TAWNG-PENG SYSTEM : BAWDWIN VOLCANIC STAGE. 57 



with that of the phenocryst, and extinguishes simultaneously with 

 it under crossed nicols. 1 On the other hand, precisely the same 

 structure has been shown by Dr. Fermor 1 to occur in the rhyolites 

 of Pavagad Hill, in the Panch Mahals district, Bombay Presi- 

 dency, which are interbedded with Deccan Traps, and are therefore 

 of Cretaceous age. The presence of this structure cannot therefore 

 be taken as evidence of similarity in age between the Bawdwin 

 and Malani rhyolites ; but in other respects, such as the prepon- 

 derance of quartz phenocrysts over those of felspar, the absence of 

 plagioclase felspar and of augite, they both resemble each other, and 

 differ from the lavas of Pavagad Hill, as described by Dr. Fermor. 



There can be no doubt that the Bawdwin rhyolites are older 

 than Ordovician, and they must therefore belong to a period not 

 very far removed from that of the outpouring of the Malani lavas. 



The felspars of the Bawdwin rhyolites are usually quite de- 

 composed, both in the phenocrysts and in the groundmass, and are 

 represented by a fibrous, felted mass of (?) sericite, which also 

 fills cracks in the groundmass. Where the felspar is not decom- 

 posed it is invariably orthoclase ; sometimes the outlines of the 

 crystals are still preserved, and in some cases traces of simple twin- 

 ning are still visible. Iron ores, exhibiting the white mesh-work 

 characteristic of leucoxene, are usually present, but I have found 

 no trace of any of the ferromagnesian minerals, or of mica. 



Mr. Coggin Brown has shown that the mineralisation of the rock 

 M . ii t - n consists in the metasomatic replacement of 

 the felspar, both of that in the groundmass 

 and the phenocrysts, by galena and other sulphides. In the final 

 stages of the process the quartz grains were also attacked, and 

 " specimens can be found showing only a very small proportion of the original 

 minerals replaced by sulphides, through others composed of sulphide ores and a 

 large amount of quartz, and finally those which consist almost entirely of 

 sulphides, with very little quartz." 2 



At Bawdwin itself sphaerulitic lavas have not been met with, 

 but in some specimens perlitic cracks are seen 



Structures in rhynhto. . 1 



traversing the quartz phenocrysts and occa- 

 sionally extending beyond their borders into the groundmass. 

 SphcTrulites occur, however, at some other localities, at Kunhawt, 



1 On the Lavas of Pavagad Hill ; Records, Geol. Surv. Ind., Vol. XXXIV, Pt. 3, pp. 154, 

 160. 



2 The Silver-lead mines of Bawdwin ; Ibid, Vol. XXXVII, p. 251. 



