Geology and Mineralogy. 149 



In Chapters xi and xiv, the law of continental evolution 

 by gradual emergence is illustrated in the geological history of 

 Europe. 



In Chapter in the author gives a classification of mountains, 

 dividing them into mountains of accumulation, mountains of ele- 

 vation, and mountains of circumdenndation.* 



The chapter on Mountains presents some very interesting 

 views in regard to the decay of mountains under erosive agencies. 

 The weakness of anticlinals, and their tendency to degradation 

 is shown in a very striking way.f The tendency to the forma- 

 tion of synclinal ridges and anticlinal valleys is, nevertheless, 

 stated, in our judgment, rather too broadly. Davis has shown 

 that the preservation or reversal of the primitive topography of 

 anticlinal ridges and synclinal valleys depends on the particular 

 relation between hard beds and the base level of the streams. \ 

 An interesting suggestion of our author is that of the frequency 

 of disastrous rock-falls in the Alps and other young mountain 

 ranges, as compared with their rarity in old mountain regions.§ 

 In the latter case there has been time for the removal of all 

 especially weak and unstable structures. 



The Evolution of Climate is discussed in Chapter xn. Our 

 author follows Heilprin in doubting the sufficiency of Neumayr's 

 evidence of the differentiation of climatic zones in the Jurassic.|| 

 Cosmopolitan species are, however, less numerous in the Creta- 

 ceous than in earlier formations; and in the Tertiary the distinc- 

 tion of zones is unquestionable. *{ The temperate climate of high 

 northern latitudes even as late as the Tertiary our author attrib- 

 utes to the submergence of a great tract in Asia, of which the 

 Aralo-Caspian depression is a remnant, permitting warm currents 

 from the Indian Ocean to find their way into the Arctic.** 



Professor Geikie maintains Croll's theory of the cause of the 

 Glacial Period. Although the small amount of dry land in the 

 early periods, and the free movements of warm waters into Arc- 

 tic regions, rendered any general glaciation impossible, he yet 

 believes that, in epochs of high eccentricity, the temperature was 

 so far reduced as to develop ice action in some localities. Traces 

 of such action, he believes, are found as early as the Devonian, 

 in the Lammermuir and Cheviot Hills. ff 



II. The Glacial Pebiod. 



It is needless to say that Prof. Geikie adheres to the doctrine 

 that the characteristic phenomena of the Drift are due chiefly to 

 land ice. 'There is so general agreement among geologists on 

 that conclusion, that argument thereon is well nigh superfluous. 

 We will, however, call attention to the exceedingly interesting 

 discussion in Chapter vn, on The Intercrossing of Erratics in 

 Glacial Deposits. The objection sometimes still offered to the 



* Page 44. f Page 52. % Science, xiii, 320. 



§ Page 51. || Page 360. \ Page 364. 



** Page 365. ft Pages 87, 371. 



