88 Reviews—G. F. Becker—Isostasy and Radioactivity. 
J. Keele. Stratigraphically the clays now exploited range from the 
Pre-Cambrian to the Pleistocene ; but while there is a lack of high- 
grade clays like fireclays or pottery clays, there is an abundance of 
material suitable for the manufacture of rough clay products. The 
report contains valuable chapters on the origin and properties of clay, 
the effects of heat in clays, field examination and testing of the 
materials, and on methods of mining and manufacture, written mainly 
for the non-geological reader. 
Memoir No. 74 is ‘* A List of Canadian Mineral Occurrences’’, by 
R. A. A. Johnston. The bulk of the book is occupied with the 
names of the minerals arranged alphabetically, with notes as to their 
occurrence under the heading of each province. The second part 
gives the names of provinces and their divisions in alphabetical order, 
with the list of minerals found in each area appended. The work is 
an exhaustive compendium of Canadian mineralogy. 
IV.—Isostasy anp Rapioacriviry. By Grorex F. Becker. Bull. 
Geol. Soc. Am., vol. xxvi, pp. 171-204, March 31, 1915. 
FY\HE object of this paper is to draw attention to certain alleged 
discrepancies between recent developments in the theory of 
isostasy, and the age of the earth as determined by radio-active 
methods. Dr. Becker returns to his own method of calculating the 
age of a cooling earth, modified by taking into consideration radio-active 
supplies of heat energy. He assumes that the depth at which the 
temperature-gradient curve most nearly approaches the diabase curve 
‘of fusion, i.e. the depth at which rock fusion is most easily - 
accomplished, is also the depth of Hayford’s level of isostatic 
compensation, 121 kilometres. Taking an earth with an initial 
temperature at the surface of 1,300°C., he finds the age to be 
68 million years, radiothermal energy maintaining only one-seventh 
of the present temperature gradient. If radio-activity supplies two- 
thirds of the earth’s heat loss, then the age is 1,314 million years, 
and the depth at which fusion most readily occurs becomes 
300 kilometres. Dr. Becker rejects such an earth as being probably 
incapable of volcanic phenomena. 
Professor Barrell (in the papers previously noticed in January 
last, p. 38) makes out a strong case for the existence of an astheno- 
sphere, extending perhaps to 600 kilometres below the level of 
isostatic compensation. He shows that the depth of easiest fusion 
must be below the level of compensation, and that it is to be looked 
for in the heart of the asthenosphere, that is, at a depth of about 
400 kilometres. Combining this result with Becker’s analysis, the 
age of a cooling earth becomes considerably greater than 1,314 million 
years, and the proportion of the earth’s heat maintained by radio- 
activity becomes much nearer to its probable value—three-quarters, — 
or more (Grou. Mae., February and March, 1915). Geological 
deductions from isostasy and radio-activity are thus easily brought 
into harmony, and the source of the discrepancies alluded to by 
Dr. Becker are at least as likely to be found in his own interpretation 
of isostasy as in the simple method of determining the ages of radio- 
active minerals based on their lead-uranium ratios. 
ArrHur Hommes. 
