BOOK NOTICES. 



Here, as elsewhere in the volume, the illustrations are drawn largely 

 from the valuable publications of the United States Government, but 

 the mountain systems of the Old World and of South America are 

 also discussed. The point chiefly urged is that the upheaval of a 

 mountain range has always been preceded by the deposition of a 

 great thickness of sediment over the area in question. 



This is the starting point of the author's theory. Babbage, in 

 1834, showed that the addition of sediment to any part of the earth's 

 crust must cause a rising of the isogeotherms in the portion of the 

 crust immediately below, or, in other words, must raise the tempera- 

 ture of the subjacent rocks and cause them to expand. The author 

 points out that since lateral expansion would be impossible, folding 

 would ensue, and the whole of the expansion would take effect up- 

 wards. The vertical expansion of the mass would therefore be about 

 three times that due to a mere linear expansion as calculated by 

 Babbage. The author develops this theory, showing how the deep- 

 seated rocks would ' flow ' laterally and vertically, being forced up- 

 ward by their expansion, and lifting the newer rocks above them. 

 These latter would undergo but little vicissitude of temperature, and 

 would be subjected to a stretching action owing to the thrust from 

 below. This force would take effect along lines of weakness, the 

 axes of anticlinals, and the arched strata would be continually drawp 

 out at the expense of their thickness as the lower rocks were forced 

 up into the loops of the folds. According to this view, the calcula- 

 tions that have been made to determine the amount of lateral com-^ 

 pression undergone by such a district as the Alps would be fallacious, 

 since they assume the original length of the strata to be the same as 

 their present length measured over all the folds. Reversed faults are 

 ascribed by the author to the same compressive forces as the folds 

 and flexures ; normal faults to a subsequent contraction and partial 

 settling of the district. 



In Chapter IX the author examines his theory with the help of 

 numerical data, taking for the expansion of the rocks the mean result 

 of his own experiments, viz., 277 feet per mile for a rise of 100^ F. 

 He then attempts to show that the ' secular contraction ' theory 

 breaks down under numerical tests. 



The author next proceeds to apply his hypothesis in detail to 

 explain the characteristic features of mountain structure, beginning 

 with the evidences of repeated compression in the Archaean or Pre- 

 Cambrian rocks. We note a leaning towards what may be called the 

 Archaean heresy, when he expresses the opinion that the occurrence 

 of gneiss and ' true schist ' m great masses is prima facie evidence 

 that they are older than the oldest Palaeozoic rocks. ; 



May 1887. 



