?0 NEW YORK STATE MUSEUM 



in protuberant areas, which became the continental platforms. 

 These original differences were maintained as the earth was built 

 outward. This theory leads to the view that the earth is composed 

 of a number of conical sectors (see postea, p. 141), whose apexes 

 lie deep in the central core of the earth, While the ocean basins form 

 the bases. These continental and oceanic sectors are. assumed to 

 be in approximate hydrostatic equilibrium, thus giving a form of 

 isostasy in which the compensation extends downward more than 

 a thousand miles (Barrell, 19 19) and which postulates a perfect 

 constancy of oceans and continents (Chamberlin, 1914; 131)- 

 Barrell (ibid.) has pointed out that the demonstration of this theory 

 depends upon competent mathematic investigation and especially 

 the quantitative measurement of the relations of the density of the 

 crust of the earth to the relief of it's surface. 



Meanwhile Barrell (191 8) has advanced a view of his own, which 

 Schuchert (19 19, p. 49) who accepts it calls the volcanic theory. 

 According to this theory the continents and oceans arose in Archeo- 

 zoic time, but from densities in ocean bottoms and continents that 

 are but " skin deep," reaching to about 150 miles in the outer shell, 

 and that result from a molten condition of the earth at the com- 

 pletion of its growth. This condition (Barrell, 1918, p. 26) is, in 

 contrast to Chamberlin's theory, derived from the chemical character 

 of the igneous rocks, the limited depth of density variations in the 

 crust, the limited amount of salt in the sea and the rotation periods 

 of the moon and planets. Barrell pictures to himself the process 

 of the formation of ocean basins in such a way, that fractional crys- 

 tallization caused first a density stratification of the crust, the heavy 

 basic crystals which formed first tending to work down. The re- 

 maining magma was then siliceous and the original crust conse- 

 quently granite. Into or through this crust broke, in certain regions, 

 dense molten matter from the depth of the earth, on a gigantic 

 scale, eruption following eruption until the widespread floods had 

 weighted down broad areas and caused their subsidence into ocean 

 basins. This action went forward with widening radius, leading 

 to the circular outlines characteristic of ocean basins and leaving 

 angular segments of the original lighter crust as continental plat- 

 forms. 



Schuchert (1916, 19 19, p. 50) estimates that the continents were 

 formerly 25 per cent larger, and that the periodic shrinking of the 

 earth causes a periodic enlarging and deepening of the oceans, that 

 can be traced back at least to Permian time. While the continents 



