334 Reports & Proceedings — Geological Society of London. 



section of a succession of lava-flows, including trachyte, trachytoid 

 phonolites, kaiwekites, trachydolerites, and basalts. It appears that 

 all the lavas were erupted from the same vent, although the surface 

 featui'es do not indicate the point from which they Avere extruded. 

 Each sheet is covered by a bed of scoria, the general coarseness of 

 Avhich proves that the centre of volcanic activity was not far distant. 

 The author gives a detailed account of the succession of twenty-five 

 flows and the field relations, which leaves no doubt as to the order of 

 extrusion ; but the main portion of the paper deals with the 

 petrography" of the lavas. The lowest lava is a trachyte composed 

 entirely of anorthoclase-felspar, and is succeeded hj a phouolite in 

 which sanidine is the most conspicuous mineral. This is followed by 

 a series of ten basalts of moderately basic character. The next flow 

 is a kaiwekite, a lava of entirely difi'erent type, in which a hornblende 

 allied to barkevikite forms the largest crystals. It also contains 

 titaniferous augite bordered with segirine, and phenocrysts of 

 labradorite and anorthoclase. The lava above contains no pheno- 

 crysts, the pyroxene is segirine, the hornblende is senigmatite, and 

 there is an abundance of nepheline. Then follows a series of basalts, 

 including two flows of trachydolerite. The lower trachydolerite 

 contains large phenocrysts of nepheline and sodalite, with only a 

 little felspar in the ground-mass. The basalts are succeeded by 

 a phonolite which contains a few phenocrysts of anorthoclase. 



The author then discusses the chemical composition and classification 

 of the lavas. He points out that in the lowest trachyte lime and 

 magnesia are practically absent, but that the second lava (phonolite), 

 although still deficient in these constituents, shows a distinct advance. 

 The basalts as a whole are low in magnesia and above the average in 

 alkalies. In the kaiwekite the alkalies advance very considerably, 

 and there is a corresponding increase in silica and decrease in lime and 

 magnesia, features which are still more marked in the succeeding 

 phonolite. He regards the kaiwekite as a connecting link between 

 the basalts below and the phonolite above. The higher basalts are in 

 general somewhat richer in alumina and poorer in lime than those 

 which occur lower in the section. 



The lower trachydolerite compares chemically with the kaiwekite, 

 but the presence of nepheline makes it somewhat richer in soda and 

 alumina, and relatively poorer in silica. The final phonolite compares 

 closely with that near the bottom of the section, but is less alkaline 

 than that near the middle of the series. 



The majority of the rocks fall into well-known and readily 

 recognized groups. The lowest lava is a trachyte which may be 

 regarded as the effusive representative of a bostonite. The lowest 

 and highest phonolites, being poor in nepheline, are of the trachytoid 

 type, but the central phonolite is almost a nephelinite in character. 

 The basalts are more or less of the normal type and call for no 

 special comment, but the afiinities of the kaiwekite and the 

 trachydolerites are discussed in some detail. 



The author considers that the porphyritic rocks of intermediate 

 composition, such as the kaiwekites and trachydolerites, may have 

 formed from an undifferentiated magma, a consideration that requires 



VI 



