66 GEOLOGY OF THE SON VALLEY, ETC. 



cleave. The lustre in a hand-specimen is dull and stony. The rock 

 never shows the glossy fracture and semi-transparency so frequent 

 in the Vindhyan varieties. Yet none of these differences are very 

 conspicuous, the resemblance, especially in the field, being sufficiently 

 pronounced to be very confusing. An instance of a very misleading 

 exposure has been mentioned in the stratigraphical portion of this 

 Memoir, with respect to the beds seen at the bend of the Son, opposite 

 Gurdah. 



Should any doubt exist as to the correctness of attributing 

 these rocks to the Bijawars, it could not be better dispelled than 

 by examining specimen yW Here we have a stratum which is not 

 more than one centimetre in thickness, interbedded with purple shales 

 of the most typical Bijawar type. It was collected amongst the hills 

 immediately north of Jungel amongst the purple slates described 

 above. The narrow siliceous band is of a dark grey colour differing 

 in that respect from the bluish rocks of Gurdah. But in all other 

 physical characters it is quite similar, and the similarity subsists 

 under the microscope. 



c— -Jasper s^ 



In some of these porcellanoid slates, much of the groundmass 

 has the appearance of devitrified colloid silica. If we imagine now a 

 siliceous rock without any of the quartz-grains or mica flakes of the 

 porcellanic slates, but offering nothing but a uniform extremely 

 fine mosaic of quartz or chalcedony, we shall obtain the jaspers that 

 form such a characteristic feature of the Agori stage and which form 

 conspicuous beds in many of the "Transition" systems of India, 

 such as the Bijawars of Bundelkhand, the Gwalior, or the Dharwar 

 series. They have frequently been noticed as pebbles in Vindhyan 

 conglomerates that have been derived from those older systems, the 

 bright-red colour which they often assume making them specially con- 

 spicuous. 



The rock is usually of a translucent bluish-grey shade which is 

 lighter or darker according to different speci- 



Hematitic rocks. , • , i 



mens; the structure appears entirely homogene- 

 ( 66 ) 



