98 GEOLOGY OF THE SON VALLEY, ETC. 



conclusion, for they resemble ordinary sediments in their forming 

 narrow beds of rapidly alternating composition, remaining con- 

 stant horizontally over wide exposures, the structure being further 

 emphasized by the very regular bands of different colours that 

 frequently give the porcellanites their characteristically striped 

 appearance. 



It seems quite clear therefore that these rocks are volcanic tuffs. 

 There remains but one difficulty to be dealt with, it is the apparent 

 absence of any volcanic centres from which this large bulk of 

 materials could have been derived. 



The only sign of the former site of a volcano that one could 



Position of volcanic ex P ect to disc <> ve r would be in the shape of an 

 centres. intrusion. So far as the Vindhyans themselves 



are concerned, the chances of finding any dykes traversing them are 

 small, for the eruptions took place in the earlier stages of this forma- 

 tion. The rocks belonging to this stage outcrop along a very narrow 

 belt of country, owing to the peculiar structure of the Vindhyans 

 whose southernmost and therefore lowermost beds are usually vertical 

 or nearly so. It would require therefore a combination of specially 

 favourable circumstances, in order that a dyke traversing rocks of 

 that age should be exhibited amongst these beds. If the volcanic 

 centres originally existed north of the line of country occupied at 

 present by the outcrop of the lowermost stages of the Vindhyans, 

 they are of course entirely concealed by subsequent formations* The 

 only chance of finding an igneous dyke of that series is therefore - 

 south of the Vindhyan outcrop, on the supposition that there ever 

 occurred any in that situation. 



About the meridian of the Rer (or Rehand) the belt of land occu- 

 pied by the Bijawar and Archaean, bounded to the north by the Vin- 

 dhyans, and to the south by the Gondwanas, has a considerable width, 

 as much as fifty miles. No acid dykes have been recorded as running 

 through these formations ; yet their examination cannot be said to 

 have been so thorough as to preclude all possibility of any such having 

 been overlooked. In a westerly direction the space between the 



( 98 ) 



