G. W. Tyrrell—The Petrography of Arran. 195 
is separated into two or three pieces by areas of fusion; but in 
extreme cases the crystal is represented by a ‘ ghost’ in greyish 
glass, in which only minute shreds of the original orthoclase have 
survived. 
The andesine crystals have apparently only suffered a little 
softening on the margins and fissuring in the interiors. The quartz 
has suffered least of all, as the crystals are only fractured. The fact 
that the broken pieces have occasionally floated away from one 
another is evidence that the glass of the pitchstone was rendered 
completely liquid by the degree of heat to which it was subjected. 
Kach xenolith is surrounded by a skin and sometimes clots of 
a black glass, which was at first regarded as a tachylyte of the same 
nature as that on the margins of the dyke. A thin section, however, 
shows that this material is really a colourless glass which is darkened 
by numberless lines, streaks, and patches of black dots, arranged with 
a perfect fluxional structure. The glass contains the same set of 
phenocrysts as the above-described pitchstone, but much smaller and 
fewer. Under a high-power objective the black dots which darken 
the glass are made out to be small, perfectly euhedral, hexagonal 
tablets of hematite, the larger ones translucent and of a blood-red 
colour. 
The difference between this rock and the pitchstone it envelops 
may be explained thus: the xenoliths have not only been perfectly 
fused, but the pitchstone glass has intermingled to some extent with 
the more basic glass of the basalt in which the xenoliths were carried 
up, the admixture of basalt glass resulting in the formation of swarms 
of hematite microlites. The flow-structure may have been imparted 
by a rolling motion of the xenoliths, due to differential velocity 
of the central and marginal parts of the basalt dyke during intrusion. 
The xenoliths are found within one foot of the margin, where 
movement would be retarded by cooling and viscosity on the marginal 
side, whilst proceeding at a faster rate in the more liquid interior. 
Conclusions.—The behaviour of the phenocrysts in the pitchstone 
xenoliths under the influence of heat gives some clue as to the 
.temperature of the basaltic magma in which they were immersed. 
The orthoclase was partially or wholly fused to a yellow or grey 
glass, andesine showed signs of softening around the edges, whilst 
quartz simply suffered some degree of fracturing. The most trust- 
worthy estimates of the fusion-points of rock-forming minerals have 
been made by A. L. Day and his collaborators in the Geophysical 
Laboratory at Washington.’ That of quartz (or rather silica, since 
quartz is unstable above 800° C.) is given as 1,625° C.; of 
oligoclase-andesine (Ab, An,) as 1,375° C. For orthoclase, however, 
we have to rely on the estimates of other experimenters.? The 
average of seven closely accordant estimates by Joly and Cusack, 
and Doelter, is 1,170° C. These figures are as would be expected 
from the effects produced by heat on these minerals. Orthoclase, 
which fuses at 1,170° C., has been converted into glass; andesine, 
1 Clarke, Data of Geochenustry, 2nd Ed., U.S. Geol. Surv., Bull. 491, 
p. 279, 1911. 
2 Iddings, Igneous Rocks, vol. i, p. 85, 1909. 
