94 GEOLOGY OF TH£ SON VALLEY, ETC. 



Kua. The appearance in the hand-specimen is not altogether unlike 

 that of an arkose, though the grains are seen to be separated from 

 one another by the porcellanic looking matrix analogous to that 

 constituting the (l porcellanites." This matrix has become so indu- 

 rated that the fractured surfaces run indifferently across it, and the 

 embedded crystals, as though they were one homogeneous substance. 

 But a glance at the microscopic section shows that the rock cannot 

 have resulted from the usual process of igneous sedimentation. 

 The grains constituting it are so different in size that they could 

 not represent particles resulting from ordinary v/eathering re-distri- 

 buted by flowing water. Their shape militates also against that ex- 

 planation, for they are generally quite angular. Some of them are 

 idiomorphic crystals, while others have their outlines rounded, not 

 in the somewhat rough manner of ordinary sand-grains, but with 

 a very smooth outline and re-entering contours which indicate 

 that the crystals have been corroded by a molten magma. Lastly, 

 these grains of varying size are imbedded in a matrix so fine- 

 grained that its detailed structure becomes visible only under the 

 microscope. 



Not only are the structures of the rock different from those 

 resulting from purely aqueous sedimentation, but the minerals are 

 of quite a different nature from those constituting the neighbouring 

 gneiss. The latter are perfectly allotriomorphic and always exhibit 

 unmistakable signs of granulation and dynamo-metamorphism. 

 The two specimens of "trappoids" alluded to consist principally of 

 felspar and quartz, the felspar being largely in excess, which would 

 agree with the analyses given in the Memoirs. So far from resem- 

 Minerals. of volcanic blin S the mi ^rals of the gneiss, all the particles 

 ori 2' n - constituting these rocks are unmistakably 



volcanic in origin. Both the felspar and quartz are corroded in the 

 manner so characteristic of rhyolites. Orthoclase and oligoclase 

 are both present. Some of the orthoclase is locally altered to micro- 

 cline, but this might result from the strains that must have been 

 produced during the folding of the lower Vindhyans. The quartz 

 ( 94 ) 



