it is 

 certain that it was a dolomite. The character and extent of this 

 marmorization indicate that the rock was subjected at the time of its 

 metamorphism to a very high temperature, in the presence of water 

 vapour under great pressure ; and this is borne out by the nature 

 of the minerals produced in the dedolomitized areas. The intrusive 



1 Determined by Mr. T. Crook, by immersion in fused silver-thallium 

 nitrate. 



2 ' On Spinel & Forsterite from the Glenelg Limestone (Inverness-shire) ' 

 Quart. Journ. G-eol. Soc. vol. lv (1899) p. 372. 



