320 McMAHON : PETR0L0G1CAL NOTES ON SOME PERIDOTTTES, ETC., 



titaniferous iron. It also contains ferric oxide. The hand-speci- 

 men has a slicken-sided appearance and under the microscope the 

 rock is seen to have been subjected to 'pressure, shearing, and here 

 and there to contortion. 



Gab br os. 



No, 94 — 210. Gabbro ; collected by the late Dr. Ferdinand Stoliczka, in the 

 Puga valley,. Ladakh ; Sp. G. 5*076. 



When examined under the microscope this rock is seen to be 

 composed of augite^ diallage and saussurized plagioclase. 



Nearly the whole of the augite has been converted into diallage 

 and only portions of the crystals have escaped change. Both 

 minerals polarize brilliantly in colours of Newton's second order* 

 They contain tabular allo-triomorphic patches of brown hornblende 

 rather suggestive of biotite. The pleochroism of the hornblende is 

 somewhat feeble ; and between crossed Nicolsit changes from a dark 

 blackish-grey to a pale orange-yellow of the first order. It shows no 

 cleavage. With convergent polarized light it gives a biaxial bar. 

 It looks like a secondary product of decomposition. 



The felspar — an altered plagioclase — has been more or less 

 metamorphosed into zoisite. In hardness it is about 6*5 of Mohs* 

 scale. A chip of it sank in cadmium boro-tungstate, its specific 

 gravity is therefore greater than 3*28. This agrees well with 

 zoisite, which has a hardness of 6 to 6*5 and a specific gravity of 

 from 3*226 to 3*381. 



Between crossed Nicols the zoisitic felspar ranged from a feeble 

 yellowish white of the first order to isotropic. 



The structure of the rock is holo-crystalline. 



No. 94 — 211. Gabbro ,- collected by the late Dr. Ferdinand Stoliczka, in the 

 Puga valley, Ladakh ; Sp. G. 2*959. 



This rock is a holo-crystalline mixture of diallage and labradorite 

 felspar. 



( '8 ) 



