DESCRIPTION OF THE CHIEF TYPES. 137 



whilst the central mass is composed of charnockite, which is uniform 

 in character up to its junctions on either side with the norite. It 

 is from this mass that the type specimens of the rock, now preserved 

 in the Geological Museum, Calcutta, were obtained (No. 9*658). 

 Through it, and the associated norite as well, run coarse-grained 

 veins — like the contemporaneous veins of ordinary granites' — in 

 which the ferro-magnesian silicate and the iron-ores are reduced to 

 mere traces, whilst the rock is made up almost wholly of microcline 

 and quartz (9*659) . 



Whilst the norite shows practically no signs of a parallel 

 arrangement of its constituents, the charnockite, forming the central 

 mass of the hill, shows by the linear disposition of the dark minerals a 

 rough foliation of north-north-east — south-south-west, being thus 

 parallel to the protaxis which has determined the Coromandel coast- 

 line. If the charnockite, the most acid member, represents the 

 residual portions of the magma whose segregative consolidation 

 resulted in the hypersthene-bearing complex at Pallavaram, it is 

 only natural to expect that one result of its probably late consolidation 

 would be a disposition of its constituents parallel to the fissure 

 it occupied and at right-angles to the direction of maximum lateral 

 pressure. That this rude foliation occurred before and not after 

 consolidation is shown by the fact that the most delicate interlocking 

 structures have been preserved in the rock : the common signs of 

 dynamo-metamorphism, such as peripheral granulation of the con- 

 stituents and the production of mylonite, are entirely wanting. 



The determination of a large number of specimens, taken from 

 the type mass near St. Thomas' Mount, gave, 



Specific gravity. 



as an average of numerous closely agreeing 

 results, a specific gravity of 2*67. 



Under the microscope, the rock is seen to be an even-grained, 

 crystalline aggregate of quartz and potash fel- 



Mineral composition. J °° ° * r 



spar as the two most abundant of the consti- 

 tuents, with smaller quantities of oligoclase, rhombic pyroxene, 

 approaching hypersthene in optical characters, opaque iron-ores and 

 an occasional granule of zircon. 



C 2 ( 19 ) 



