182 DR. C. E. TILLET OJf THE PETROLOGY OF [vol. lxxix, 



(b) Petrography. 



(i) Chlorite -Epidote -Albite- Schists. 



The constituent minerals of these rocks are chlorite, epidote and 

 elinozoisite, albite, titanite ; and (accessorily) calcite, hornblende, 

 quartz, also in some examples a few flakes of white mica. The 

 petrographical character of this group of the Green Schists can be 

 exemplified by the description of a Green Schist from Seacombe 

 Sands, Prawle coast. Macroscopically, this is a silky, grey-green, 

 distinctly schistose rock, showing white grains of albite. Under 

 the microscope the constituents seen to be present are chlorite, 

 epidote, albite, titanite, hornblende, and magnetite. 



The chlorite forms plates with which the fibres of amphibole 

 are intergrown, and yielding a green to yellow-green pleoehroism. 

 Sections examined show almost uniaxiality and positive optical 

 character, with low double refraction. 



The epidote is a ferriferous type giving third-order carmine 

 tints, and a strong pleoehroism in yellow-green tints. It is optic- 

 ally negative, the birefringence having a value of O038 corre- 

 sponding to a ferric-oxide content of 14 to 16 per cent. 



The felspar is an albite with refringence less than Canada 

 balsam, and extinction on 010 of 19°. It is optically positive. 

 Twinning is rare, and cleavage is often absent. The most common 

 inclusions in the albite are grains of epidote and fibres of 

 amphibole. 



Titanite occurs in rounded grains of high relief, associated 

 with epidote and chlorite. 



The amphibole is present in long needles with cross-parting, 

 associated with chlorite or enclosed in albite. The colour is bluish 

 green to almost colourless, and the maximum extinction from the 

 prism axis is 20°. 



A very small amount of quartz is interstitially found among 

 the albite grains, characterized by its higher refringence, and 

 proved by optic uniaxiality. A single grain of apatite is present. 



The crystals of chlorite and amphibole are oriented in parallel 

 enclosing grains of epidote, and such bands may be separated by 

 areas of albite. 



This Green Schist was selected as a type-rock for analysis. It 

 is free from calcite, and was so selected, as it is not clear how far 

 the calcite in these rocks represents migration of material. The 

 results of this analysis are set forth in the accompanying table 

 (p. 183). Quoted with it are the analyses of a number of com- 

 parable basalts, Prof. P. A. Daly's average for all basalts, and 

 two analyses of dynamically metamorphosed rocks which are 

 mineralogically similar. 



The agreement between these analyses confirms the essentially 

 igneous character of the rocks. 



