MEMOIR OF J OSEPH LE CONTE 51 



The hypothesis assumes sufficient heterogeneity of composition of the 

 globe and unequal density as the causes initiating the greater subsidence 

 of the oceanic areas, and unequal heat conductivity and static equilibrium 

 as the conditions necessary to progressively increase the surface relief. 

 As a corollary to this, it follows that the continental masses and the 

 oceanic depressions are permanent features, as taught by Dana. 



The original heterogeneity, in efficient degree, would seem improbable 

 in a rotating globe built from superheated gaseous material commingling 

 and diffusing through long eons of time. This objection Le Conte met 

 by claiming that the efficient differences in density and conductivity were 

 very small and the resultant surface relief of the globe comparatively 

 insignificant. It is impossible to confidently contradict the assumption, 

 but it would seem as if the heterogeneity in a sphere of superheated 

 matter would not be represented by such large unit areas as the present 

 continents and oceans. 



According to the planetesimal hypothesis, the planet was slowly built 

 by accretion of cold particles and was always solid and always cold at the 

 surface. Whatever heterogeneity and irregularity of figure was produced 

 by unequal infall of the planetesinials, or by differences in their com- 

 position, could not be destroyed by liquid convection and diffusion ; but 

 original heterogeneity is not an important factor under this conception. 

 Professor Chamberlin postulates only such slight inequality or small de- 

 pressions in the surface of the embryo planet as would contain the shallow 

 waters of the primitive ocean. The exposed areas, subjected to weather- 

 ing processes, became relatively lighter in weight, while the suboceanic 

 areas, by protection from weathering and by concentration, became su- 

 perior in density. This difference in specific gravity of large areas by 

 epigone processes began when the growing globe was small, probably 

 smaller than the planet Mars, or as soon as it was able to retain its 

 atmosphere and hydrosphere; and the differential density and surface 

 relief has been perpetuated to the present time. Isostatic equilibrium 

 and comparative permanence of the continental masses follows under this 

 hypothesis the same as under the Laplacian. 



The contraction of the earth's crust as an effect of interior cooling and 

 shrinkage has been the subject of philosophic discussion since its recog- 

 nition by Descartes in 1644; but the first description and mapping of 

 extensive folding of strata in actual demonstration of great horizontal 

 compression was by the Kogers brothers, about IfilO, in their Appalachian 

 sections in Virginia and Pennsylvania, .\inerican geologists, and par- 

 ticularly Dana and Lo Conte, wore sndi active supporters of fhe con- 

 tractional theoi7 of mountain fnniKilion that it lias sometimes been called 



