50 LA TOUCHE : GEOLOGY OF WESTERN RAJP'UTAtfA, 



angles to the south or south-west. These rocks are very fresh looking and 

 seem to be scarcely, if at all altered, the crystals of orthoclase which 

 they contain having remarkably well defined outlines. 



Further to the west in the neighbourhood of Thob, the prevailing 

 rock is of the common strongly porphyritic type of rhyolite, with dark 

 grey and red colours. Seen from the north-west side in profile the 

 large hill (837 feet) appears to be built up of successive flows dipping 

 to the south-west at various angles up to 45 , each flow being marked 

 by columnar jointing. Some breccias and fluidal rhyolites are exposed 

 in the low ground about three quarters of a mile to the north of the 

 village, apparently dipping beneath the porphyritic rocks. 



In the group of hills between Thob and Patodi, still further west, a 

 regularly stratified series of beds is exposed in a scarp facing north-east, 

 the beds dipping to the south-west. Red, slightly porphyritic rhyolite 

 is exposed at the base ; above this comes a band of an almost black 

 rock, crowded with quartz grains and weathering with a shaly. struc- 

 ture, probably an ash bed. Then there is another band of the red 

 porphyritic rhyolite, and above this a light greenish tuff, with a sandy 

 texture- and somewhat shaly. With this are associated some more 

 coarsely fragmental beds studded on the surface with small nodules 

 resembling lapilli Above this there is a band of a dark grey amygda- 

 loid rock which looks like a contemporaneous flow of basic rock, and 

 is greatly decomposed. Finally, forming the crest of the ridge, is a 

 thick flow of compact blue rhyolite, with well marked flow-structure, 

 and containing fresh looking crystals of felspar. The reck is tra- 

 versed by a system of fine black veins, probably coloured with iron 

 oxide, and roughly following the lines of flow. This series of beds is 

 about 150 feet in thickness and can be traced for a considerable 

 distance along the scaTp. The main mass of the hills to the south- 

 west of the scarp is composed cf a thick flow of dark grey and red 

 porphyritic rhyolite, like that forming the hills near Thob, with a 

 roughly columnar "structure. This flow forms a dip slope gradually 

 descending to the level of the plain on the south-west, 

 ( 50 ) 



