PETROGRAPHICAL NOTES. J I 



&nd in which the southern outcrop of the Agori stage contains no vol- 

 canic rocks, I have had no opportunity to examine the southern por* 

 tion of the Bijawars. But the description given in the Manual, of 

 the Rer section (ist edition, page 35, 2nd edition, page 56) leads one 

 to believe that here again the Agori stage stretches right across the 

 outcrop, and that volcanic rocks have reappeared. 



Owing to the disturbed condition of the Bijawars, it is some* 

 times difficult to decide whether these igneous 



Absence of dykes. 



rocks are interbedded or intruded. If any 

 considerable proportion of them were of an intrusive character, we 

 might expect to find a profusion of dykes in the immediately adjoin- 

 ing archaean gneiss. This not being the case, it is probable that most 

 of the basic rocks found in the Bijawars of the Son region are in 

 reality interbedded flows, but the examination of neither gneiss nor 

 Bijawar has been complete enough to make.any decisive affirmation, 

 In a great many cases, however, there is no doubt as to the crystalline 

 lavas being interbedded. 



The underlying archaean series does contain, it is true, a profusion 

 of basic dykes, but these are of archaean age and differ in their 

 characteristics from those of the Bijawars. Hypersthene and garnet 

 which occur in the archaean are not to be found in the Bijawar rocks ; 

 while the reverse is the case with pyrites. Leucoxene and chlorite 

 are extremely abundant in the Bijawars, but seldom met with in the 

 archaean. It is important to draw this distinction, because as there 

 has been sometimes some confusion in determining the exact boundary 

 of the Bijawars, rocks really belonging to the gneiss have sometimes 

 been reckoned as part of the Bijawars. Hence by studying the sped* 

 mens preserved in the Museum, one might be led to believe that 

 some of the basic rocks in the two formations are identical. 



Basic volcanic rocks are never absent from the northern portion 

 of tne Bijawar outcrop throughout the whole length of the Son out- 

 crop far beyond the limits of my own observations : the collection 

 contains specimens from localities as far west as Hardi, beyond 

 which point the Bijawars are exposed only for a short distance. My 



( 7i ) 



