THE 
GEOLOGICAL MAGAZINE 
NEW SERIES. DECADE VI. VOL fiisonian lngzj-s 
No. V.—MAY, 1916. { 
ORIGIN AL ARTICLES. 
Laren Ae 
On al Muse % 
I.—Tue PrrrocraPHy oF ARRAN. me eels Us 
3. Prreustone XeEnouiTHs in Basatr Dyker, Drepin, ARray.' 
By G. W. TYRRELL, A.R.C.Sc., F.G.S., Lecturer in Mineralogy and Petrology, 
University of Glasgow. 
‘{\HE phenomenon of which this paper treats occurs in a basalt 
| dyke exposed in a small quarry by the roadside north-east of 
Dippin, near the twelfth milestone from Brodick. This dyke runs 
from north-west to south-east, and is intrusive into the great 
teschenite (or crinanite) sill of Dippin. It is probably the one 
referred to in the Geological Survey Memoir on North Arran, South 
Bute, and the Cumbraes (1903), p. 119. It averages 12 feet in 
thickness, although it is extremely irregular, owing perhaps to the 
difficulty experienced in penetrating the tough, coarse, massive rock 
of the sill. The contacts are not plane, but abut against the crinanite 
in a very intricate manner, sometimes along vertical or horizontal 
joints, sometimes along irregular surfaces unconnected with jointing. 
Both contacts show thin films of tachylyte, which, as the rock is 
quarried, are left adhering to the irregular surfaces of the sill rock. 
Another dyke-like mass, 1 foot thick, is seen in the same quarry, and 
is doubtless an offshoot from the larger dyke. 
Within a foot of the northern contact of the larger dyke there are 
found enclosed several lumps of brown and black glassy rocks con- 
taining a few large white crystals of felspar. These lumps range 
from 1 to 8 inches in diameter, and are each surrounded by a thin 
film of tachylyte, while small clots of a black glass are to be found 
in their vicinity. Other xenoliths found near by consist of a medium- 
grained crystalline rock resembling the crinanite of the Dippin sill. 
These are larger than the glassy xenoliths and are quite evidently 
affected by heat. ‘They are also surrounded by a skin and sometimes 
clots of tachylyte. 
The Dyke.—The dyke belongs to a distinctive group of the Tertiary: 
north-west dykes, which appears to be abundantly distributed about 
the Clyde Estuary. These dykes have frequently been called 
‘basaltic andesite’ or ‘andesitic basalt’, and are distinguished by 
containing beautifully fresh phenocrysts of anorthite or bytownite, 
in a ground-mass composed of laths of labradorite, augite frequently 
intergrown with enstatite or hypersthene, and an abundant mesostasis 
1 The first two papers of this series appeared in the Gro. Maa., Dec. V, 
Vol. X, pp. 305-9, 1913. } 
DECADE VI.—VOL. III.—NO. V. 13 
4 OCT 211916 % 
