2 14 Notices respecting New Boohs. 



up, and often enclose in their folds strata of a newer age, which 

 have become thereby considerably metamorphosed. 



" It is only in the great or in the old mountain-ranges that these 

 old gneisses and schists are seen, because it is by denudation alone 

 that they become exposed " (p. 84). 



While agreeing with his predecessors as to the proofs of great 

 sedimentation prior to the formation of a mountain-chain, the 

 author to some extent differs from many of them in questioning 

 the necessity for that progressive subsidence which the majority 

 of geologists believe must have gone on side by side with the depo- 

 sition. In support of this view, Mr. Mellard Eeade cites the case 

 of the accumulation of strata probably of great thickness in deep 

 water off the mouth of the Amazon ; but he would probably himself 

 admit that such an explanation is only capable of being applied to 

 the occurrence of great thicknesses of clays, and not to alternating 

 strata of coarse- and fine-grained sediments, such as so constantly 

 constitute the materials out of which mountains are made. 



The second stage of mountain-making is explained by the 

 author as arising from the upward displacement of the isogeo- 

 therms, consequent upon the sedimentation in a particular area. 

 In this he follows the line of reasoning previously suggested with 

 greater or less precision by Scrope, Eabbage, Herschel, and other 

 authors. 



It is in applying this well-recognized principle to the explanation 

 of the contortion and crumpling of the thick masses of sediments 

 that the author shows much novelty in his treatment of the 

 question. Mallet and many other authors have insisted that the 

 tangential thrusts, by which the folding of the strata was evidently 

 produced, must have resulted from the contraction following from 

 the secular cooling of the globe, whereby the outer crust is con- 

 tinually tending to accommodate itself to the central nucleus. Our 

 author not only combats this view with many arguments that de- 

 serve to be very carefully weighed, but offers an alternative hypo- 

 thesis, which does not appear to be open to the objections to which 

 the older theory is liable. We cannot do better than allow the 

 author to explain this hypothesis in his own words. After insist- 

 ing that the rise of the isogeotherms is the necessary consequence 

 of excessive sedimentation, he goes on to say : — 



" The rise of temperature exerts a tendency to expand the new 

 sedimentaries, in every direction, in proportion to their extent and 

 mass. The tendency to expand horizontally is checked by the 

 mass of the Earth's crust bounding the locally heated area. The 

 expanding mass is therefore forced to expend its energies within 

 itself ; hence arise those foldings of lengthening strata, repacking 

 of beds, reversed faults, ridging up, and elevatory movements which 

 occur in varied forms, according to the conditions present in each 

 case. 



" The upper layers of the Earth's crust being less and less 

 affected by these variations in temperature as the surface is neared, 

 are by the ridging-up thrown into a state of tension, while the 

 lower beds of the sedimentaries are in a state of energetic com- 



