95 



position, their specific gravity was found to be 3*18. 

 Tscliermak's and Giimbers picrites are 2-93 to 2*96*. 



The Inchcolm picrite = 281. The sp. gr. of the non.- 

 felspathic picrites of Eum is stated by Juddf as about 3*20. 

 The hornblende-picrite from Anglesey, described by Prof. 

 Bonney,J has a sp. gr. of 2 88. Accordingly the sp. gr. of 

 the Mt. Horror rock is high enough for a picrite containing 

 some felspar. 



The most important mineral is the augite, and not the 

 mono- silicate olivine as in the more basic peridotites. It is 

 present in large sections more or less oblique to the vertical 

 axis. Its tint in plain light is light purplish, deepening to ar 

 dark mauve shade at the edges. A feeble pleochroism is 

 perceptible where the purple shade is most pronounced. The 

 purple colour (according to Knop) results from the presence 

 of titanic acid ; this colouration is well seen by transmitted 

 light and a hand lens in thin section of the rock. In some 

 of the crystals traces of the cleavage cracks parallel to the 

 prismatic surfaces are visible. Zonal lines are very marked 

 and frequent. The low extinction angle indicates that the 

 pyroxene is not very ferriferous. There are inclusions of 

 colourless grains of olivine after the manner which gives rise 

 to lustre-mottling or poicilitic appearances. In gabbros and 

 diabases we find felspars included in the bisilicate in this 

 way ; the characteristic inclusion in ultra-basic rocivs is the 

 mineral olivine. 



The augite is repeated in the ground mass in the form of 

 small crystals, often with defective ends ; many of these 

 crystals show sections out of the prismatic zone. There is a 

 tendency to grouping, and then a somewhat granular con- 

 dition of augite prevails. Tliis augite of the ground mass is 

 of the same colour as the large phenocrysts, slightly pleo- 

 chroic when deeply tinted. The extinction angles of vertical 

 sections vary from 28° to 40°. 



The next constituent is olivine in grains and defective 

 crystals with rounded corners, at once distinguishable in the 

 sections by its freedom from colour. Its fissures are filled 

 with yellow serpentine, sometimes showing minute honeycomb 

 structure. It is filled with enclosures containing bubbles and 

 opaque grains (chromite ?) ; besides these, there are large 

 glass cavities with inclusions. 



The felspars are narrow, long, lath-shaped, and by their 

 extinction angles may be referred to the labradorite-anorthite 

 group. They are tolerably fresh and altogether play a minor 

 role in the constitution of the rock. 



* Teall, British Petrography, p. 103. 

 i Q. J. Geol. Soc, 1885, p. 392. 

 J Q. J. Geol. Soc, 1883, p. 256. 



