194 G. W. Tyrrell—The Petrography of Arran. 
of dark glass. From their abundance in the island of Great Cumbrae, 
situated in the Firth of Clyde, it is proposed to distinguish them 
as the Cumbrae type of basalt. The Tynemouth dyke of the North of 
England appears to be an outlying member of the same series. 
In thin section the rock from the centre of the dyke shows small 
but rather numerous phenocrysts of plagioclase felspar which are 
occasionally euhedral, but are more often worn and corroded into 
curious irregular shapes. Whatever the shape, all the crystals 
possess a narrow marginal zone of different composition orientated 
similarly to the main mass. ‘I'he transition between the two parts 
is usually quite sharp. The measurement of extinction angles shows 
that the crystals are bytownite (Ab,An,) with marginal labradorite. 
There are also a few phenocrysts of fresh enstatite, showing a good 
positive biaxial figure in basal section ; and fewer still of a yellowish 
augite. The ground-mass is composed of zonal labradorite laths, 
intermixed with prismoids of augite and grains of iron-ore, with an 
abundant mesostasis apparently of a felspathic substance, crowded 
with black hair-like microlites, and probably representing a de- 
vitrified glass. 
This rock is an andesitic basalt or basaltic andesite generally 
similar to the Tynemouth and Cleveland dykes of the North of 
England. Other dykes of the same character, with slight variations 
in texture, amount of glass, or abundance of phenocrysts, occur in 
the vicinity, cutting the Dippin sill and the adjacent Triassic 
sandstones. 
Traced towards the margin of the dyke the rock becomes denser, 
and contains fewer and smaller felspar phenocrysts. A specimen 
taken 3 inches from the margin shows a beautiful variolitic texture in 
thin section. The felspars of the ground-mass are reduced to the 
size of large microlites, and are arranged in irregular radiating 
bundles or sheaves which involve numerous long prisms of augite 
and grains of iron-ore. 
At the actual margin the rock is tachylytic for a thickness of 
+ inch. In thin section the glass is dark, opaque, and almost 
structureless, with only a few obscure microlites. It contains 
numerous small spherical amygdales of calcite, and a few small 
phenocrysts of labradorite with the usual rounded or irregular 
outlines indicating magmatic corrosion. 
The Xenoliths.—The great majority of the xenoliths are of pitch- 
stone. In thin section the bulk of this rock is seen to consist of 
a pure, almost colourless glass, in which hair-like microlites are 
aggregated into clots or patches, leaving -large areas of glass 
absolutely free from microlites. The phenocrysts are of euhedral 
quartz, orthoclase, and andesine (Ab;An,). The effects of the heat 
to which this rock has been subjected are striking. Of the three 
principal minerals orthoclase has suffered the most, andesine much 
less, and quartz hardly at all. The orthoclase in all cases shows 
some degree of fusion, which has occurred around the margins and 
along the cleavages, producing a yellow or greyish glass which 
contrasts with the lighter glass of the ground-mass. The resulting 
shapes of the crystals are highly irregular; occasionally the crystal 
ee ee ee eS 
