

CLASSIFICATION OF THE ROCKS. IO7 



II.— CLASSIFICATION OF THE ROCKS. 

 The rocks exposed in the immediate neighbourhood of Salem 

 may be conveniently divided into the following groups, which are 

 arranged in probable order of age : — 

 4. Younger igneous intrusions. 



c. The "White Elephant rocks". 

 b. Peridotites of the Chalk hills. 

 a. Basic dykes. 

 3. The " pyroxene-gr anulites" or charnockite series. 

 2. Schists of the Salem-Ahtur valley. 

 1. Fundaments! biotite-gneisses. 



The old biotite-gneisses have the composition of granitic rocks, 

 but no more direct evidence as to the nature of their origin is ob- 

 tainable. The schists and leaf gneisses of the Salem-Ahtur valley 

 include the deformed products of probably both igneous and aqueous 

 formations, whilst the u pyroxene-granulites "■ (charnockite series) 

 present intrusive relations to both the above groups, and are con- 

 sequently considered to be the youngest of the foliated rocks within 

 this are*. These rocks are considered to be Archaean in age, purely 

 because they resemble petrologically the old gneisses and schists of 

 Europe and America; they have been invaded and traversed by the 

 younger igneous rocks forming group 4. 



(1) Fundamental Biotite-gneisses. 



The biotite-gneisses of this area present the usual characters 

 of those which are generally considered to be members of the older 

 (lower) division of the Archaean gneisses. The position given to these 

 gneisses, merely on petrological grounds, is confirmed in the present 

 instance by comparatively definite geological evidence which is 

 detailed below (pages 109 and 121). 



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