Sottas—On Pitchstone and Andesite. 89 
occasionally the rays develope into minute lozenge-shaped plates. 
From these bodies we pass downward in size to the almost. 
ultra-microscopic globulites which are isotropic. No crystals that 
could be mistaken for amphibole occur in any of the slices I 
have examined. It isin the stellate character of the crystallites. 
that our pitchstone differs from that of Arran, in which the 
erystallites are represented by tufts of branching plumose micro- 
liths. 
Specimens are in the Museum taken from the very edge of the 
dyke, where it came in contact with the surrounding granite. — 
These differ in structure from the rest of the rock; under the 
microscope they reveal an irregular banded structure due to alter- 
nations of layers almost colourless, with others rich in brown glass, 
and dusty with globulites. The feather-lke pyroxene prisms 
are absent, but their absence is compensated by the excessive 
development of stellate crystallites which attain a larger size 
than in the rest of the rock, at the same time remaining isotropic, 
or rarely appearing feebly birefringent. 
Phenocrysts of quartz are present, and curious elongated 
streaks of colourless glass devoid of crystallites and structureless, 
save for certain problematic spherical and tubular bodies near the 
edge, which are colourless or faintly bluish and transparent ; 
around them is an aureole of different refractive index to them- 
selves, and to the surrounding glass, but also colourless and trans- 
parent. One of the little spheres gives a black cross between 
crossed nicols, and it is probable that both spheres and tubes 
consist of chalcedony. In some cases additional structures are 
present, such as rhombohedra, apparently of calcite, and long 
filaments, with an axial row of highly refringent granules, looking 
very like Oscillatoria. It is possible that these lenticular patches 
or streaks were formed by a splitting of the pitchstone, while in 
the viscous state, as a consequence of cooling, the prisms being 
subsequently filled up with glass and other material. 
The specific gravity of the central part of the dyke, as deter- 
mined by a Walker’s balance, was found to be 2°41, of the selvedge, 
2°42. . 
Dr. Haughton’s published analysis is given in Column [.; 
that by its side in Column II. is of an Arran pitchstone, by M. P. 
