Reviews—New Geological Maps. 571 
makes light deviate from a rectilinear path in passing through it. 
Now on the undulatory theory light is composed of vibrations of the 
ether in all possible directions. When, however, it passes through 
a crystal belonging to any other than the regular system all the 
vibrations are made to take one or other of two definite directions at 
right angles to each other and to the direction of propagation. The 
light is then said to be polarised.’?’ There is also much confusion in 
the use of the terms ‘refractive index’ and ‘indices’. Again, on 
p. 31 he says: ‘‘ The mica gives a typical biaxial figure showing two 
dark hyperbole (brushes), which unite four times during a rotation 
(at the extinction positions) to form a cross of which one arm is 
broader than the other. The thin arm joins the optic axes. The 
broad arm is called the bisectrix, in the case of mica the acute 
bisectrix as it bisects the acute angle between the optic axes.” On 
the following page, speaking of the optical sign of minerals, he 
remarks: ‘‘ Some minerals have their interference tints heightened 
when covered with a quartz wedge having its long axis in the same 
direction as their own, while others have them lowered. In the first 
case the sign of the double refraction is said to be posetive, in the 
latter it is negative.’ We are left to guess, however, what 
constitutes the ‘long axis’ of either the crystal or the quartz wedge. 
The whole of the introductory portion dealing with the crystallineform . 
and optical properties of minerals is put into twenty pages, and is 
almost valueless to any serious petrological student. Chapters VII-XI 
deal with the rock-forming minerals, and these constitute a useful 
portion of the book. However, it would have been better if the 
minerals had been arranged according to their crystalline system or 
their optical behaviour rather than their chemical composition. 
Their descriptions are often somewhat unscientific, and occasionally 
there are insufficient data for the identification of a certain species. 
It is to be remarked that such valuable mensurate constants as the bi- 
refringence in anisotropic media are disregarded. Chapters XII-XXI 
refer to igneous and sedimentary rocks, and the processes and products 
of metamorphism and weathering. The description of the various 
simple rock types is by far the best portion of the work, but it suffers 
from incompleteness, and there is occasionally a lack of definition of 
tock names and structures. Many of the illustrations are admirable, 
especially those which are reproduced from line-drawings, but some 
of the photomicrographs are too smudgy and out of focus to be worth 
the space they occupy. 
Fe Hee 
TV.—Somr New Geotocicat Maps. 
1. Gxotocican Map or Eeypr.—We have received from the Survey 
Department, Cairo, a geological map of the country issued 1910, in 
six sheets, on the scale of 1 : 1,000,000, on which the various 
formations are very clearly shown in colour-prints. No record, 
however, appears of the names of the geologists who are responsible 
for the field-work. The area extends from the Gulf of Salum and 
Alexandria to Port Said on the north, and from Wadi Halfa to Ras 
Hadarba on the Red Sea coast on the south. The southern part of 
