88 LA TOUCHE : GEOLOGY OF WESTERN RAJPUTANA. 



also very similar to Nos. 1 1508 and 1 1*532, the first of which is from 

 the same group of hills, and the other from the neighbourhood of 

 Thob, at the south-western end of the chain of hills in which the Aeolai 

 group is situated. 



The felsites of Tusham hill described by Col. McMahon, 1 

 and compared by him with the rhyolites of Barmer and Pokaran 

 collected by Mr. Blanford, 2 differ from the latter in containing mica 

 among the constituents of the groundmass, and although the structure 

 is very similar, the presence of this mineral makes it more than doubt- 

 ful whether the Tusham rocks are identical with the Malanis. 



Through the courtesy of Dr. Callaway, who has devoted much 

 time to a study of the pre- Cambrian rocks of Great Britain, I have been 

 enabled to compare these rhyolites with a few slides cut from the 

 ancient devitrified pitchstones and perlites of the Wrekin in Shrop- 

 shire, which were microscopically described by Mr. S. Allport. 8 The 

 similarity in the structure of the groundmass in the two sets of rocks 

 is very remarkable, and indeed it would be difficult, if not impossible, 

 if the specimens from the two localities were mixed together, to pick 

 out those from the Wrekin from among the Rajputana rocks. The 

 identity is especially striking in a specimen from Lyd's Hole, which 

 shows the peculiar structure of the groundmass that I have designated 

 hy the name of " quartz mosaic. " The sphaerulitic and other structures 

 too in o'iher slides are exactly similar, but perhaps they are more clearly 

 defined in the Rajputana specimens, since these have not been subjected 

 to so great an amount of disturbance and crushing as the Wrekin rocks. 

 It would of course be rash to conclude that this similarity implies any 

 strict contemporaneity between the volcanic outbursts in two localities 

 so widely separated ; but when we consider that the Malani rhyolites 

 were poured out in pre-Vindhyan times, it is probable that there is no 

 very great disparity in age between them. 



> Rec. Geol. Surv. Ind., Vol. XVII, Pt. 3, p. 10S. 

 3 Ibid, Vol. XIX, Pt. 3, p. 161. 

 3 Quart. Journ. Geol. Soo, Vol. XXXIII, p. 449. 

 ( 8S ) 



