It is a far cry from an "earth established so that it may not be moved" to the unstable earth described by the Sturgis-Hooper professor of geology at Harvard University in this thought-provoking volume. Professor Daly is one of America's most speculative and unorthodox geologists; most of us among the younger generation of geologists, are prone to be more or less bound by the splendid traditions which we have received from Chamberlin, Barrell, Gilbert and the other "great Masters" who were in their prime at the opening of the present century. But Professor Daly has never been a "conformist;" no mental shackles which could hold him have ever been forged. And in his latest book he gives full rein to his speculative sprit.

The trained geologist recognized that on many pages the author makes no "rigorous distinction between the ascertained facts of geology and of the many speculative considerations" of his thesis; and the author wisely and carefully calls frequent attention to his method of attaining "simplicity of statement," so that the non-geologist will not be misled.

Briefly stated, the most important conjecture is that early in the history of the juvenile earth there was a single continent composed essentially of a granite crust floating on a glassy basaltic substratum. This unique continent was domed and furrowed in response to strains due chiefly to contraction of the cooling earth and to decrease in rotational velocity because of tidal retardation. "At the close of the Paleozoic Era, the east-west geosyncline of the northern hemisphere was intensely crumpled by the sliding-together of the North Polar and the Equatorial dome. The result was the Appalachian-Hercynian system of mountains...Meantime, the continent was being domed as a whole. In this way sliding-slopes, directed toward the primitive Pacific Ocean, which was then larger than now, were generated...At last, about the middle of the Mesozoic Era, the unique continent finally succumbed to the pull toward the Pacific. It broke into huge fragments, the Americas, Eurasia-Africa. Australasia, and Antarctica. The last, Antarctica, seems not to have moved much horizontally. The others slowly slid, with haltings and renewed sliding, toward the Pacific. The great mountain arcs of the Circum-Pacific-Zone were born and slowly gained in strength as the continental fragments slipped down over the earth's body. During the process of folding along the border of the invaded Pacific, basaltic floods were pouring out through the tension-cracks of the continental interior. The fissures, through which the lavas rose, became sealed with frozen basalt...A new, heavy, deep-lying crust became ready to receive the water running in from the ancient Pacific Ocean; the Atlantic, Arctic, and Indian Oceans thus came into being."

Professor Daly's thesis, thus briefly summarized, is a synthesis of the cosmogony of Jeans and Jeffreys, the Wegener-Taylor hypothesis of drifting continents, and his own ideas concerning mountain-building and the origin of igenous rocks. No intelligent verdict concerning its accuracy can be made until the many speculations on which it is frankly based have been properly evaluated and adequately tested. The book now in hand is a transcript of a course of semi-popular lectures delivered by its author to a non-technically-trained audience at the Lowell Institute, and it therefore does not contain full statements of the data and reasoning processes which enabled the author to reach the conclusions therein suggested.

Pending publication of such data, we may welcome "Our Mobile Earth" as a stimulating contribution, well designed to provoke geologists to a new examination of many of the fundamental tenets of their science. To any one interested in the evolution of the physical environment which has played so large a part in the history of life upon the globe, it will prove to be exceedingly interesting reading.