The Field of Geology— McGee. 205 



telligible. While yet the planet was young and its surface approx- 

 imately homomorphic and homogeneous, slight warping of the prim- 

 eval surface occurred, and low islands emerged, either synchronously 

 or independently, to form the continental nuclei; for under the hy- 

 pothesis this slight initial cause sufficed to set the entire process of 

 earth-differentiation in operation. The sun beat upon the low isl- 

 ands and checked the chilling of the rocks, which thus rose higher 

 and higher, while the neighboring seas grewdeeper and deeper; anon 

 the rain fell upon the rocks, triturated them, and carried their debris 

 to the perimeters of the infant continents which they absolutely de- 

 press3d though always less than their own thickness by reason of 

 their relative lightness, and as the sediment-choked sea shoaled, the 

 continents grew; then the isogeotherms rose under the continental 

 margins, and the sediments were still further lifted by consequent 

 deformation; with the heating from below the strata expanded lat- 

 erally as Avell as horizontally, and so slipped seaward on the sloping 

 bottoms and crumpled their perimeters which thus rose still further 

 above sea level and formed mountain ranges overlooking the sea on 

 the one hand and the original continental nuclei on the other; and 

 thus the continents expanded and the derived rocks being ever lighter 

 than the original, they have maintained approximately their original 

 positions, while the contin3nt-building movement has never ceased 

 to operate. With increase in area, the continents were separated into 

 tracts within which the geologic processes varied in activity, some of 

 the nuclei merged and the land masses became asymmetric; some 

 were temporarily submerged, and others may have become lost; +he 

 shores were shifted through long distances with the successive oscil- 

 lations of the growing land; and as the land area increased, isostatic 

 stresses were developed which affected the entire globe and perhaps 

 gave birth to new continents or brought death to old. At the same 

 time great mountain ranges were upheaved and great valleys exca- 

 vated, particularly about the continental margin, and consequent 

 diastatic movement supervened; in the interior of the continents the 

 primitive rocks were slowly degraded, modified in density and conduc- 

 tivity, and here, too, consequent deformation progressed; and various 

 subordinate movements occurred, which need not be considered here. 

 But in general the continents grew peripherally, their margins were 

 thezonesof activity while their centers were more stable, and the gen- 

 eral process was one of differentiation and levitation ; while the ocean 

 floor simultaneously shrunk and sank, the general tendency was to- 



