J. Allan Thomson—Incelusions in Volcanic Rocks. 491 
dolerite of Portrush, and studied them under his guidance. The 
present paper consists of a discussion of rocks from these two localities, 
with descriptive notes on a few other inclusions that I have in my 
collection. 
Two things stand out clearly from a cursory examination of this 
book, the frequency, almost the universality, of inclusions in igneous 
rocks, and the importance of the aid which they afford to the student 
of volcanic phenomena. Under these circumstances, it is remarkable 
how long this branch of study has escaped attention from British 
geologists. Lacroix’s useful nomenclature has not been adopted in 
our literature, although Phillips, Teall, Sollas, Harker, Judd,’ and 
others have paid considerable attention to some varieties of inclusion, 
and Harker has established a slightly different classification of these 
phenomena. 
Betore proceeding to actual descriptions it will be of advantage to 
outline Lacroix’s terminology and most important conclusions, and to 
correlate the terms already in use. Lacroix establishes two classes :— 
A. Enelaves enallogenes. 
B. Enclaves homeogenes.* 
A.—The first, enallogenous inelusions, are without any genetic 
relation to their host, but are merely accidental inclusions of rocks 
which the magma has been able to pick up in its progress to the 
surface. 
B.—The second class, homeogenous inclusions, are not mere accidents, 
but are considered to be related to their host in that they have 
erystallized from the same magma in depth. The best known type 
are the olivine nodules of basaltic rocks. 
Zirkel, following Sauer, uses the terms Linschliisse endogene in the 
sense of Lacroix’s enclaves homeogénes, and insists on the difference 
between such inclusions and the basic secretions (Schlieren) of granitic 
rocks. Doelter makes a similar division into exogene and endogene, 
correlating these with Lacroix’s enallogéne and homeogéne.* But 
in many recent descriptions German authors have preferred the 
latter terms. 
In English scientific literature the term zenolith has practically 
established itself in the sense of an enallogenous inclusion. It was 
proposed by Sollas in his paper on the rocks of Carlingford,’ whiie 
individual crystals derived from foreign sources were called xenocrysts 
to distinguish them from phenocrysts. 
Harker, in a paper on mixed rocks,® and in his memoir on the 
Skye rocks,’ distinguishes acctdental and cognate xenoliths. In the 
latter ‘‘there is a genetic relationship between the enclosed and the 
1 For a discussion of the subject in 1893 and previous literature see Judd, 
Q.J.G.S., vol. xlix (1893), p. 175. 
* “Evaddos = different ; duoos=similar; yeryaw=to beget. 
8 F. Zirkel: ‘‘ Lehrbuch der Petrographie,’’ 2nd ed. (1893), vol. i, p. 794. 
4 C. Doelter: ‘‘ Petrogenesis,’’ 1906, ch. vii, p. 101. 
5 Trans. Roy. Ir. Acad., vol. xxx (1894), p. 493. 
6 A. Harker, ‘‘Igneous Rock Series and Mixed Igneous Rocks’’?: Journal of 
Geology, Chicago, 1900, p. 389. 
7 A. Harker, ‘‘ The Tertiary Igneous Rocks of Skye’’: Geol. Surv. Mem., 
1904, p. 351. 
