Reviews—Geology of the Lomagundi Mica Deposits. 133 
the geological aspects of the science, and chapters 7 and 8 (dealing 
with the reservoirs of oil and gas, and with the laws of migration 
and accumulation of petroleum respectively) are especially good. 
In the discussion of maps and their uses (chapter 9) the author is 
rather bound down by American convention, and he has not treated 
this phase of the subject as widely as its universal importance merits ; 
in this connexion brevity may very easily lead to ambiguity or 
inaccuracy, and the dismissal of “ structure maps > and “‘ isochore 
lines” with two short paragraphs is to be deplored. The chapter 
on oil structures is somewhat fuller and includes a classification of 
American oil-fields on the basis of varying tectonic features. The 
fields are classed with reference to folded structures, monoclinal 
dips (homoclines), dome structures, faults, and unconformities ; such 
a classification is, of course, open to the criticism that in one field 
two or more structures may occur, as in the case of Wyoming, where 
all five of the foregoing features are known, but usually one main 
type of structure predominates and thus justifies the attempt. 
The volume also includes some remarks on the kind of surface 
-ndications to be looked for in prospecting for oil ; in this connexion 
it is gratifying to note that the author does not follow the usual lines 
suggested by some writers who advocate drilling a test well wherever 
two or more likely seeps of oil and gas can be correlated with supposed 
anticlinal structure. However favourable the surface indications 
may be, the geological structure must be proved first of all, and the 
superficial evidence of oil taken at its true value, i.e. as confirmatory 
evidence. The sentence in the final paragraph of this section might 
well have been italicized in order to accentuate its importance, since 
it is a golden rule for all prospectors : “Drilling should be under- 
taken only on a favourable geological structure in rocks proved 
petroliferous.” Adherence to this policy not only condemns “ wild- 
catting ”’, but makes for success in oil finding by minimizing risks and 
eliminating that undesirable factor of “ chance eh 
Although the volume can be read through easily in an hour or 80, 
it contains so much information, concisely arranged and carefully 
illustrated, that one keeps a copy near to hand for repeated reference 
where general principles rather than textbook detail are required. 
We would draw the attention of both author and publisher, however, 
to the somewhat excessive price charged for a book of this size and 
description ; in its first edition at Ils. 6d., and in its more recent 
edition at 16s. 6d., the volume may be said to be anything but 
“popular”. That is the most general criticism made since its 
publication in 1918. H. B. MILNER. 
Tur GroLtocy oF THE LOMAGUNDI Mica Deposits. By H. B. 
Mavre. Short Report No. 10, Geological Survey of Southern 
Rhodesia. pp. 8, with 2 figs. 1920. 
(fats country rock of the district consists of mica-schists 
and quartzites, believed to be an unusually highly meta- 
morphosed facies of the Lomagundi system, which is considered to be 
