2$ GEOLOGY OF THE SON VALLEY, ETC. 



In the Son valley the analogy with the Himalayas is, however, 



Absence of great imperfect. There are no great faults with a 

 boundary faults. downthrow to the north to represent the great 



boundary fault of the Himalayas and the minor boundary faults which 

 divide the tertiary zone along their southern foot. On the contrary 

 the principal faults of the Son area, though none are comparable in 

 size to the boundary fault of the Vindhyans with the rocks of the 

 Aravalli mountains, have their upthrow to the north, and the dips 

 of the lower Vindhyans in their outliers is also northwards against 

 these faults. The conditions are therefore at first sight the reverse 

 of what prevails along the southern margin of the Himalayas, and in 

 a previous paper I pointed 1 this out as a possible difficulty in the way 

 of the explanation of the relation of the upper and lower Vindhyans 

 in the outliers south of the Son adopted in the preceding paragraph. 



A fuller examination of the Son valley area and a wider con- 

 . f It t the sideration of the general conditions of the prob- 

 west - lem have convinced me that there is no weight 



in this objection. On the one hand, just such a boundary fault as is 

 required by the analogy is found just west of the area included in 

 the map, whence it can be traced for some distance into the Narbada 

 valley. To the eastwards, in the area covered by the map attached to 

 this memoir, the run of this fault would carry it into the area covered 

 by the rocks of the Gondwana system. In this case we might suppose 

 that the main boundary fault exists, but is hidden to view, and that 

 the area of transition and lower Vindhyan rocks exposed between the 

 Kaimur scarp and the Gondwana boundary is part of the upraised 

 floor of the old area of deposition north, i.e., outside, of the main 

 boundary fault. 



Against this supposition there might be put the degree of disturb- 

 , , „ ance of the lower Vindhyans, which is greater 



Dying out of the bound- ' ° 



ary fault. than we have any reason to suppose has taken 



place among the deposits of tertiary or later age south of the outer- 

 most boundary fault of the Himalayas. Besides this the occurrence 



I Rec., Geol. Surv. Ind., Vol. XXVIII., p. 144. 

 ( 28 ) 



