STRATIGRAPHY. 1/ 



generally rises into hills. Mr. Vredenburg has suggested that 

 the absence of outcrops may be due not to a disappearance of the 

 porcellanites, but to their outcrop being in the ground covered by 

 the bed of the Son and therefore hidden. 



It is true that the outcrop of the rocks of the horizon occupied 

 elsewhere bv the porcellanites runs under the bed of the Son at 

 Hurma and again from about Silpi to Gurdah, but between these 

 points there is a stretch of about 25 miles where it must run over the 

 ground south of the S n, yet throughout this extent no exposures can 

 be seen. When we r ollect that sewhere the rocks of this stage 

 almost invariably rise to r^iges 01 hills it is difficult to account for 

 the complete absence of an outcrop of the stage as a whole without 

 supposing that it has either thinned out, or that the characteristic 

 hard beds have become so few and small in comparison with the 

 softer in^erbedded shales that the stage as a whole has no longer the 

 power of resistance which it elsewhere possesses. In either case 

 there is a marked diminution of the amount of foreign material intro- 

 duced among the purely aqueous deposits which may be considered 

 as a thinning out of the volcanic ashes of the porcellanite stage. 



From the above it will be seen that the rocks of this stage have 

 their maximum development near the western 

 en reS eru°ption° ° an,C anc * beyond the eastern limit of the area included 

 in the map. Further the regions of maximum 

 thickness are also those of the greatest development of the coarse- 

 grained beds. These two facts taken together point to an approach, 

 in either direction, to the centres of volcanic activity from which 

 these ashes were ejected. 



It is not possible to say whether these centres lay to the south 

 of the present lower Vindhyan boundary or to the northwards under 

 the country now covered by the Vindhyans. The fact that the 

 exposures of the porcellanite stage in the outliers seem to be less 

 in thickness and on the average finer in grain than in the corre- 

 sponding parts of the main area to the north is, to a certain extent, 

 an indication that the volcanic centres lay in that direction rather 

 to the south. 



c ( 17 ) 



