10 REPORT 1886. 



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-crnst 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-coloured, 

 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 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 Walters- 

 hausen, 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 basalts, 

 affords a probability that the inner magma is in part metalHc.^ 



(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 



> Phil. Trans. 1884. Also Crosby in Proc. Boston Soo. Nat. Hist. 1883. 



2 Phillips says {3Ianval of 6eolo(/y, 1855, p. 493): 'If we regard them (the 

 internal crystalline rocks) as acquiring solidification by cooling in zones parallel to 

 the surface, we should have sheets of granitic and basaltic rocks generated below, 

 the first uppermost, the last undermost, while above the several strata were produced 

 in a series beginning at the bottom. In this sense the rocks of fusion may be called 

 with Lyell liypogene. Certainly under particular areas of country are found evidence 

 of the liquefaction of one set of igneous products after the solidification of others. 

 Many dykes of basalt traversing granite show themselves to have been in fusion after 

 the solidification of the granite.' In various forms Phillips returns to this idea, as at 

 pp. 556 and 564, in that unpretending manner which was his wont. Dr. Sterry Hunt 

 has kindly directed my attention to the fact of Phillips's right of priority in this 

 matter. Durocher in 1857 elaborated the theory of magmas in the Annales des Mines, 

 and we are indebted to Dutton, of the United States Geological Survey, for its 

 detailed application to the remarkable volcanic outflows of Western America. 



3 These basalts occur at Ovifak, Greenland. Andrews has found small particles 

 of iron in British basalts. Prestwich and Judd have referred to the bearing on 

 general geology of these facts, and of Lockyer's suggestions. 



