GEOLOGY OF THE ATLANTIC OCEAN. 43 



2. We have reason to believe, as the result of astronomical investi- 

 gations, that, notwithstanding the plasticity or liquidity of the under- 

 crust, the mass of the earth — its nucleus, as we may call it — is practi- 

 cally solid, and of great density and hardness. Thus we have the 

 apparent paradox of a solid yet fluid earth ; solid in its astronomical 

 relations, liquid or plastic for the purposes of volcanic action and 

 superficial movements. 



3. The plastic sub-crust is not in a state of dry, igneous fusion, but 

 in that condition of aqueo-igneous or hydro-thermic 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 tempera- 

 ture. 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 Daubree and of Tilden and 

 Shenstone. 



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 siliceous or acidic, of low specific gravity and light- 

 colored, and corresponding to such kinds of plutonic and volcanic 

 rocks as granite and trachyte ; and a lower, less siliceous or more 

 basic, more dense, and more highly charged with iron, and correspond- 

 ing 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 re- 

 cent spectroscopic investigations of Lockyer, that there is evidence of 

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

 covery by Nordenskiold of native iron in Greenland basalts affords a 

 probability that the inner magma is in part metallic. 



5. Where rents or fissures form in the upper crust, the material of 

 the lower crust is forced upward by the pressure of the less supported 

 portions of the former, giving rise to volcanic phenomena either of an 

 explosive or quiet character, as may be determined by contact with 

 water. The underlying material may also be carried to the surface 

 by the agency of heated water, producing those quiet discharges which 

 Hunt has named crenitic. It is to be observed here that explosive 

 volcanic phenomena and the formation of cones are, as Prestwich has 

 well remarked, characteristic of an old and thickened crust ; quiet 

 ejection from fissures and hydrothermal action may have been more 

 common in earlier periods, and with a thinner over-crust. 



6. The contraction of the earth's interior by cooling and by the 



