Colonel McMahon — Granite of the Himalayas, 217 



vol. xvii. affords a striking illustration of a microlith thus ruptured. 

 Cracks of the character aHuded to do not in any case extend from 

 the microlith into the matrix. 



Thirdly, opacite embracing and riding on the backs of previously 

 formed microliths. As illustrations I would point to the right-hand 

 figure of fig. 6, plate ii. p. 144, vol. xvi., compared with fig. 13 of 

 plate at p. 158, vol. xvi. Records G.S.I. The former is a representa- 

 tion of a microlith in a Himalayan gneissose-granite, and the latter 

 of one in an Aden lava. The last-mentioned illustration could be 

 exactly matched by numerous illustrations from the gneissose-granites. 



Fourthly, stone cavities,^ in which mineral matter has been depo- 

 sited on cooling. Compare fig. 18, plate, p. 72, vol. xvii. (a 

 Dalhousie specimen) with figs. 4 to 10, inclusive, plate, p. 158, vol. 

 xvi. Records G.S.I., Aden lava specimens. The presence of clusters 

 of minute microliths sticking to and fringing the outside of the 

 Dalhousie stone cavity shows conclusively that this is not a case of 

 schillerization. 



Fifthly, stone cavities in which either crystals have been deposited 

 on cooling, or in which mineral matter has caught up and enclosed 

 previously formed crystals. Illustrations, figs. 1, 6, and, possibly, 

 the npper portion of fig. 18, plate, p. 72, vol. xvii. Records G.S.I. 



These instances, which are selected by way of sample of the kind 

 of evidence applicable, indicate I think that the gneissose-granite at 

 one period of its history was in a condition of what may be termed 

 — roughly speaking — igneous fusion. That water was, however, to 

 a considerable extent mixed up with the constituents of the rock 

 when it was in a plastic state, and that its condition may be more 

 correctly described as one of igneo-aqueous fusion, is made plain by 

 the examination of thin slices under the microscope. Liquid cavities 

 abound in the quartz of the granite ; whilst, it is important to observe, 

 they are entirely absent from the slates in contact with it. The heat 

 produced by contact with the granite appears to have been sufficient 

 to drive the water out of the quartz of the slates and of the siliceous 

 rocks brought within its influence. An interesting illustration of 

 this was afforded by an examination of the crystals of schorl found 

 in the slates in contact with the granite. When these crystals were 

 first formed, they appear to have contained inclusions of air, gas, or 

 liquid ; but as the heat increased, and the included matter expanded 

 under its influence, the included gas, or liquid, was forced out to the 

 surface of these crystals and driven off. An interesting illustration 

 of this will be found at fig. 7, pi. ii. p. 144, vol. xvi. Records G.S.I. 



Cause of tlie foliation of the Gneissose- Granite. 



A realization of the eruptive character of the rock described in 

 the above pages removes many difficulties from the way of the 

 Himalayan geologist. " Despite the wonders performed by flexure 

 of strata in mountain regions," wrote Mr. Medlicott, the Director of 

 the Geological Survey of India, in his Annual Report for 1883, " the 



^ This is an awkward term ; but, as it was introduced by Sorby, I retain it : micro - 

 scopists know what it means. 



