574 



THE DISTRIBUTION, ETC., OF ALKALINE ROCKS, 



;5 



greatly faulted. The Mt. Flinders area near -Ipswich is an 



analogous instance. 



If a basic rock-mass (B) be intruded into an alkaline one 



(fig. 9, iv.) and remelts the alkaline body (A) and thereby loses its 



heat and solidifies, the 

 alkaline lavas expelled will 

 contain inclusions of the 

 basic rock. This might 

 account for the basic xeno- 

 liths in The Spire ortho- 

 clase basalt (W.67) and in 

 the Dingo Creek (Nande- 

 war) lamprophyric monchi- 

 quite, and the common 

 occurrence of basic inclu- 

 sions in alkaline rocks as 

 observed in Brazil, Portu- 

 gal and elsewhere. 



On these hypotheses the 



succession can easily be 



explained and understood. 



The volcanic succession 



observed in the Nande- 



wars, the Fassifern district, 



is exceedingly common in alkaline areas. 



Island and in many Antarctic 



Fig.9 (i. ii. iii. iv.). — Diagramatic repre- 

 sentation of the formation of mixed mag- 

 mas by the development of a subsidence- 

 area (rift valley) in a plateau undergoing 

 secular contraction. 



a. Zone of sedimentary rocks— b. Zore of alkaline 

 rocks and Archaean schists— c. Zone of acid igneous 

 rocks— d. Zone of basic igneous rocks— e. Nucleus — 

 X and y. Points of eruptions of alkaline, mixed and 

 basic magmas — A. Alkaline rock — B. Basic rock. 



Kerguelen 



Mittagong, etc. 

 It occurs at 



islands, also in Bourbon. " Observations by R. von Drasche," 

 says Geikie* " have shown that at Bourbon (Reunion) during the 

 early submarine eruptions of that volcano crystalline rocks 

 (gabbros) were erupted, that these were succeeded by andesitic 

 and trachytic lavas, but that when the vent rose above the sea- 

 level basalts were poured out." A similar sequence has been 

 shown to exist in the islands of Palma and St. Paul.f 



* Text-Book of Geology, 3rd Ed., p.253. 

 t Tschermak's Min. Mitth. 1876, pp.42 and 157. 



