PETROGRAPHICAL NOTES. 65 



There are some highly calcareous slates which are common 

 amongst the volcanic strata to be mentioned 



Calcareous Slates. , . 



hereafter. 

 The Agori stage of the Bijawars often contains bands of 

 siliceous rock in which the particles of quartz are 



Resemblance to Porcel- ■ , . , „ . 



Janite beds of Lower as fine as or even finer than in thennest-grained 

 rocks hitherto mentioned, but owing to the 

 absence or smaller proportion of associated mica, the rocks do not 

 exhibit any distinct cleavage. They bear a remarkable outward re- 

 semblance to the Vindhyan porcellanites. It will be mentioned when 

 discussing this latter formation that rocks of different origin could in 

 the course of recrystallization assume a very similar appearance. 

 Thus a fine silt consisting mainly of finely divided quartz, a siliceous 

 ooze originally consisting of particles of colloid silica, a band of chert, 

 or an acid volcanic ash might finally become very similar in outward 

 appearance if devitrification and recrystallization have been acting 

 long enough. In the case of these Bijawar porcellanites their 

 complete gradation into the fine slates indicates that they are 

 merely a variety of these rocks resulting from a slightly different 

 mineralogical composition. The minute quartz particles being 

 more abundant, while the micaceous minerals are fewer or want- 

 ing, so that the rock does not possess any distinct schistosity. The 

 conclusion drawn from the mode of occurrence of the rocks is confirmed 

 by their microscopical examination. The rock is much finer-grained 

 than any of the Vindhyan porcellanite slides which I have examined 

 (with the exception of slide 1890, specimen y 1 ^, collected in the 

 Samdin river) ; it always contains a fair quantity of minute mica- 

 ceous flakes which have every appearance of being of derived 

 character; it never shows any of the tuffaceous characters exhibited 

 by the Vindhyan specimens. 



The peculiar rhombohedral jointing of the Vindhyan porcellanites 

 is very imperfectly developed or entirely wanting in these Bijawar 

 varieties. In place of it we find the rock irregularly cracked and 

 faulted in such a manner as to render it almost impossible to obtain 

 specimens. In addition to this, the rock may have a tendency to 



F ( 65 ) 



