3:6 MCMAHON : PETROLOGICAL NOTES ON SOME PERIDOTITES, ETC., 



last slice. The felspars have in part been converted into chlorite 

 and otherwise much altered. 



The rock is probably a highly altered ash. 



/ 0. 8—275. Volcanic Ash, from Puga valley, Ladakh ; collected by 

 R. D. Oldham, F.G.S. ; Sp. G. 2873. 



The hand-specimen of this rock is greenish grey in colour and 

 is dotted with black augite crystals. Viewed macroscopically it 

 appears to be an undoubted ash. 



The microscope confirms this verdict : but at first sight thirt 

 slices of the rock when examined under the microscope are in some 

 respects suggestive of a lava. 



Numerous crystals of augite, pale brown in transmitted light, 

 which contain inclusions of the groundmass, are penetrated round 

 their edges by tongues, and closely resemble corroded phenocrysts> 

 This impression is rendered stronger by the fact that the material 

 which forms the inclusions and the tongues is identical with the 

 groundmass itself; and all of it is uniformly dotted with minute 

 spots, or patches, of white and opaque mineral matter resembling 

 leucoxene. The tongues, moreover, are continuous with the ground- 

 mass. There is no physical break between them suggestive of 

 clastic structure. 



This pseudo-corrosion appears to have been produced in the 

 following way. The original rock, I take it, was composed of augite 

 and felspar phenocrysts imbedded in a glassy or felspathic base, and 

 the phenccrysts of augite contained inclusions of this base. Then 

 came the volcanic explosion that formed the ash. The large augites 

 were broken into fragments and were in their passage through the 

 air abraded at their edges by collision with each other. The material 

 of the groundmass, on the other hand, broke up more easily and 

 formed a very fine-grained dust which by subsequent pressure was 

 forced into tongue-like abrasions in the augite crystals. Lastly, 

 there followed aqueous infiltration that caused a segregation of the 

 ( *4 ) 



