14 GEOLOGY OF THE SON VALLEY, ETC, 



The thickness of the stage as a whole varies no less than that 

 of the basement sandstones and conglomerates and as a rule in the 

 same direction. That is, on those sections where the basement 

 beds are thickest, the overlying shales and limestone are also 

 at their thickest ; while on those sections where the coarse-grained 

 basement beds are reduced to a few feet the overlying shales 

 are also thin, and the porcellanites are brought close down to the 

 boundary. 



For the porcellanite stage I have retained the name originally 



applied to these beds, though their true nature 



has now been fully established by Mr. Vreden- 



burg's careful study of their petrology which forms Chapter V of this 



memoir. 



The typical rock of the stage is a hard fine-grained porcellanic 

 rock, varying in colour from almost black to a pale green, compact, 

 and breaking with a conchoidal fracture. It forms extremely sharp- 

 edged splinters, so keen that I have seldom obtained a specimen with- 

 out receiving one or more cuts from the flying chips. This feature, 

 combined with its evenness of grain, made it a favourite material 

 with the men of the stone age in this region for the manufacture 

 of stone implements, both large sized axes and choppers and small 

 flakes, of which many singularly perfect specimens were found. 



Associated with these fine-grained rocks are others to which small 

 fragments of quartz and felspar give a porphyritic aspect, and in 

 places these fragments become so numerous that the rock assumes 

 the form described by Mr. Mallet as trappoid. In the western 

 area where these rocks are most developed, hornblende is a much 

 less common ingredient than in the similar rock east of the Mirzapur 

 district to which Mr. Mallet's description 1 more especially applies. 



The thickness of this stage near the western boundary of the 



Rewah State cannot be less than 800 feet, but the boundaries are 



very uncertain. Rocks of very similar type are found interbedded 



among the lower Vindhyans both above and below that portion which 



» Mem., Geol. Surv. Ind., Vol. VII., p. 36. 



( 14 ) 



