lo8 GEOLOGY OF THE SON VALLEY, ETC. 



stage. It may be concluded from this distribution that an exten- 

 sive series of acid eruptions began simultaneously with, or just 

 before the commencement of, the Vindhyan era. At the time of 

 maximum activity, showers of fine volcanic dust together with 

 coarser debris were ejected in enormous quantity giving rise to the 

 tuffs forming the " porcellanic stage *\ The beds occasionally found 

 at higher horizons may represent stray eruptions belonging to a 

 period of declining activity. The eruptions were confined to the 

 region occupied by the Son outcrop and adjoining districts, for it 

 appears that the " trappoid beds " do not reappear in Bundelkhand 

 (Mem., Vol. VII, page 45). By far the greatest majority of the speci- 

 mens indicate subaqueous conditions. It is not inconceivable that 

 acid lava-flows emerging from subaqueous volcanic vents would, on 

 coming in contact with the oceanic water, become superficially so 

 rapidly cooled as to reduce them to a fragmentary condition by 

 irregular, violent and sudden contraction of the molten glass. In 

 this way, the true tuffs — that is those ejected in a fragmentary state 

 by explosive action — would be augmented considerably by such 

 disintegration of the materials which actually appeared at the vol- 

 canic outlet in the form of ordinary acid lava. This suggestion 

 possibly accounts for the fact that the fragmentary materials in these 

 beds so greatly exceed the true lavas in quantity, and in fact are the 

 predominating variety of the volcanic products. The rearrangement 

 of silica in the rocks which has given them their porcellanic character, 

 is a feature which might well be expected to be accentuated in vol- 

 canic rocks of subaqueous origin. Such conditions of formation are 

 naturally favourable to the rapid chemical action of water on the 

 unstable volcanic glass. 



t 108 ) 



