LOWER VINDHYAN SERIES. IOI 



a coarse granite with no trace of foliation, this being probably ap- 

 parent only in the field. Strain phenomena are, however, manifest in 

 the microscopic section. The rock has the composition of an ordi- 

 nary biotite-granite. It consists of quartz, orthoclase, oligoclase. 

 biotite and sphene. All the minerals are perfectly allotriomorphic, 

 The quartz shows not only strain shadows, but even the initial stages 

 of mosaic formation. The orthoclase shows microclinic structure. 

 The felspars are largely altered to saussurite and epidote. 



The minerals of such a rock are sufficiently characteristic to 

 be recognised in a fragmentary condition in a microscopic section 

 of the overlying rock, if it were true that this is an arkose made up of 

 their debris. Far from its being sedimentary, however, it appears 

 in the microscopic sections of both the specimens f| and T -|-g-, as a 

 rock so thoroughly crystalline, that it can only be regarded as an 

 intrusive rock, for it is doubtful whether rhyolitic lavas can ever 

 be so completely holocrystalline. 



In the hand-specimen the rock is whitish-grey with a greenish 



Micrograms. tin S e > and the crowded porphyritic crystals give 



it a granitoid appearance. One of the speci- 

 mens, T ^, when examined microscopically, appears most remarkably 

 crystalline. The large porphyritic constituents, usually quite idio- 

 morphic, are plagioclase felspar in crystals that are admirably zoned 

 when seen in polarised light, and biotite in hexagonal prisms. These 

 lie in a groundmass of plagioclase, orthoclase, and quartz, all of 

 which are locally idiomorphic, while elsewhere the structure is micro- 

 granitic. Lastly, the remaining interstices between the grains of this 

 growth of smaller crystals are occupied by micropegmatite. The 

 felspars are sometimes altered with formation of epidote, otherwise 

 they are perfectly clear and transparent. - 



The rock is therefore totally unlike the under-lying gneiss, while 

 all its constituent minerals are identical with those observed in the 

 11 trappoids," with this difference that the structure is thoroughly 

 igneous instead of being partly clastic. The only mineral not seen in 

 this section, although so common in the "trappoids," is the porphyritic 



( ioi ) 



