THE HISTORY OF THE NORTH ATLANTIC 63 



plastic for the purposes of volcanic action and superficial move- 

 ments. 



(3) The plastic sub-crust is not in a state of dry igneous 

 fusion, but in that condition of aqueo-igneous or hydrothermic 

 fusion which arises from the action of heat on moist substances, 

 and which may either be regarded as a fusion or as a species 

 of solution at a very high temperature. This we learn from 

 the phenomena ^of volcanic action, and from the composition 

 of the volcanic and plutonic rocks, as well as from such 

 chemical experiments as those of Daubre"e, and of Tilden, and 

 Shenstone. 1 It follows that water or steam, as well as rocky 

 matter, may be ejected from the under-crust. 



(4) The interior sub-crust is not perfectly homogeneous, but 

 may be roughly divided into two layers or magmas, as they 

 have been called ; an upper, highly silicious or acidic, of low 

 specific gravity and light-coloured, and corresponding to such 

 kinds of plutonic and volcanic rocks as granite and trachyte ; 

 and a lower, less silicious or more basic, more dense, and 

 more highly charged with iron, and corresponding to such 

 igneous rocks as the dolerites, basalts, and kindred lavas. It 

 is interesting here to note that this conclusion, elaborated by 

 Durocher and Von Waltershausen, and usually connected with 

 their names, appears to have been first announced by John 

 Phillips, in his " Geological Manual," and as a mere common- 

 sense deduction from the observed phenomena of volcanic 

 action and the probable results of the gradual cooling of the 

 earth. It receives striking confirmation from the observed 

 succession of acidic and basic volcanic rocks of all geological 

 periods and in all localities. It would even seem, from recent 

 spectroscopic investigations of Lockyer, that there is evidence 

 of a similar succession of magmas in the heavenly bodies, and 

 the discovery by Nordenskiold of native iron in Greenland 



1 Phil. Trans., 1884. Also Crosby in Proc. Boston Sac'. Nat. Hist. , 

 1883. 



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