8. Allport — On the Pitchstones of Arran. 3 



tween crossed prisms ; the pyroxenic belonites, on tlie contrary, 

 possess double refraction, and exhibit colours. 



In one of the specimens from the Clauchland Hills the appearance 

 is somewhat different : in the greater part of the section the glassy 

 base is clear and colourless, the needles of pyroxene are sharply 

 defined, and the plant-like forms also occur, but are here broad 

 leaf- like expansions of green colouring matter, which is not resolved 

 by an -l-inch objective into anything like structure ; nevertheless, 

 it is not clear and transparent, and might possibly be resolved under 

 a still higher power. This mode of occurrence would appear to 

 represent the first stage of separation of the ferruginous from the 

 other silicates of the base. The grains seen in the specimens are 

 spherical, or egg-shaped bodies of a yellow substance, which 

 polarizes light and exhibits a minute radial structure ; some contain 

 crystals of felspar, or short prisms of augite, also radiating from 

 the centre, and not unfrequently extending beyond the circumference 

 into the glassy base (Fig. 1). These spherules are clearly not 

 secondary formations filling cavities, like the zeolites in other rocks, 

 but are evidently related to the spherolites common in several 

 varieties of perlitic pitchstone ; it is a remarkable fact, that although 

 they are clearly and sharply defined, yet the fern-like groups and 

 other crystals frequently traverse the spheres as well as the base, 

 penetrating both indifferently ; their formation has not interfered 

 in any way with the general texture of the rock ; in other words, 

 the crystalline forms are grouped, or distributed, without any 

 relation whatever to the presence of the spheres-. 



Besides the forms just described, crystals of felspar, quartz, 

 augite, and magnetite are porphyritically imbedded in the base ; 

 although comparatively rare in the rock from this locality, they are 

 very numerous in others, and as their character and mode of occur- 

 rence is precisely the same in all the specimens examined, the 

 following description will apply to the other varieties. 



The felspar crystals' are of two kinds, readily distinguished from 

 each other by their action on polarized light ;. when examined in the 

 polariscope, some are seen to be finely striated, or striped with 

 coloured bands, indicative of the laminated structure of the triclinic 

 group ; others are of one colour only, or in the case of twins have 

 two colours, sharply divided by the plane of junction of the two 

 halves (Fig. 4). These crystals are orthoclase, and agree in their 

 optical characters and fractured condition with the sanidine of many 

 trachytes ; they occur far less frequently than the former. 



The two kinds of felspar have frequently grown together in one 

 group, which may appear perfectly uniform in ordinary light, but in 

 the polariscope is at once resolved into a number of separate crystals, 

 with their axes at various angles, often penetrating each other, but 

 each one sharply defined by a difference in shade or colour ; this is 

 due to the position of the optic axis relatively to the plane of polar- 

 ization (Fig. 5). In ver}^ thin sections the crystals exhibit little or 

 no colour, only dark and light shades, but when thicker they display 

 fine colours. Some of the crystals are completely and regularly 



