The Field of Geology— McGee. 201 



the rocks which it is reasonable to assume — the arches are too loDg 

 and rest too heavily upon the terrestrial nucleus to convey crushing 

 strains to their extremities without greater compression about their 

 keystones. The "contraction hypothesis" must therefore be rejected, 

 at least as a quantitatively adequate cause of terrestrial deformation. 



There is an alternative hypothesis. A dozen geologists have 

 shown that lines of mountain growth commonly coincide with 

 zones of rapid deposition during former times, and that in these 

 zones the deposition was accompanied by depression (thus fore- 

 shadowing the later conception of consequent diastatic movement); 

 they have shown further that in consequence of the combined 

 sinking and thickening of the crust the surfaces of equal tempera- 

 ture within the earth — the isogeotherms — unquestionably rose 

 through the sediments until strata formed at the temperature of 

 the sea bottom were heated to hypogeal temperatures; and they 

 inferred that the consequent expansion of the sediments developed 

 stresses whereby further heat was generated, and that the rocks 

 were thus corrugated, flexed and sometimes metamorphosed. This 

 hypothesis has had currency for a generation. It has been com- 

 monly questioned, however, whether the assumed cause is commen- 

 surate with the observed effect, whether the expansion of sedimen- 

 tary beds by local rise of isogeotherms from time to time and from 

 place to place is sufficient to explain the extensive and profound 

 corrugation observed in the mountains of the earth, the shortening 

 of the Alpine arc by 120,000 metres as measured by Heim,and the 

 shortening of the Appalachian arc by 60 miles as estimated by 

 Ciaypole. Quite recently, however, Reade has pointed out what 

 the early advocates of the hypothesis had overlooked, viz: that 

 since the strata are confined in two directions, any expansion due 

 to rise of temperature must take place all in one direction, and that 

 a given rise of temperature would produce thrice the elevation and 

 perhaps thrice the corrugation inferred by the older geologists; 

 and the hypothesis has thus been rendered more acceptable. 



Singularly, Reade and all of his predecessors, save J. Herschel, 

 practically neglect the most important factor in the series of move- 

 ments contemplated in the hypothesis; and even Herschel's case 

 is hardly the general one: lines of sedimentation are the margins 

 of continents; in each case the sediments are laid down not upon 

 a horizontal surface but upon a seawardly sloping bottom; more- 

 over the sediments do not form a horizontal surface, but take a 



