376 Reviews — A. Holmes — Age of the Earth. 



classified according to their acidity and mode of formation, whereby 

 nineteen divisions are obtained, and the names of these divisions, with 

 suitable prefixes, are made to suffice. The book is written to meet 

 the needs not so much of students as of working geologists, and for 

 such simplicity of nomenclature is a great advantage. Nevertheless, 

 it is very difficult to place many important types in so simple 

 a scheme ; thus, borolanite and kentallenite are found under the 

 dolerites, while such well-known groups as the essexites and 

 monzonites do not appear. 



There are short chapters on the sedimentary rocks, weathering, the 

 chemistry of rocks, radio-activity, and lastly a useful chapter on the 

 collection of material and the preparation of thin sections. 



Chapter ix on the origin and variation of igneous rocks is full of 

 interest, as it contains the author's ideas resulting from a large amount 

 of field work, and in it he states a very strong case for the origin of 

 igneous rocks from 'refusion'. The chapters on the metamorphic 

 rocks also contain much original matter and discussions of many 

 controversial questions. 



Throughout the book a great many of the rocks taken as examples 

 are from South African and Australian occurrences, so that the book 

 will doubtless be found most useful by geologists working in those 

 parts of the empire. 



V. — The Age of the Eahth. By Akthub Holmes. 8vo ; pp. xii + 



195, with 4 plates, 5 text-figures, and 5 diagrams. London and 



New York : Harper & Brothers, 1913. Price: cloth, 2s. 6d. net; 



leather, 3,9. 6d. net. 



ri1HE problem of the age of the earth has occupied the attention 



J_ of scientists repeatedly since the fourteenth centur}', and has 



been a frequent source of keen controversy. This little book has 



for its aim the criticism, in the light of recent work, of the various 



methods which have been applied to the solution of the problem. 



After a brief historical account the several methods are treated in 



detail. 



Kelvin's estimate, based on the effect of tidal retardation on the 

 shape of the earth, is passed over as being based on astronomical 

 theories not generally accepted. 



As instances of attempts to connect sedimentation with astronomical 

 data, the work of Croll and of De Geer on the date of the culmination 

 of the Glacial period is quoted. In the same chapter is a review of 

 Gilbert's attempt to connect the regular alternations of limestone 

 and shale in the Upper Cretaceous of Colorado with the position 

 of perihelion. 



The work of denudation is considered in the light of the available 

 data for several large rivers, and of Dr. F. W. Clarke's recent 

 estimates of solvent denudation. It is estimated that 9,000 million 

 tons of material are deposited annually on the continental shelves. 



The time-worn method of determining the age of the oceans from 

 measurements of the ratio of accumulated sodium to the annual 

 increment is discussed in detail. By introducing numerous corrections 

 it is possible to arrive at ages varying from 80 to 340 million years. 



