MKTA.MolM'lllr I.IMKsToNKs TIU.KY 



It will In- sufficient to describe the nature <if the minerals uf these rocks as a 

 whole, points of particular interest shown l>y any of them being refererd to in the 

 course of this description. 



The constituent mineral are dolomitf. cnlcitf. forxh-rite. <-/t<nlr<nlit<-. 

 hornblende, dm/mnlr. pUogoptte, .-//*'/////'. and 



The minerals which characterise the class are dolomite, calcite and forsterite. 

 the remainder forming a subordinate group, which, while often abundant, yet rank as 

 accessory constituents to the class. 



Dolom ite. While the ordinary method of differentiation between this mineral 

 and calcite is the staining method of Lemberg, yet the criteria which have been found 

 to operate in metamorphosed dolomites of other regions are often of use here. These 

 iiii hide the different types of twinnning shown by. dolomite and calcite respectively, 

 and the degree of turbidity. 



As earlier noted (') the dolomite twinning on the 0221 plane is sufficiently 

 distinctive in favourable sections, and the turbid character of the calcite also affords a 



further means of discrimination. This poly synthetic twinning in dolomite suggests 

 that a secondary twinning along a glide plane is involved. 



Calcite is quite frequently twinned in these rocks and the turbidity often observed 

 is due to the presence of minute inclusions which are probably carbonaceous. 



Forsterite. The usual habit is in rounded grains or prismatic crystals, in which 

 the trace of the 010 cleavage parallel to the elongation is imperfectly developed. In 

 rocks Nos. 137 and 318 the olivine is almost completely free from decomposition, but 

 in the remainder all stages of serpentine development are revealed. Usually this is a 

 colourless type, but in some cases it appears of a pale yellowish green tint. In No. 992 

 the serpentine is accompanied by granules of secondary magnetite. Where developed 

 as an inclusion in the serpentine, the carbonate is usually calcite. 



Chondrodite. Orange yellow crystals of a member of the humite group ol 

 minerals are developed in rocks Nos. 135 and 137. In No. 135 they are partly 

 arranged along a plane through the rock, as if indicating the passage of fluorine bearing 

 vapours along a bedding plane or other .surface < I interruption. 



In thin section the mineral is noticeably pleochroic, varying from golden 

 yellow (X). to colourless (V, Z). Twinning is faintly developed in some sections. 

 The extinction measured from the 001 cleavage, the plane of lamellar twinning, 

 corresponds to that of chondrodite in the section available, reading 20 degrees. 

 Clinohumite is thus excluded. 



There is no trace of intergrowth with forsterite, and serpent inisation proceeds in 

 the same manner as in normal olivine. 



1 Oe.il. Mag., Vol. Ivii. 1020. p. 453. 



