ALKALINE ROCKS 307 



and Wichmann * have respectively collected and described alkaline rocks, such as 

 syenite and foyaite, from Fiji. Nephelinite occurs at Kaiatea, phonolite lava and 

 scorife at Raratonga, and typical nepheline basalt at Itutaki. Alkaline plutonic 

 rocks, such as nepheline-syenite, are also stated by Marshall, on the authority of 

 La Croix, to occur at Tahiti. Marshall states (oj). cif., p. 445) tliat the classi- 

 fication of coast-lines as Pacific and Atlantic in type is not yet accepted by 

 many authorities. He makes the very fair comment that the argument for con- 

 sidering the east coast of New Zealand to be of Atlantic type is at least as strong 

 as that for classing it as Pacific. He considers (op. cit., p. 446) that it apjaears 

 somewhat heroic to regard such isolated islands as Raratonga and Tahiti as frag- 

 ments of a fractured and shattered continent. Obviously Marshall concludes that 

 there is no conclusive evidence to justify the coast structure of New Zealand being 

 classed as wholly Atlantic or wholly Pacific, and he does not consider Pacific and 

 Atlantic types of rock a useful classification as applied to the petrology of New 

 Zealand. 



Dr. H. I. Jensen has argued f that alkaline rocks are chiefiy developed in areas 

 where there has been heavy crust foundering as the result of great faulting. He 

 considers that these serious displacements of the earth's crust may bring about a 

 fusion and extrusion of extremely ancient portions of the earth's crust containing 

 in them an excess of alkali derived from the heated waters of primaeval seas. Cer- 

 tainly the neighbourhood of the great horst is one of very vast tectonic disturbances 

 of the nature of block fiiulting. Dr. Jensen states that as the result of his study 

 of the sequence of Australian alkaline lavas he infers that "basic lavas may rise 

 along fractures and intrude themselves into the alkaline zone " of the earth's crust. 

 " Here they will assimilate alkali and also help to bring about an elevation of the 

 whole alkaline zone." He states, in the 23etrology of the alkaline rocks of Mount 

 Erebus in this Memoir, " In this paper therefore the origin of basic alkaline rocks 

 is ascribed to stoping, assimilation, and solution of alkaline sedimentary and meta- 

 morphic rocks by basaltic magma from the zone of basic igneous rock which is sup- 

 posed to underlie the whole of the earth's crust." 



Mr. J. Allan Thomson, in his chapter in this Memoir on the inclusions of the 

 volcanic rocks of the Ross Sea, shows that the keny tes of Ross Island contain not 

 only inclusions of Beacon Sandstone floated up from a strongly down-faulted, deep- 

 seated area, but also fragments of the quartz-dolerites of the sills, presumably of 

 Cretaceous Age. He states that " all the other inclusions (in tlie lavas of Mount 

 Erebus) are of igneous origin, and genetically connected with the alkaline rocks of 

 Ross Island. Many of them are types not met with at the surface as separate rocks, 



* A. Wichmann, " Ein Beitrag zur Petrographie des Viti Archipels," Tocher. Min. und Petiog. 

 Mitth., V. (1883). 



t Proc. Linn. Soc. N.S.W., vol. xxxiii. part 3, 1908, " On the Distribution, Origin, ic, of the 

 Alkaline Rocks." 



