Vol. 70.] LAVAS AT THE NOETH HEAD, OTAGO HABBOUB. 393 



nearly all different from those of the previous lava. The felspar is 

 sanid'ine, the pyroxene is aegirine, the amphibole is cossyrite, and 

 there is also an abundance of nepheline. 



There is again a succession of several basalts, the lowest of 

 which is, however, distinctly less basic than the rest. After six 

 basalt-flows another complete change recurs, and a trachydolerite 

 is encountered. In this are large phenocrysts of nepheline 

 and sodalite, and very little felspar, which is entirely restricted 

 to the ground-mass. There is much indication of the former 

 presence of hornblende, which has been entirely resorbed, and the 

 pyroxene is segirine-augite. 



* Again, there are two more basalts, one of which is an exception- 

 ally large flow, and a trachydolerite is then again found. In this 

 rock there is a little anorthoclase and an abundance of small crystals 

 of barkevikite, although they still show conspicuous resorption and 

 have the same intimate relation with the violet-tinted pyroxene. 

 Finally, there is another phonolite, but in this case it contains a 

 few- crystals of anorthoclase. 



The order of succession, which is so conspicuous here, is found to 

 be not infrequent elsewhere in the neighbourhood. Thus, on the 

 west side of Wickliffe Bay, at the See House and at Long Beach, 

 there is a similar succession from an intermediate rock, with very 

 large crystals of hornblende, followed by a dense phonolite and 

 afterwards by a basalt. 



III. Chemical Composition. 



The actual composition of each of the rocks is shown in the 

 accompanying table (pp. 394-95). The following statement 

 emphasizes those points that appear to call for special notice. 



The lowest rock, a trachyte, is practically wanting in lime and 

 magnesia, and contains little iron. The second lava, a phonolite, 

 shows a distinct advance in those constituents, but is still somewhat 

 deficient in them. The basalts, as a whole, are low in magnesia 

 and above the average in alkalies, though Xo. 6 is an exception 

 and is distinctly the most basic rock of the whole series. In the 

 kaiwekite (Xo. 13) the alkalies advance very distinctly, with a 

 corresponding increase in silica and decrease in lime and magnesia. 

 These features are still more marked in the succeeding phonolite — 

 in fact, chemically the kaiwekite is simply a connecting-link 

 between the basalts and the phonolite. This phonolite is the most 

 alkaline rock of the whole series. 



The basalt (Xo. 15) which succeeds the phonolite is distinctly 

 less basic than the rest of the basalts, and in some measure marks 

 a transition between the alkaline and the basic rocks. The rest of 

 the basalts are, as a whole, a little . higher in alumina and lower in 

 lime than those that lie beneath the middle phonolite. 



The trachydolerite (Xo. 21) is comparable with the kaiwekite, 

 although the soda and alumina are somewhat higher and the silica 

 distinctly lower, as would be expected from the presence of 



