16 GEOLOGY OF THE SON VALLEY, ETC. 



The thinning out of the porcellanites from a maximum develop- 

 ment at either extremity of the map to a disappearance in the 

 stretch between Hurma and Gurdah is for two reasons less con- 

 spicuous in the map than it otherwise might be. In the first 

 place, the apparent thinning out of the rocks of this stage may 

 be due not to an actual thinning of the beds of the same age 

 as the porcellanites at either extremity of the area included in 

 the map, but to a disappearance of the hard beds and a correspond- 

 ing increase of the intercalated shales which cannot be separated 

 from those of the under and overlying stages. Consequently when 

 the hard rocks cease to be present in sufficient proportion to give 

 a special character to the stage and enable it to stand out by its 

 superior resistance to denudation, there is an appearance of compara- 

 tively sudden extinction of the stage. In the second place, the 

 wide expanse mapped as porcellanites west of Markundi is due 

 to an expansion of the outcrop owing to its being exposed along the 

 crest of a very open anticlinal, while the other wide expanse west 

 of the Gopat is due to the fact that the portion of this area lying 

 north of Lat. 24 30' was surveyed by Mr. Datta, who has included 

 and placed the upper limit of this stage at a higher horizon than that 

 adopted by Mr. Grimes and myself in the country south of this parallel. 

 Where the boundaries between the stages are so indefinite as in 

 the lower Vindhyans, and where the division between one stage 

 and another depends on whether a certain form of rock may be 

 regarded as the characteristic or merely as adventitious, such differ- 

 ences between different observers is inevitable, and will also be 

 found in the mapping of the same observer in different areas. 



The complete disappearance of the stage between Hurma and 

 Gurdah is largely a matter of cartographical convenience. The expo- 

 sures here are too few to allow of its presence or absence being defi- 

 nitely established, but it is difficult to believe that, if the porcellanites 

 were present in anything like the development they attain to the east 

 and west of these limits, there could be a complete absence over 

 so large an area of any outcrop of a stage which, through its hardness, 



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