BY H. I. JENSEN. 583 



of the ground-mass, the micrographic intergrowths of quartz and 

 felspar in the base, the occurrence of minerals produced by pneu- 

 matolysis such as riebeckite, arfvedsonite, etc., and titanium, 

 zirconium, and fluorine minerals, and the occurrence of chloride 

 and sulphate minerals in the phonolites are all strong arguments 

 for believing that old hydrous, sulphate- and chlorine-rich 

 sediments have been remelted or assimilated to produce the 

 magma. 



The rare minerals which occur so commonly, though it be in 

 small amount, in alkaline rocks are in every instance minerals 

 which do not typically belong to pure plutonic magmas. They 

 are best developed in regions of contact-metamorphosis and in 

 pegmatite-veins. Such is the case with scapolite, vesuvianite,. 

 guarinite, analcite (vide anted) t arfvedsonite, etc. The peculiar 

 dendritic (paisanitic) or mossy (poikilitic) structure of the 

 alkaline amphiboles, and the corroded nature of any stray pheno- 

 crysts of them which one may meet with, and their frequent 

 alteration to haematite or ferrite, show that crystals of these 

 minerals can only form under very peculiar circumstances, and 

 are again destroyed on immersion in a moving fluid or pasty 

 magma. 



The occurrence of spinels, garnets with kelyphitic borders and 

 similar minerals in alkaline trachytes and tinguaites is also a 

 feature pointing to the assimilation-theory. 



The foregoing features observed in Australian alkaline rocks 

 are equally well observed elsewhere in other parts of the world. 



Of all the alkaline areas outlined in this exposition only some 

 of the following are definitely known to be older than the Triassic: 

 Australia — The Kiama-Jamberoo alkaline basalts (shonkinose 

 and monzonose). 



Europe — None are known to be Pre-Triassic, but the Mon- 

 chique, Laurvig, Kola, and Miask areas may be. 



Asia — A Paleozoic leucite rock has been described from Tun- 

 guska, Siberia, by K. von Chrustchoff,* and the same author has 



* See Min. Mag. x. 



