94 PEOP. J. W. JTJDD ON" TEETIAEY 



(" Eorellenstein") ; the rock, indeed, appears like an ophitic dolerite 

 through, which portions of a troctolite are scattered, though, as pointed 

 out in the text (p. 71), this may not be the real explanation of the 

 origin of the structure. 



Fig. 4. Augite crystal from the gabbro passing into dolerite seen on the south 

 side of the West Glen, St. Kilda. Magnified 180 diameters. The 

 central part of this crystal presents the ordinary characters of augite, 

 the whole extinguishing together ; but along its edges the augite 

 becomes " granulated" or broken up into masses of granular particles, 

 each of which has a difierent orientation. This structure may have 

 been produced by crystallization having gone on to a certain extent, 

 while the mass was in a state of perfect internal equilibrium , but, before 

 final consolidation, some internal movement or strain having been set 

 up within it (see p. 76). 



Fig. 5. Zoned crystal of augite from the piperno of Pianura near Naples. Mag- 

 nified 100 diameters. The central portion of this crystal is of a pale green 

 colour and exhibits the characteristic cleavage-cracks of augite ; it is quite 

 destitute of pleochroism. The outer zone, which is not sharply divided 

 from the central mass, but appears to graduate insensibly into it, is of 

 a rich yellowish-brown tint, and exhibits the characteristic pleochroism 

 of a moderately ferriferous enstatite (proto-bronzite), reddish brown 

 to bluish green. The outer surface of the crystal, where it is in contact 

 with the surrounding magma, exhibits the irregular black band so 

 often seen in hornblende and mica-crystals under similar circum- 

 stances. Although the outer zone of this crystal is so strikingly 

 distinguished from the central part by its strong pleochroism, yet, so 

 far as I can make out, they extinguish together ; some of the cleavage- 

 cracks of the central mass also seem to pass without change into the 

 outer zone. Owing to the narrowness of this outer zone, I have not 

 found it practicable to compare the interference-figures given by the 

 two portions of the crystal with convergent polarized light. This 

 example is very instructive when compared with the augite of the 

 ophitic dolerite of the Shiant Isles (see Plate Y. fig. 1). In this we 

 sometimes find the purplish -browji non-pleochroic augite shading 

 into a material of a yellowish-brown tint, exhibiting in a very marked 

 manner the characteristic pleochroism of hornblende ; this pleochroic 

 portion of the augite crystals still exhibits the characteristic cleavage 

 and the extinction-angle of the augite. These examples seem to 

 indicate that in the paramorphic changes of augite into hornblende 

 and enstatite respectively, the peculiar pleochroism of the paramorphic 

 minerals is the first character which is acquired (see p. 65, footnote). 



Fig. 6. Altered augite from the gabbro (olivine gabbro) Isle of Mull. Mag- 

 nified 25 diameters. This example is very interesting on two grounds. 

 In the first place, we have twinned crystals like those occurring 

 commonly in the Whin Sill and so well described by Mr. Teall 

 (Quart. Journ. Geol. Soc. vol. xl. (1884), p. 647 &c., pi. xxix.). Such 

 twinned augites do not appear to be at all common in the rocks of the 

 district we are describing. The crystals have been perfectly schiller- 

 ized and have assumed the foliation of diallage. In the second place, 

 it is noteworthy that though the augite material _^has been converted 

 into " viridite " substance, or, in other words, the* crystals are under- 

 going uralitization, yet the enclosures still remain which were formed 

 daring the secondary foliation of the mineral. These enclosures 

 appear to have contained so much titano-ferrite that, by weathering 

 action, they have yielded the characteristic white product known as 

 "leucoxene," giving a very peculiar appearance to the altered product 

 (see p. 85). 



Fig. 7. Two grains of serpentinized olivine in a gabbro (olivine-gabbro) from 

 Portsoy, Scotland. Magnified 30 diameters. The expansion of the 

 olivine during its hydration is seen to have prodiiced series of radiating 

 cracks traversing the surrounding crystals of felspar and augite, and 

 these cracks have been injected with the serpentinous matter. The 



