Page:The Journal of geology (IA journalofgeology11893univers).pdf/440

The preface to the third edition of this standard text-book states that it has been entirely revised and in some portions recast and re-written, so as to bring it abreast of the continuous advance of geological science.

A careful comparison of the third edition with the second indicates that this claim is fully warranted. The general plan of the volume is unchanged, but there are few discussions in which modifications do not appear. In many places the changes consist of nothing more than the addition or modification of a sentence or a paragraph. Even these minor modifications and additions are of great value, since in them are embodied many of the newer facts and ideas which recent research has developed. Thus we find the newer estimates of the average elevation of the continents; new suggestions concerning the age of the earth; the introduction of new descriptions of minerals of petrographical importance, and the modification of some upon which new light has been thrown by recent investigations; the adoption of Rosenbusch's terms for certain rock structures; the use of the word megascopic in place of macroscopic; a re-arrangement of rocks upon a genetic basis, as sedimentary, massive or eruptive, and schistose or metamorphic, and a better subdivision under these heads; throughout the descriptions of rocks, additions and improvements incorporating the more essential facts brought out in recent publications. The possibility of the metamorphic origin of some granites is minimized, and the probability that the greater number of them are eruptive is emphasized; the processes of metamorphism are elaborated, and the kinds of mineralization of common occurrence are pointed out. We find, too, new facts as to the amplitude of earthquake waves; the results of the more recent mathematical calculations concerning the distortions of the sea level by the attractive influence of land elevations; fuller statements as to the possibility of changes of sea level, and concerning the causes of oscillations of the level of land and sea; the conclusions to which experiment has led concerning the effect of hot water on the fusion temperature of rock; new ideas concerning the flow of rock as the result of crushing and pressure; clear cut statements growing out of recent discussions concerning the efficiency of glacial erosion; a multitude of facts at one point and another drawn from the reports of the Challenger, and from the reports of other deep