PETROGRAPHICAL NOTES. gi 



Unaltered examples of the lavas have been found in Bundelkhand, 

 and represent doleritic and basaltic types of varying degrees of 

 coarseness, consisting principally of labradorite, augite and ilmenite 

 or magnetite, with varying quantities of olivine. They are remark- 

 able, moreover, for the frequent presence of micrographic quartz. 



Owing to the great compression which the Bijawars have suffered 

 in the Son region, the rocks there are much metamorphosed. The 

 augite is almost always altered to uralite, and it seems also to have 

 combined with some of the substance of the felspars giving rise to a 

 less ferriferous hornblende. These changes are accompanied by the 

 formation of chlorite, calcite, and epidote. The felspar is usually 

 saussuritised, its chemical alteration being generally inversely pro- 

 portional to the amount of fracturing which it has suffered. 

 Fresh olivine is never met with, and one rock shows an interesting 

 transformation of that mineral into regularly oriented chlorite. 

 Ilmenite is universally changed into leucoxene. Pyrites is usually 

 largely developed. 



Far more abundant than the lavas are the tuffs in which trans- 

 formations of an exactly similar nature have taken place. The great 

 preponderance of chlorite gives them generally a schistose structure. 

 They contain calcite in large proportion and are interbedded with 

 great limestone masses whose true nature is perhaps that of 

 travertine. 



It has been sometimes stated that basic volcanic rocks are rare 

 or absent amongst the older sediments. This impression may 

 have resulted from the fact that owing to their great liability to 

 considerable metamorphism, the older basic lavas have not been 

 recognised as such and have remained unnoticed. The data at 

 present available are probably insufficient to estimate whether the 

 proportion between acid and basic volcanic rocks throughout the 

 world has varied in successive geological periods, but the existence 

 of considerable quantities of basic lavas in many pre-cambrian 

 systems is well established. 



In the present case, we find no well-defined law in the succession 



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