122 



GEOLOGY. 



when its meaning shall be demonstrated. The climax of the phenomenon is 

 found in the iron-bearing basalts of Greenland, in which little masses of the metal 

 are scattered promiscuously through a stony base in meteoritic fashion, as shown 



Fig. 326. — A pronounced example of igneous rock composed of minerals of very dif- 

 ferent specific gravities that show no signs of separation by differences of gravity. 

 The light portion is native iron, which has more than twice the specific gravity of 

 the basalt, the dark portion, through which it is promiscuously scattered. From 

 Kaersut, Nugsuaks peninsula, Greenland. (After Phalen, Smithsonian Misc. 

 Coll., Vol. I, p. 199.) 



in Fig. 326. In this case the specific gravity of the metal is more than twice 

 that of the embracing stony matter. 



II. Deformations under the Planetesimal Hypothesis. 



Relative to deformation two great eras are to be recognized, that of growth 

 and that following the cessation of growth. During the stages of growth the 

 chief cause of change of bodily form is assigned to the accession of material. 

 This enlarged the geoid, and at the same time tended to compress it by the addi- 

 tional gravitative pressure. Heat arose from this compression, and, until it was 

 dissipated, tended to prevent further compression. There was thus an antago- 

 nism between the opposing processes of growth and shrinkage, but during this era 

 the former dominated. 



When growth ceased to be appreciable, this type of diastrophism ceased to be 

 dominant, but its effects were more or less perpetuated. In the subsequent era, 

 the chief agencies of deformation are thought to have been a re -aggregation of 

 material, with some extrusion, a re -distribution of heat in the interior, and a loss 

 of heat from the surface. Incidentally there was some generation of new heat as 

 in the previous era, but this is not presumed to have equalled the loss. 



This twofold division of earth-history may be regarded as having given rise 



