Geology. 663 



ters may be found " examples " (200 in all) for solution by the 

 reader. The index is immediately preceded by a set of tables of 

 useful optical data. The publishers have been very successful in 

 printing the numerous mathematical formulae clearly and accu- 

 rately. The volume should be useful to teachers and graduate 

 students as well as to undergraduates. h. s. it. 



II. Geology. 



1. The Strength of the Earth\< Crust ; by Joseph Barrell. 

 Jour, of Geol.. vol. xxii, Nos. 1-8, vol. xxiii, Nos. 1, 5, 6, 1914 

 and 1915.* — The reprinting of these papers in collected form for 

 circulation by the author puts them in very convenient shape for 

 use by geologists, and merits calling attention to them. They 

 constitute probably the most serious and profound discussion, 

 which has yet been attempted, of the facts which are known and 

 of the theories which have been deduced from them, concerning 

 the strength of the earth's outer shell. Upon this physical fea- 

 ture so many conclusions drawn by geologists ultimately depend 

 that Professor Barrell has done a valuable service in applying 

 quantitative data from the geologic side and in discussing from 

 the standpoint of the geologist the results obtained by physical 

 measurements by the geodosists, and the inferences derived from 

 them by analysis. This combination of geological and mathe- 

 matical lines of attack, by one fully competent to deal with both, 

 gives a certain authority to the conclusions drawn in the final 

 papers. The author finds that the crust is very strong when 

 measured by its capacity to support great deltas, mountain ranges 

 or large internal loads due to variations in density not in accord 

 with topography, while on the other hand the altitudes of the 

 continents as a whole or in large sections show nearly perfect isos- 

 tasy. The maintenance of such isostatic conditions through geo- 

 logic time, in spite of opposing geologic activities, is held to imply 

 the existence of a zone of undertow below the zone of compensa- 

 tion, which is both thick and weak to shearing stresses. Geo- 

 logical^ such a zone, called the asthenosphere, — the shell of 

 weakness — must have important bearings, which are treated in the 

 later portions of the work. l. v. p. 



2. A Text- Book of Geology. Part I, Physical Geology ; by 

 Louis V. Pirssox, Professor of Physical Geology in the Sheffield 

 Scientific School of Yale University. Part II, Historical Geol- 

 ogy ; by Charles Schuchert, Professor of Paleontology in Yale 

 University and of Historical Geology in the Sheffield Scientific 

 School. Pp. x, 1,051, figs. 522, appendix, index, and folding 

 colored geological map of North America. New York, 1915 

 (John Wiley and Sons, $4. On net. Parts I and II sold also in 

 separate volumes, $2.25 and §2.75 net).— This book, "dedicated 



* Copies of this memoir cf about 250 pages may be purchased from the 

 Yale Cooperative Store for 75 cents, parcel post extra. — Ed. 



