84 LA TOUCHE : GEOLOGY OF WESTERN RAJPUTANA, 



inclined to think that it is an original feature, otherwise it would be 

 difficult to account for cases where the secondary quartz does not 

 surround the whole phenocryst. If this growth had taken place after 

 the consolidation of the rock, there seems to be no reason why it 

 should not have spread through the whole of the groundmass, as it has 

 done in some instances (PI. IX, figs. 4, 5). In other cases also, for 

 instance, No. U'537 (PI. IX, fig. 2), there is no sign cf such a growth 

 round the quartz phenocrysts and yet there is no reason to suppose 

 that these flows have been subjected since their consolidation to 

 different conditions. There is no evidence of their having been greatly 

 disturbed or folded to such an extent that some of them night have 

 been acted upon by great heat and pressure, to which others were not 

 subjected. I have observed that some of the flows which show no 

 trace of the formation of a " mosaic", bear evidence in the shape of 

 included fragments of rhyolite probably showered down upon them 

 from above, that at the time of their eruption they were at once ex- 

 posed to the atmosphere, and therefore presumably cooled more rapidly 

 than the flows which do not contain such fragments, but which do 

 possess the mosaic structure. Thus the formation of this structure 

 seems to have depended to some extent at any rate on the conditions 

 under which the rock solidified. For this reason, although I have 

 called the quartz of the " courts " surrounding the phenocrysts 

 " secondary," I do not consider that it is " secondary ' in the sense of 

 having been produced by devitrification subsequently to the consolida- 

 tion of the rock, but merely that it was of secondary growth in com- 

 parison with the original quartz of the phenocrysts. 



It is difficult to say why the groundmass of some of the lavas 

 should present the appearance just described, while in others instead 

 of the " quartz mosaic " we have a microcrystalline aggregate composed 

 of minute but perfectly distinct crystals of quartz and felspar. Pro- 

 bably this variation in the character of the groundmass is due to some 

 difference in the conditions and rate of cooling, or perhaps, in the 

 ( 84 ) 



