L04 PKOF. P. MA11SHALL OX THE SEQUENCE OF [Dec. 1 9 1 4, 



and Mesozoic times. ... (4) Very old rocks, often the Archaean complex, 

 occur in or very near every alkaline province. ... (7) For reasons given, the 

 theory is advanced that alkaline rocks are derived from, in the first place, 

 Archaean saline beds which, by chemical attacks on the adjacent sediments, 

 have given rise to an alkaline magma in the process of metamorphosis. This 

 magma has been squeezed laterally into the continental areas and has under- 

 gone differentiation, or it has mixed with other mag-mas, chiefly basic, and has 

 then been differentiated. 



It appears heroic to attempt to apply No. 1 to the case of Tahiti. 

 for the soundings round the island are all of great depth, and no 

 structure that can possibly be regarded as the old plateau exists 

 nearer than Australia. In Dunedin it is true that the alkaline 

 volcanic rocks occur on the border of a schist-area. I have, how- 

 ever, eke where given reasons for the belief that these schists are of 

 Mesozoic age. 1 As to i, xcvy old rocks are not known anywhere 

 near the alkaline rocks of the Pacific Islands. In New Zealand 

 the supposed Archaean gneisses are 150 miles distant. With regard 

 to 7, the statement of the origin of alkaline rocks is partly based 

 on 1 and 4, and must stand or fall with them. 



Dr. \i. A. Daly tentatively ascribes the presence of trachytes 

 and trachydolerites among the basalts of Hawaii to changes in the 

 normal basalt arising from its solution of small quantities of sedi- 

 mentary limestone cut by the respective lava-conduits. Generally, 

 he has stated more recently that 



' The segregation of alkalies and the observed desilication of subalkaline 

 magmas are largely explained by the assimilation of the natural carbonates. 

 It is conceived that the whole of the carbonates may take part in the appro- 

 priate chemical reactions, or that sometimes the subalkaline magma is affected 

 only by the carbon dioxide that is given off by magmatic heat from invaded 

 limestone into the volcanic vent or other magmatic chamber." - 



In Tahiti no included fragments of limestone have yet been 

 found in the volcanic rocks. In the magmatic chamber syenites, 

 essexites. monzonites, theralites, peridotites are all found, and it is 

 probable that all these divergent types result from the differentiation 

 of one original basic magma with a high content of alkalies. So 

 far as the required limestone is concerned, appeal would have to 

 be made to the Globigerina ooze that certainly covers the sea-floor 

 in the neighbourhood of these islands. 



Near Dunedin the only limestone known is a highly siliceous 

 rock. No fragments of this have been found in the volcanic rocks, 

 and there is a notable absence of types of siliceous rocks in the 

 volcanic series. In addition, the relations of the rocks of this series, 

 as described above, seem to demonstrate that the phonolites and 

 basalts have both been derived from an essexitic magma by mole- 

 cular differentiation. 



1 ' New Zealand & Adjacent Islands ' in the ' Handbuch der Ixegionalen 

 (ioologie' vol. vii, pt. 1, Heft 5, p. 18 (Heidelberg, 1911). 



2 ' Origin of the Alkaline Rocks ' Bull. Geol. Soc. Am. vol. xxi (1910) 

 p. 115. 



