﻿Vol. 66.] DEDOLOMITIZATION IN THE MAKBLE OF PORT SHEPSTONE. 515 



felspar. This felspar exhibits well-marked twin-lamellation, with 

 an extinction-angle corresponding to oligoclase. The boundary- 

 edges of the felspar grains are irregularly crenulated, and occasion- 

 ally a granular structure is to be seen. 



In the marginal portion of the rock the felspar is associated with 

 scapolite and diopside. The scapolite, which is fairly abundant, 

 occurs in crystals which often have rectilinear boundaries. It shows 

 a rather low refractive index, moderate birefringence, and, in suit- 

 able sections, rectangular cleavage-cracks parallel to the prism 100. 

 Sections parallel to the principal axis show of course only one set 

 of cleavage-lines, and in these sections the birefringence is at its 

 strongest, giving reds and greens of the third order as compared 

 with the greys and pale yellows of the sections that lie more 

 transversely. A favourably oriented section enabled the optically 

 negative character of the mineral to be determined. 1 



The diopside occurs in colourless, rather isolated grains, pos- 

 sessing a high refractive index, strong birefringence, well-marked 

 prismatic cleavage-cracks, and oblique extinction. 



(6) The spinel-forsterite rock. — A hand-specimen of this 

 rock consists of a pale pink central core, a narrow grey band, and 

 an outer white portion, the latter being part of the normal dolomite. 

 The central core owes its colour to the abundant presence of a pink 

 spinel in small octahedral crystals, which are embedded in a white 

 carbonate (calcite). The latter occurs in very large crystals, in 

 which the spinel is so embedded as to interrupt the reflecting 

 cleavage- surfaces in the manner characteristic of the pcecilitic 

 structure of igneous rocks. Besides the spinel there is present 

 in isolated grains a dark red to black mineral of brilliant adaman- 

 tine lustre ; this mineral, as will be shown later, has been deter- 

 mined as rutile. The grey layer owes its colour to the presence 

 of forsterite in round translucent grains of a dull grey colour and 

 vitreous lustre. 



The minerals of this rock were isolated, and the separated 

 material examined by the following method : — The rock was treated 

 with cold dilute hydrochloric acid until effervescence ceased; the 

 solution was then filtered, and the residue washed and dried. The 

 removal of the forsterite was effected by flotation in methylene iodide 

 (of density 3*3), in which both the spinel and the black mineral 

 (rutile) sank. The black mineral was separated from the spinel by 

 hand-picking. Its density, determined by Mr. Campbell Smith 

 on -0448 gram by means of a small specific-gravity bottle, is 

 approximately 5*0. The refractive index is greater than 2*0 ; 

 the colour, reddish brown to black ; the lustre, metallic to adaman- 

 tine. The fragments are rather brittle, and translucent to opaque. 

 In thin sections the mineral appears as irregular translucent grains 

 of a light brown colour and strong birefringence. There are 



1 We have to thank Dr. J. S. Flett for assistance in the determination of 

 this mineral as scapolite. 



