472 Reviews—F. W. Harmer—COrag Mollusca. 
this part of the theory is tentative and the details are liable to 
modification. 
When the theory is considered in relation to the actual intrusion 
or extrusion of the rocks, several difficulties arise. The first of these 
is concerned with the sequence of igneous intrusion. Bowen 
considers that the intrusion of basic into acid material is due to 
movements during cooling, and that the absence of granitic upper 
layers is sometimes due to denudation. In the ‘‘ Younger Igneous 
Rocks”? of the Scottish Highlands the sequence is from peridotite to 
granite, with the acid rocks always intrusive into the basic. As 
several of the former are found in contact with the schist roof, 
the acid layers cannot have been denuded away, and it is difficult 
to understand the mechanics of the differentiation and intrusion, 
particularly if the former be considered to have occurred in situ. 
A related point is concerned with the intrusion of batholiths, as the 
theory takes no account of the material into which the batholith is 
intruded. It also seems to the writer that some of the later igneous 
rocks must partly owe their composition to refusion, and possibly 
absorption, and that they are not all direct differentiates of a primary 
basaltic magma. Yet, on the whole, the theory may be regarded as 
the most satisfactory advanced so far, particularly with regard to 
those parts of it which do not depend on any great extrapolation from 
the synthetic work. AS: 
Il.—Tue Puriocene Moxtusca or Grear BriraIn, BEING SUPPLE- 
MENTARY TO S. V. Woon’s MonocrapH oF THE Crac Mortusca.' 
By F. W. Harmer, F.G-S., F.R. Met.S., ete. 
‘]\HE rich remains of Pliocene Mollusca which characterize ae Crag 
deposits of East Anglia have long been familiar to the palzo- 
conchologist through Searles Wood’s classic memoir on that subject, 
which was issued in parts by the Paleontographical Society between 
the years 1848 and 1882. Few fossiliferous regions have been so 
popular with the collector and student as the counties of Norfolk, 
Suffolk, and Essex, which have yielded the Crag fauna, doubtless by 
reason of their easy accessibility from London and the numerous 
open crag-pits from which specimens could readily be obtained. 
It ies been inevitable, therefore, since the completion of Wood’s 
work, now more than thirty years ago, that a large amount of 
further material has been acquired, the study of which has rendered 
necessary the recognition of many new forms and a revision of the 
older nomenclature. This is the special task which the author of the 
present memoir has set himself to accomplish, and for which, we 
think, he is particularly well qualified to undertake. It is interesting 
to note that Mr. Harmer was the great friend of Searles Wood, the 
younger, and a co-worker with him in unravelling the history of the 
Crag formations, he himself becoming the author in later years of 
many valuable contributions on the same subject, so that at the present 
day he may be regarded as one of our best authorities on the Pliocenes 
1 Pt. i, pp. 1-200, pls. i-xxiv; pt. ti, pp. 201-302, pls. xxv—-xxxil. Published 
by the Paleontographical Society, 1914; 1915. (Pt. iii now in the press.) 
