320 Scientific Intelligence. 



The next two chapters, VI and VII, will be, perhaps, the most 

 important and interesting of the work to geologists. They pre- 

 sent the author's conception of the nature of magma, beginning 

 with it in its original seat, and following it upward until it 

 appears in various forms of intrusion in the upper lithosphere or, 

 as in the last one, VIII, in extrusions of lava upon the earth's sur- 

 face. In accordance with the conditions imposed by astronom- 

 ical considerations, and shown to exist, by seismical investigations, 

 with respect to the physical status of the earth, magma in its 

 original place toward the base of the lithosphere must be con- 

 ceived as a condition of a zone of the latter, not as any mass of a 

 liquid nature. Although highly heated, it is rigid, but potentially 

 mobile, and may shift its position slowly as adjustments of the 

 lithosphere cause differential changes in pressure. In the begin- 

 ning it can have no definite boundary, but as it moves upward 

 its viscosity diminishes with lessening pressure, and it enters the 

 zone of intrusions. The possible cases of such intrusions are fully 

 discussed ; thus the bathylith is conceived by the author as a 

 broad flattened body in nearly horizontal position, where magma 

 has entered the fractured lithosphere in regions of major thrust- 

 ing, as on the Pacific border. It is when the magma leaves what 

 we may term the zone of speculation and enters that one where 

 observation is possible that the ideas of the writer will be espe- 

 cially open to testing by other geologists. His conceptions seem 

 logical, and are based upon geological observations and buttressed 

 by reference to the known laws of physics and chemistry, but 

 some of them will probably not be accepted without discussion. 

 Thus, while it would seem as if Prof. Iddings had presented pow- 

 erful arguments against the hypothesis of the upward movement 

 of magma into the outer lithosphere on a large scale by the 

 sinking and assimilation of crust blocks in the depths, his view 

 that spalling and sinking of blocks cannot probably occur at all 

 (p. 210) on account of the viscosity and pressure of the magma, 

 places one on the horns of a dilemma. For if the magma were 

 always of this nature it is difficult to see how differentiation, 

 under any terms of that process, could take place. It would seem 

 to the reviewer as if only highly siliceous, and, therefore, viscous 

 magmas, were in the author's mind, but in the less siliceous ones 

 greater fluidity might be expected, which would give opportuni- 

 ties for differentiation, and in these at times, under suitable con- 

 ditions, the sinking of crust blocks might be expected to a limited 

 extent. 



The volume is a most important contribution to the theoretical 

 side of geology ; it is very stimulating and should be read 

 by every worker in the physical fields of this science. It is hand- 

 somely printed and abundantly illustrated with halftone plates 

 and fine photogravures of nebulae. l. v. p. 



6. Pleistocene Mammals of Iowa ; by Oliver P. Hat. Annual 

 Report, Iowa Geological Survey, for 1912, 1914, pp. 1-499, pis. 

 I-LXXV, 142 text figs. — This important contribution by the well- 



