EDITORIAL 157 



and this furnishes a working basis for such inferences as are dis- 

 cussed in Professor Schwarz's article. These are representative of 

 the lines of interpretation that have most prevailed under the domi- 

 nance of the doctrine of a primitive molten condition. 



If, however, the earth be supposed to have been built up by the 

 accretion of planetesimals, with a concurrent differentiation into 

 continental and oceanic segments through the agency of weatheriDg 

 and transportation, as recently suggested,' the differentiation of 

 specific gravities between the sub-oceanic and the sub- continental 

 segments may extend to a possible depth of 1,500 to 1,800 miles. 

 With such a depth of differentiation, a difference of surface pro- 

 trusion of three miles only requires an average differentiation of 

 specific gravity of about one-fifth of one per cent., a difference quite 

 beyond detection lithologically. If the depth of effective differentia- 

 tion were much less, as is not improbable, the average specific gravity 

 of the sub-oceanic rocks would still need to be only slightly greater 

 than that of the sub-continental ones to meet the requirements of 

 the case. 



When we consider the wide range of variation that was hkely 

 to be introduced by selective fusion and by the magmatic differentia- 

 tion of the extruded rocks of both the sub-oceanic and the sub- 

 continental segments, it does not seem safe to infer that any spe- 

 cific class of rocks would be excluded from either area. The probable 

 differences between the rocks of the two areas would be detectable 

 merely by a greater preponderance of heavy rocks in the sub-oceanic 

 areas and of lighter ones in the continental. If a compilation of 

 available data, made some years ago at the suggestion of the writer, 

 is to be trusted, the preponderance of basic rocks in the extrusions 

 of the oceanic volcanoes, present and past, is quite as high as the 

 planetesimal hypothesis requires, and the discovery of a notable 

 percentage of granitic and other acidic rocks is quite consistent 

 with this hypothesis. 



Under the planetesimal hypothesis the segments now beneath 

 the ocean were, at an early stage, parts of the land surface alike with 

 the continental segments. They are presumed to have been gradu- 

 ally converted into ocean bottoms by surface differentiation, leading 



I Chamberlain and Salisbury, Geology, Vol. II, pp. 106-11. 



