Presidential Address. 211 



the composition of the volcanic and plutonic rocks, as well 

 as from such chemical experiments as those of Daubr^e 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-coloured, and correspond- 

 ing 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 cor- 

 responding 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. 2 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 dis- 

 covery by Nordenskiolcl of native iron in Greenland basalts, 

 affords a probability that the inner magma is in part me- 

 tallic. 3 



(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 



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



2 Phillips, Manual of Geology, 1855, p. 493. Dr. Sterry Hunt has 

 kindly directed my attention to the fact of Phillip's right of pri- 

 ority 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 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. 



