LOWER VINDHYAN SERIES. 97 



of the * trappoids. " The debris are all exceedingly angular and ir- 

 regular, and owing to their fragmentary condition, the grains cannot 

 always be identified as quartz or felspar ; but as a considerable pro- 

 portion can be recognised for certain as felspar, it is most probable 

 that here again, as in the case of the trappoids, felspar is in greater 

 abundance than quartz. 



In these porcellanites, just as in the trappoids, the felspar is 

 ~. . t , usually perfectly clear. On the supposition that 



Characters of felspar. . rr 



the rocks were an aqueo-sedimentary one, the 

 felspar must have travelled over great distances to be comminuted 

 into such small fragments ; it would be very surprising then that it 

 should remain unaltered, showing no trace of the atmospheric alter- 

 ation to which it is so liable. Fine-grained sediments do not, as a rule, 

 contain felspar : they may consist largely of such minerals as quartz 

 or mica, or iron ores, but the felspar is represented entirely by 

 its atmospheric decomposition products, principally kaolin. Unal- 

 tered felspar is usually found only in sediments of the coarseness of 

 sandstone ; and even then, except in the case of an arkose produced 

 in immediate proximity to a granite, it is quite subordinate to quartz 

 in quantity. 



Fn addition to felspar and quartz, the porcellanites consist to a 



large extent of fragments of more or less devitrified glass, and grains 



of micro-crystalline rhyolite which often contain small idiomorphic 



Apatite, hornblende, apatites. Grains of calcite and small crystals 



etc - of hornblende are also met with. Fragments 



of pumice are abundant, often of comparatively large size. 



Observations on a larger series of specimens than I have been 

 able to examine would no doubt bring out many interesting points. 

 Still the evidence is sufficient to show that these rocks are certainly 

 not derived from the disintegration of gneiss, which had been 

 hitherto the most generally accepted theory. 



At the same time, microscopic examination shows that the rocks 

 are not purely igneous : that is, they are not of the nature of lavas* 

 Their mode of occurrence in the field also leads to the same 

 H ( 97 ) 



