128 MIDDLEMISS : THE GEOLOGI OF IDAR STATE, 



the rock becomes quite crowded with many forms of xenoliths 

 that almost remind one of an agglomerate. The Sabli xenoliths, 



.25 25 n9.1Q1-2 PI 15 fi". 3) are particularly rich both in hom- 



4 5 8 4 5 1) v ' ' J 



blende and in sphene. There are long laths of plagiOClase in 12191. 

 The ^round-mass varies considerably in size of grain. Ordinarily 

 the quartz-felsj)ar mosaic, as in , 2 r ;\ (12487) 

 || Ground-mass. from north . ( , lst ()f Jam]a< ( , m , hy suitable 



illumination in the thin section and with a sufficiently high magni- 

 fication, be resolved into quartz and felspar, but the kind of felspar 

 cannot be distinguished. The minute flakes of biotite can also 

 be equally well examined as regards colour, pleochroism. cleavage, 

 etc. Hornblende also in some cases, as in the rock | mile south of 

 Likhi, in blue-green and yellow tints can be seen among the larger 

 nests of biotite. 



A series of more and less fine-grained representatives, as regards 

 the ground-mass, can be easily sorted out, among which at last we 

 reach a stage where it is almost or quite amorphous and crypto- 

 crystalline. An excellent example of the last is afforded by the rock 

 of the little hill west of Malasa, 5 a o° (12493. PI. 15, fig. 1), which is the 

 most compact and blackest as to base that I have collected among 

 these porphyries. A certain fluxional structure in this case may 

 be detected by the trains of iron ore, biotite and hornblende flakes. 

 Another' feature that links these porphyries with the micro- 

 granites and granophyres, is the prevalence 

 D JMicropcgmatito. ^ micropegmat it e in the ground-mass of some 



of the coarser-grained examples. It occurs surrounding the pheno- 

 crysts, of orthoclase, and microcline, as in 4 2 r ;V,, from the centre, of 

 the Jamla hill mass. In this case it is evident that the action of 

 the ground-mass on the felspar phenocrysts at least has not been 

 in the direction of solution of them with production of the usual 

 corroded outlines, but has resulted in continuous crystallisation 

 round the phenocrysts as nuclei, producing a very narrow " court " 

 as in the many cases cited by LaTouche in the Western Rajputana 



rhyolites. 1 



On the whole, and in a general way, there is a considerable 



resemblance between these quartz-porphy- 



eqriinSent of^heffl ries and some of the Malani rhyolites de- 



ani rhyolites. scribed by LaTouche (Joe. cit. pp. 78-88). 



The presence of frequent microcline among the phenocrysts, and 



i Mem., Geol. Surv. India, Vol. XXXV, pt. 1. 



