﻿112 



ME. Q. W. TYRRELL ON THE 



[vol. lxxii, 



the mass, contain some felspar and analeite, and are richer in the 

 metasilicates. These are true picrites in the original sense of 

 Tsehermak. In hand specimens the peridotite is a fresh, medium 

 to coarse-grained, hlackish-green, heavy rock, in which olivine may 

 occasionally be distinguished, and especially hornblende in large, 

 lustre -mottled plates. The rock is variable in granularity, and 

 may be devoid of poikilitic hornblende. The picrites are distinctly 

 finer-grained, and show white or pink specks of felspar and 

 analeite. 



Microscopicall}-, the peridotite consists of olivine, titanaugite, 

 hornblende, biotite, and iron-ores in the proportions shown in 

 Table V, col. hi (p. 113). The olivine, which may form 60 to 

 70 per cent, of the rock, occurs in more or less rounded, subhedral 

 grains, ranging up to half an inch in diameter. It is occasionally 

 quite fresh, but is usually in all stages of alteration to blue, green, 

 yellow, and colourless varieties of serpentine. The coloured ser- 

 pentines are almost entirely devoid of separated magnetite ; but 

 the unaltered olivine and the colourless variety of serpentine 

 contain irregular streaks of magnetite, indicating that the colour 

 of the serpentine depends on whether the iron-oxide is thrown out 

 of combination or not during the process of alteration. The next 

 most abundant constituent is augite, which occurs in polysomatic 

 clusters of small euhedral grains wedged in between the olivines, 

 and Avhere the latter mineral is altered, embedded in serpentine. The 

 granular habit of the augite is distinctive of this type of peridotite, 

 which is accordingly distinguished as the Lugar type. The 

 earliest described picrite or peridotite of the teschenite series, that 

 of Inchcolm, contains titanaugite in large plates which mould and 

 poikilitically enclose olivine. 



A red-brown hornblende, belonging, like the arnphiboles of the 

 preceding rocks, to the barkevikite group, occurs in large, irregular, 

 poikilitic plates, enclosing small olivines, and groups of granular 

 augite-ciystals. It usually forms about 10 per cent, of the rock. 



The remaining constituents, biotite and iron-ores, form only 

 about 4 per cent, of the rock. The two minerals are, as usual, 

 closely associated. The rock is almost devoid of apatite : this 

 mineral appears to be associated with analeite, and increases in 

 abundance along with that mineral. 



The picrites which occur towards the top of the ultrabasic 

 stratum are divisible into two varieties. One, almost devoid of 

 recognizable felspar, contains much analeite, and very abundant 

 augite. It approximates to the lugarite type, and represents the 

 modification of the peridotite at the contact with lugarite. In 

 thin section this rock consists of numerous, euhedral, purple 

 augites, embedded in a turbid cryptocrystalline or isotropic ground- 

 mass containing zeolites, occasional clear analeite, and ' ghosts ' of 

 felspars, identical with the ground-mass of lugarite. Barkevikite 

 occurs in ragged plates exhibiting the poikilitic habit and inter- 

 grown with ihnenite. Olivine is comparatively sparse, and is 

 completely serpentinized. 



