122 HOLLAND: GEOLOGY OF THE NEIGHBOURHOOD OF SALEM. 



specific gravity of 2*8o. The biotite-gneiss contains much quartz and 

 is distinctly acid in composition. In penological characters also 

 the two rocks are quite distinct : the charnockite is a compact, blue- 

 grey, fresh-looking rock, whilst the biotite-gneiss is well foliated and 

 mottled by patches of a dark-green, micaceous mineral lying in 

 dirty-white felspar and pale-blue quartz with, frequently, shreds 

 of pyrite. Under the microscope the charnockite is found to be 

 composed of hypersthene, pale blue-green augite, felspar and a little 

 quartz, with lumps of opaque black iron-ores, all showing a com- 

 mon type of rock amongst the charnockite series, and displaying 

 practically no signs of dynamo-metamorphism. The biotite-gneiss, 

 on the other hand, is not only highly crushed, but its minerals all 

 show signs of alteration of a kind not seldom found in definite 

 contact cases]: epidote and muscovite are formed in the felspars, 

 pyrite and rutile are fairly abundant, and the ferro-magnesian silicates 

 have completely lost their individuality, being replaced by an 



Fig. i.— Altered biotite-gneiss near its contact with charnockite, j| miles south 

 of Salem. (Section magnified by 20 diameters.) 



( 20 ) 



