THE GRANITOID GNEISS. 5 



mean that (i) some at least of these rocks which look like gneisses 

 should really be regarded as foliated granite which on its intrusion 

 metamorphosed the slates, and (2) that they are post Cuddapah in age • 

 Whether this holds only for the gneisses in the immediate vicinity of 

 Chachanbali or for the wide area here described, is at present impos- 

 sible to decide. (Specimens 15*202 to 15*209.) 



III.— THE GRANITOID GNEISS. 



In the vicinity of Bondesor occurs a very coarse-grained granitoid 

 gneiss in which large pink felspar crystals and broken red garnets are 

 contained in a dark groundmass. Usually the felspar crystals, which 

 are sometimes nearly a foot long, are approximately parallel, the direc- 

 tion being north-westerly. At times these giant phenocrysts are not 

 bounded by polyhedral surfaces, but are more or less ovoid in form 

 causing the rock to become an augen gneiss (specimen 15*162), 

 resembling at first glance a coarse conglomerate with rounded pinkish 

 pebbles in a dark fine-grained cement. In addition to the north-west- 

 erly parallelism of the phenocrysts, which is probably the direction 

 of the flow prior to consolidation of the magma, a north-easterly 

 system of joints and slickensides, caused by subsequent earth-move- 

 ments, is often well developed. 



On examination of thin sections of this rock with the microscope 

 it is seen that in addition to large crystals of felspar and grains of 

 garnets, which are easily distinguished macroscopically, the chief 

 constituents of the dark groundmass are quartz, twinned and untwin- 

 ned felspars, biotite and small grains and crystals of garnet and 

 pyroxene. The pyroxene is usually pleochroic with the colors of 

 hypersthene — rose and greenish blue — but it is sometimes seen to be 

 distinctly monoclinic, shewing a maximum extinction of 15 when 

 measured against the cleavage lines. In some varieties brown 

 hornblende and a pale green monoclinic pyroxene are very frequent. 



The history of this rock as revealed by the microscope agrees 



