376 DYNAJMICAL GEOLOGY. 



Origin of the Earth's Form and Features. 



This embraces first, the origin of the shape of the earth's mass ; secondj 

 the origin of continental plateaus and oceanic depressions, and of all move- 

 ments in the earth's crust through geological time not involving orogenic 

 work ; and, third, the origin of the movements producing the upturning of 

 formations and the making of mountains. 



The first of these subjects, geogenic work, pertains to astronomy. The 

 movements referred to under the second, by which wide changes of level 

 have occurred without special orogenic results, except displacements along 

 old or new fracture-planes, have been termed by G. K. Gilbert epeirogenic, or 

 continent-making (1890). The work included under thethird head is orogenic, 



1. GENERAL CONSIDERATIONS BEARING ON THE EARTH'S FORM. 



1. Solidification of the earth. — The earth solidified from the center out- 

 ward. This conclusion is established on the evidence that pressure raises 

 the fusing point of rocks. The globe was, therefore, never in a state of 

 complete liquidity. According to Clarence King, experiments made for him 

 by C. Barus with reference to the question as to the earth's rate of cooling 

 (see page 1026), lead collaterally to the conclusion that the depth of the 

 liquid exterior of the globe has at no time exceeded 50 miles. 



The study of meteorites has led some astronomers and writers on the constitution of 

 the globe to the opinion, in view of the iron in these bodies, and the fact that their place 

 in the solar system is to a large extent near that of the earth, that the earth's interior con- 

 sists, for the greater part, of iron. This view is favored, also, by the high percentage (10 

 to 14) of iron oxide in most igneous rocks ; the existence of much native iron in doleryte 

 at Disco Island, Greenland ; and the occurrence of the greatest of iron-ore beds of the 

 world in the oldest rocks, the Archsean. Platinum, gold, silver, and copper are heavier 

 metals ; but it is remarkable that they are not brought up among the constituents of erup- 

 tive rocks, as iron is, but are obtained from the supercrust and its veins : as if these metals, 

 in consequence of being in vaporizable combinations, or those of comparatively little spe- 

 cific gravity, were near the surface of the fused globe, while below these were the iron and 

 whatever, under the conditions, could form alloys with it. If the earth is two thirds iron, 

 or iron to within 500 miles of the surface (without much increase in the density of the iron 

 downward), and the rest were made chiefly of basaltic, or dolerytic, material, it would 

 have about its ftresent specific gravity, 5-5. 



The complete solidification of the earth is held to be its present condition 

 by most physicists who have recently discussed the subject. This implies that 

 the crust that was formed over the surface of the liquid stratum by cooling 

 had continued to thicken until the whole was solid. The evidence favoring 

 the earth's essential solidity has been obtained by investigating mathemati- 

 cally the amount of deformation which the sphere, if a liquid mass enveloped 

 in a thin crust, should undergo during its revolution ; and also the effect of 

 such tidal movement in the earth's mass on the height of the oceanic tides. 

 Kelvin concludes, on these grounds, that the earth must have an effective 

 rigidity at least as great as that of steel (1862, 1872). G. H. Darwin has 



