

Il8 HOLLAND: GEOLOGY OF THE NEIGHBOURHOOD OF SALEM. 



fairly complete list of the known varieties of the charnockite series, 

 garnetiferous and non-garnetiferous. Those of " intermediate " com- 

 position are, however, by far the most abundant, and are typically 

 represented in the Shevaroy mass, where they are as a rule non- 

 garnetiferous. 



The varieties which are distinguished as u intermediate " are not 



only intermediate in specific gravity and bulk 

 Distinctive characters. 



analysis between the acid and basic extremes, 

 but microscopic examination shows each specimen to contain practi- 

 cally all the minerals which are generally found in the complete 

 series ; even in the same section we find hypersthene, augite and 

 'the peculiar brown-green hornblende, with quartz, potash-felspar, 

 plagioclase and iron-ores, and these minerals exhibit a great 

 tendency to aggregate into groups of granules of the same class, 

 which gives the rock a very blotchy appearance. But notwithstand- 

 ing this apparently composite character which the microscope thus 

 exaggerates, a series of large hand-specimens taken from different 

 parts of the Shevaroy mass are remarkably uniform in average 

 composition. Even by taking small specimens, such as one uses 

 for determination by a Walker's specific gravity apparatus, the local 

 divergences from the mean result are not great. To obtain an idea 

 of the average composition of the Shevaroy mass of rock, 48 

 specimens were taken indiscriminately from different parts of the 

 hills and found to have an average specific gravity of 2*777, with 

 very closely agreeing results for the averages of each exposure. 

 This result is practically identical with the average (2'775) obtained 

 for a smaller number of specimens collected at random by Dr. H. 

 Warth in the South Arcot district. It is not difficult, nevertheless, 

 to obtain in any large exposures small specimens which are basic 

 in composition and others which are distinctly acid ; but the rock 

 most frequently met with in the Shevaroys is one in which this 

 separation into distinctly basic and acid portions is not evident in 



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