DESCRIPTION OF THE PRINCIPAL EXPOSURES. 171 



CHAPTER V. 



Description of the Principal Exposures. 



It has already been pointed out that notwithstanding the great 

 variations in silica percentage which the members of the char- 

 nockite series present, they exhibit distinct characters, both mi- 

 croscopic and macroscopic, which readily serve to group them 

 together, and which point to the unmistakable consanguinity of 

 the different varieties. Partly because their true mineralogical 

 characters were not recognised before the microscope was used in 

 South Indian petrology, and partly because the earlier surveyors 

 were compelled for economic reasons to confine their attention to 

 isolated areas, this very important fact was not recognised before 

 1892. Some members of the charnockite series have been referred to 

 as (l quartzo-felspathic gneiss," others have been spoken of as 

 " hornblendic gneiss," whilst in other localities they have been mapped 

 as "syenitoid gneiss." This system of nomenclature has resulted 

 firstly, in the separation from one another of the different members 

 of one series of related rocks, and secondly, in the grouping together 

 of rocks, which, though composed of the same mineral species, are 

 not genetic relatives of one another. In consequence of these 

 unfortunate circumstances, for which only the great petrological 

 progress of recent years is to blame, only a small portion of the 

 voluminous literature of South Indian geology can be made use of 

 in summarising the facts concerning the geological relations and 

 geographical distribution of the rocks now grouped together under 

 the name " charnockite series." 1 



The type-exposures near Madras. 

 On account of their proximity to the city of Madras, the 



1 Mr. R. B Foote has, by referring to the rock at Cape Comorin as a type, given a 

 convenient means for identifying the members of the charnockite series in the districts of 

 Madura and Tinnevelly. ("On the Geology of the Madura and Tinnevelly Districts/' Mem. 

 Geol.Surv. /wc?., Vol. XX, p. 28.) It is unfortur.ate that this practice of supplementing the 

 mineralogical description of a rock series by reference to a local type has not been universally 

 followed in our publications. 



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