﻿Vol. 66.'] DEDOLOMITlZATlOX IX THK MAKliLE OF TORT SHEPSTOXE. 513 



iron-ore (magnetite) in those grains which are most serpentinized. 

 It is the presence of this finely divided iron-ore that gives to the 

 olivine its coaly-black appearance. 



The spinel occurs in well-formed crystals, which frequently show 

 square or triangular sections. Although not present in every 

 section made from the dark band, the spinel is in places quite 

 abundant. It has a faint greenish tinge, and is perfectly isotropic. 

 A conspicuous feature is a black margin, due to a coating of powdery 

 iron-ore, which is probably derived from the decomposition of 

 the olivine. Incipient alteration to a fibrous product is visible 

 in places. To make certain that the mineral was really spinel and 

 not periclasc, the black material (which after crushing was retained 

 on a 90-mesb, but passed a 60-mesh sieve) was digested for 2k hours 

 in hot concentrated hydrochloric acid. 1 A product was obtained 

 consisting almost entirely of isotropic crystal fragments freed from 

 the iron coating. This material is pale purplish grey in reflected 

 light, but possesses a faint green tinge in transmitted light. In 

 methylene iodide it is nearly invisible, indicating a refractive index 

 near to 1*74. A qualitative analysis showed that it contained a 

 large proportion of alumina. 



The mica, viewed in the slice, is pale brown with decided pleo- 

 chroism (pale brown to colourless) and very strong birefringence. 

 Flakes of this mica were picked out from the hand-specimen and 

 examined by Mr. Campbell Smith with the following results: — 

 The dark-brown flakes are quite brittle, and too small for the deter- 

 mination of the percussion figure. The refractive index was found 

 by the method of Schroeder van der Kolk to be between 1-56 and 

 1'59. The mineral is biaxial (2E = 24°) and optically negative ; 

 the pleochroism on cleavage-flakes is dark brown to pale brown 

 (light vibrating parallel to the plane of the ootic axes giving pale 

 brown). 



Mr. T. Crook, of the Imperial Institute, isolated by means of 

 the electromagnet about 2 grams of the mica, and, after cleaning 

 this material with weak acid, determined its percentage of water to 

 be 1-72 and its density to be over 2-85. The summation of these 

 characters, with those determined by Mr. Campbell Smith, safliccs 

 for its identification as phlogopite. 



In the course of the isolation of the mica, it was found to be 

 associated with a considerable amount of pyrrhotite, which was 

 separated by means of a heavy solution of a density of 3-34. An 

 examination of a thin section in reflected light reveals the pyrrho- 

 tite grains embedded in the phlogopite. 



(c) The light-mica-forsterite zone. — This zone, which is 

 about 2 inches in width, consists of a pale mica (phlogopite), a 

 pale-brown member of the olivine group, and a small amount of 

 pale-green serpentine, derived from the last-named mineral, 

 together with interstitial calcite. 



An examination of a slide shows clearly the nature of the change 



1 Periclase is soluble in hot concentrated hydrochloric acid. 



