198 O. C. JONES, GEORGE D. HUBBARD Vol. XXII, No. 7 



Hayford^'' and Bowie give the crushing strength of granite 

 as about 1,425 kilograms per square centimeter, which place 

 the zone of no cavities at ^bout 8 kilometers. Van Hise, work- 

 ing on this problem, using the crushing point of rock as 1,700 

 kilograms per square centimeter, obtained adepth of 10,000- 

 12,000 meters. 



GEOLOGIC AND vSEISMOGRAPHIC DATA. 



Geologic Climates. — It was the problem of variability of 

 geologic climate that caused the final overthrow of the Laplacian 

 hypothesis. There have been in geologic history seven periods 

 of climatic change. ^^ Four of these have been glacial periods: 

 the first two, early and late proterozoic, are well down the 

 geologic column, and indicate a cold climate early in geologic 

 history not at all reconcilable with a cooling globe, which is 

 implied by a molten interior. The other periods of climatic 

 change are the Silurian, Permian, Triassic, Cretaceous, Eocene, 

 and Pleistocene, of which the Permian and Pleistocene are 

 glacial. 



''Original Crust' \ — The great granite embossments of Scan- 

 dinavia, Canada, and other countries, formerly believed to 

 be the exposed surface of the original crust, have been shown 

 to be but intrusions in older sedimentary rocks, or in some 

 cases greatly altered sediments. ^^ The fact that there are now no 

 known exposures of the original crust, certainly none that 

 could have served as an original source of the sedimentary 

 rocks — such a lack of original feeding grounds, certainly mil- 

 itated against the idea of an original crust at all. 



It is obvious, too, that if our earth has been built up from 

 planetesimals which have been gathered in upon a nucleus there 

 will be no uniform gradation of material in any direction, nor any 

 zoning into concentric layers so that any one layer will be essen- 

 tially homogeneous as to density, melting point and composition. 

 Presumably this heterogenity is not as great as it was ages ago 

 when the infall of planetesimals was more frequent and when 

 the larger part of the accretionary accumulation was not so far 

 remote. 



"Hayford and Bowie: Spec. Rept., U. S. C. & G. S. 

 i^Pirsson and vSchuchert: Text Book of Geo!. 

 '^Chamberlin, T. C, 1916, .Smithsonian Rpt. 



