go LA TOL'CHE : GEOLOGY OF WESTERN RAJPUTANA. 



rectangular and triangular or wedge shaped chips of glass, in which 

 bundles of delicate fibres have been developed, projecting inwards 

 from the edges and at right angles to them, not meeting in the centre 

 but leaving a clear space along which there is usually a line of black 

 granules (PI. XI, figs. 4, 5). Some of the chips contain minute globu- 

 lites round which the fibres curve as shown in fig. 5. The 

 structure is exactly similar to that developed in a piece of artificial 

 glass exposed after its first consolidation to great heat, an example 

 of which is given in Professor Bonney's anniversary address to the 

 Geological Society of London in 1885. 1 The matrix of this breccia, 

 No. 1 1524, is a fine grained microcrystalline rhyolite, and there is no 

 doubt that it was molten when the chips of already solidified glass from 

 another flow were showered into it, and that they were then partially 

 devitrified by the heat of the surrounding molten mass. 



The volcanic ash, No. 8-480, described by Mr. Middlemiss in the 

 paper above referred to, from one of the boulders in the Salt Range 

 boulder bed, is very similar to one of the tuff beds at Korra, No. 

 11-501. 



3. Granite. 



The granites associated with the Malani rhyolites do not present 

 many features of particular interest under the microscope. The Siwai'.a 

 granite contains an abundance of actinolitic hornblende, usually of a 

 bright green colour but sometimes with a bluish tinge; thepleochroism 

 is usually very strong. It is commonly interstitial, but occasionally 

 occurs in idiomorophic crystals, especially in the veins and dykes pro- 

 truded from the main mass into the surrounding rhyolites. The felspar 

 and quartz frequently form a micropegmatitic intergrowth or granophyric 

 structure of great beauty, and the rock is then indistinguishable from 

 the specimen No. 8-472 described by Mr. Middlemiss in the paper 



1 Quart. Journ. Geol. Soc, Vol. XLI, p. 92. 

 ( 90 ) 



