Page:Popular Science Monthly Volume 22.djvu/875

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last and largest work of the distinguished Scotch professor will be a welcome addition to the small list of really valuable text-books on geology. Although on this side of the Atlantic it will not take the place of our American manual as a textbook, it will be widely read by those who, whether geologists or not, take an interest in the past history and present composition of our globe. The treatment is such as will interest the general reader, and enable him to acquire a good grasp of the general principles of the science without tiring him with detail. The author's article on "Geology" which appeared in the "Encyclopædia Britannica" in 1879 forms the basis which has since been expanded into the treatise before us.

The volume is divided into seven books, of unequal length, devoted respectively to "The Cosmical Aspects of Geology"; "The Materials of the Earth's Substance: Dynamical Geology"; "Structural Geology, or, the Architecture of the Earth's Crust"; "Paleontological Geology"; "Stratigraphical Geology"; and "Physiographical Geology." Under the first of these heads, which will be for the general reader the most interesting of all, the various theories that have been advanced to account for the alterations of climate and the occurrence at intervals of cold cycles are discussed. The chapter closes with an interesting abstract of Dr. James Croll's researches upon the physical causes on which climate depends. In the next chapter, that on geognosy, the probable condition of the interior of the earth is considered, the author favoring the view that "the substance of the earth's interior is probably at the melting-point proper for the pressure at each depth." The age of the earth and the measures of geological time are next discussed. This chapter contains a brief description of the rock-forming minerals, as also the macroscopic and microscopic characters of rocks, and a classification and description of the more important kinds of rocks, with the methods used in determining them.

The processes of change at present in progress upon the earth are very fully considered in the third chapter, that on dynamical geology, which covers nearly three hundred pages. This forms one of the most interesting and instructive features of the book, and will be widely read. The short chapter on paleontological geology is far from being dry or uninteresting, containing as it does a description of the conditions essential for the entombment of organic remains on land and sea, their preservation in mineral masses, the relative value of such remains, and their uses in geology. This is a most essential preliminary for a correct understanding of the following chapter, on stratification. Nearly one third of the volume is devoted to a description of the various strata, with illustrations of the principal fossils found in each, and concluding with the flint implements of the recent or human period.

Being designed as a text book for use in Britain, local examples are chosen for illustration, but the author is fully alive to the fact that America furnishes many unequaled illustrations of some of the most important facts of geology. For his frontispiece he selects the plateau and canons of the Colorado, the drawing being a reduction of one made by Mr. W. H. Holmes, and several cuts are introduced into the body of the work representing views in Colorado, Idaho, and Montana. In his preface he says: "Comparatively few of us have any adequate conception of the simplicity and grandeur of the examples by which the principles of the science have been enforced on the other side of the Atlantic." And further on he says: "If the student is led to study with interest the work of our brethren across the Atlantic, and to join in my hearty regard for it and for them, another important section of my task will have been fulfilled. And, if, in perusing these pages, he should find in them any stimulus to explore nature for himself, to wander with the enthusiasm of a true geologist over the length and breadth of his own country, and, where opportunity offers, to extend his experience and widen his sympathies by exploring the rocks of other lands, the remaining and chief part of my aim would be attained."

Geology is such a progressive science