204 The Irish Nat7i7-alist. October, 191 3. 
REVIEWS. 
THE STUDY OF ROCKS. 
A Manual of Petrology. By F. P. Mennell, F.G.S. London : Chapman 
and Hall, 19 13. Pp. viii -1-256. Price ys. 6d. 
This clearly written and well printed work is an enlargement of the 
author's " Introduction to Petrology," published in 1909. Like its 
predecessor, it introduces the student to a wider area of observation than 
is usual in such text-books, since the author's experience has been largely 
gathered in the open-air study of rocks in southern Africa. That is 
outlook is by no means restricted will be seen by his description and 
illustration of the rhyolites of the county of Antrim on p. 133, and of the 
altered chalk of Scawt Hill on p. 202. 
The introductory chapters on rock-forming minerals are of necessity 
slight, but form an important adjunct for those who have to limit their 
libraries when travelling, or in the prospector's camp. The serious 
student will want to know a little more of the relations of the optical pheno- 
mena that are utilised in thin slices under the microscope to the forms of 
minerals studied. On p. 16 it is implied that an ordinary ray arises in 
all cases of double refraction ; and on p. 15 the danger of relying on mean 
refractive indices is not pointed out. In the descriptions of such im- 
portant minerals as quartz, tourmaline, and calcite, we should like to 
find some reference to their different types of trigonal symmetry. The 
statement that the two former are hexagonal, and that calcite is rhombo- 
hedral is surely somewhat out of date. As usually happens in petro- 
graphic works, the microscopic characters of minerals are relied on almost 
exclusively. Even in the case of muscovite (p. 52), the mention of 
straight extinction shows that the forms seen in thin sections are being 
considered, and not those so easily recognised in the field. Tourmaline 
(p. 70) is stated to be transparent ; but few geologists will note this 
character when hanimering at common cry.stals of schorl. In this treat- 
ment, Mr. Mennell has followed what is customary, rather than his own 
good advice given later on p. 82. 
It is not of much avail to quarrel with the names for rocks that have 
received acceptance through the profound authority of Rosenbusch, 
but a "glassy granophyre " (p. 131), instead of a " vitrophyre," seems 
an undue defiance of Vogelsang, the inventor of the term. To pass from 
such details, there is much that is unconventional in the author's discus- 
sion of types of rocks. We welcome his treatment of assimilation by 
igneous masses, of the origin of the much discussed banded ironstones 
(p. 192), and especially of contact-metamorphism on a regional scale 
(p. 204). The book concludes with a good review of the geological aspects 
of radioactivity 
G. A. J. C. 
