310 MCMAHON : PETR0L0G1CAL NOTES ON SOME PER1D0T1TES, ETC., 



I regard this rock as a hypogene representative of a volcanic rock 

 rather than a volcanic rock itself. The hornblende-rock, said to be 

 an altered pyroxenite, is also a hypogene rock. It is doubtful 

 whether any of the rock specimens described in this paper include 

 a true lava. 



The samples of ash sent me are from the Wangat and Puga 

 valleys. As these rocks have been much altered by the combined 

 effects of pressure, aqueous infiltration, and the contact action 

 of intrusive igneous masses, it is in some cases difficult to say 

 decidedly whether one is dealing with an altered ash, or a metamor- 

 phosed lava : and, in other cases, whether the rock is an ash or a 

 very fine-grained fragmentary rock containing some splintery pieces 

 of acid lavas, such as felsites and rhyolites. 



One of the serpentines from Shigar (4 — 210) collected by 

 Mr. Lydekker is of especial interest as it proves to be the rare 

 variety called bowenite or pseudo-jade. The description of this 

 rock is given at page 11. 



Peridotites. 



No. 8—278. Peridotite (Lherzolite) from the Puga valley, Ladakh ; col- 

 lected by R. D. Oldham, F.G.S. ; Sp. G. 2865. 



This is a much altered rock composed of olivine, monoclinic 

 pyroxene, a little enstatite, bastite, diallage, grains of magnetite, and 

 secondary decomposition products. 



The olivine is traversed by very numerous canals of serpentine 

 and is cut up into rounded grains. 



The pyroxene has also suffered much from decomposition. It is 

 dull and polarizes very feebly. Like the olivine it is penetrated by 

 infiltration canals, but they are not so numerous as those in the 

 former mineral. They are filled with a colourless serpentinous 

 mineral inert in polarized light. The diallage and bastite possess 

 hardly any double refraction. 



The olivine grains are sometimes fringed with radiating tufts of 

 fine needles, with oblique extinction, which are evidently some form 

 l 8 ) 



