BY H. I. JENSEN. 523 



This theory, which, I think, it will be admitted, has a strong 

 scientific basis, is, of course, hardly capable of proof, but workers 

 on alkaline rocks are invited to test it for their special regions. 

 Its importance lies in the fact that it is consistent with the 

 Laplacian theory of earth-origin, and diametrically opposed to the 

 planetesimal. It therefore suggests another means of testing the 

 relative merits of the two theories. 



The view just advanced is in harmony with some of the ideas 

 of Durocher and G. T. Prior on alkaline rocks. 



Durocher divided magmas into two principal groups. Those 

 which drew their lavas from primary magma-basins he styled 

 "magmas of the first order"; those which drew their supplies 

 from smaller reservoirs he called "magmas of the second order." 

 Into the latter group he placed, with particular emphasis, magmas 

 which give rise to trachytes and phonolites. If we consider the 

 magmas of these smaller reservoirs to have arisen from the 

 refusion of primitive sedimentary rock, his view is in complete 

 accord with my hypothesis. 



In his paper comparing the foyaitic rocks of Abyssinia with 

 other alkaline rocks,* G. T. Prior describes them as conforming 

 to the Atlantic type, because such rocks are rather plentiful in 

 the Atlantic region. Although the term is unsuitable because of 

 its geographic significance, there lies a germ of truth in it, namely, 

 that the Atlantic Ocean is, for the most part, a senkungsfeld 

 which until as late as perhaps Middle Tertiary times constituted 

 one of the oldest and most durable of continents. Its disappear- 

 ance was due to trough-faulting and Assuring. The Atlantic is 

 therefore a destroyed continental plateau, just as the East African 

 province is one on the high road to destruction. 



The theory which I have put forward to account for the origin 

 of alkaline rocks is, however, at variance with the "primitive 

 fresh ocean " hypothesis advanced by Professor Joly in his 

 exceedingly pretty paper on an " Estimate of the Geological Age 

 of the Earth."f 



* Min. Mag. Vol.xiii. No. 61, Feb. 1903. 

 t Ann. Report Smithsonian Institution, 1899. p. 247. 



