J. Allan Thomson—Inclusions in Volcanic Rocks. 493 
viscous and cooling more slowly, have not had their gases disengaged 
so fast, so that these have had time to act chemically on the inclusions 
and to determine the formation of many minerals—mica, orthoclase, 
etc.—that cannot be reproduced artificially by pure fusion alone. 
It is noteworthy that this division of volcanic rocks corresponds 
with that established by the work of Fouqué and Levy on the 
synthesis of igneous rocks. The basaltoid rocks can all be reproduced 
by fusion and suitable re-cooling, the trachytoid cannot. 
The study of homceogenous inclusions is of interest because of the 
light they throw on the relations between volcanic and plutonic 
rocks. The inclusions may be divided into two classes, those which 
possess the same mineral assemblage and practically the same chemical 
composition as their host, and those which are formed only of the 
large minerals of the intratelluric crystallisation and are more basic. 
The former kind are called by Lacroix the formes grenues or granular 
form of the eruptive rock, the second enclaves basiques or basic 
inclusions. As examples of the first may be cited biotite sanidinites 
in biotite trachyte, and nepheline syenite in phonolites. Sometimes 
instead of the plutonic rock corresponding to the volcanic host there are 
found rocks which otherwise occur in association with the former; 
e.g., camptonites, which so often accompany nepheline syenites, are 
found in the phonolites of Mont Dore. As examples of the: basic 
inclusions, the olivine nodules of basalts may be taken. The 
generalisation may be made that in trachytoid rock the ‘formes 
grenues’ of the host are more frequent and the basic inclusions rare, 
whilst in basaltic rocks the reverse is the case. For a discussion of 
the mode of origin of homceogenous inclusions reference must be made 
to Lacroix’s book. He admits that here similar phenomena may be 
arrived at in different ways. 
The following descriptions are all of inclusions in basaltoid rocks: — 
I. Inclusions of the Dolerite Sill of Portrush, Co. Antrim, Ireland. 
This rock is so well known and has been so often noticed that 
a description of it would be out of place here. Sir A. Geikie! has 
remarked the fine grain assumed by the otherwise coarsely crystalline 
dolerite near its junction with the Lias shales, and Professor Cole has 
given a description of the microscopic structure of both types in his 
suggestive account of the contact phenomena.” The inclusions 
examined by me are all from the coast section, and include holo- 
crystalline nodules from the coarse dolerite at the north end of the 
cape and inclusions of dolerite and Lias from the east side near the 
junction. Sir A. Geikie mentions the difficulty of distinguishing 
the basalt and the Lias rocks at the contact; this difficulty is 
accentuated with regard to the inclusions, for in these the shale has 
assumed a more crystalline appearance, and inclusions of a flinty type 
of igneous rock in the dolerite show a great resemblance to sedimentary 
rocks. 
1 Loe. cit., vol. ii, p. 299. 
2 G. A. J. Cole, ‘‘ On Contact-Phenomena at the Junction of Lias and Dolerite at 
Porirush’’: Proc. Roy. Ir. Acad., vol. xxvi, sect. B, No. 6, p. 62. 
