8 REPOHT — 1886. 



arises from tho action of heat on moist suhstances, and which mcy either 

 bo regarded as a fusion or as a species of solution at a very high tempera- 

 ture. This we learn from tho phenomena of volcanic action, and from 

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

 chemical experiments as those of Danbree and of Tilden and Shenstone.' 

 (4) The interior sub-cmst 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 ' <w specific gravity and light coloured, 

 and corresponding to such kinds of >Tutonic 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 t)ie dolorites, 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 conformation 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 magmao 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 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 



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



* rhillips say3 (Manval of Geoloyj, 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 hypogetie. Certainly under particular areas of country are found t lence 

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

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

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

 pp. 5r)6 and 564, in that unpretending manner which was his wont. Dr. Sterry Hunt 

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

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

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

 detailed application to the remarkable volcanic outflows of Western America. 



' 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. 



