GENESIS OF THE ALKALINE ROCKS 107 
thick limestones do underlie the lavas of Oahu, and it is scarcely 
credible that during the slow growth of the volcanic piles through 
2,500 fathoms of warm Pacific water there should be no inter- 
bedding of calcareous oozes, coarser shell deposits, or coralliferous 
limestones. The magma of a volcanic vent, locally fluxed through 
such a composite, could not fail to incorporate some calcareous 
material. The remaining question is as to how much assimilation 
is ‘“‘sufficient to produce any notable result.” The answer is— 
comparatively little. Since alkali-rich rocks are very rare in Hawaii 
and, so far as known, of small individual volumes, the absolute 
amount of limestone assimilation need be very slight and in any 
case quite local. 
The syntexis of basalt and limestone is positively suggested 
by the several occurrences of nephelite-melilite basalt in and 
around Honolulu, where deep borings have proved the existence 
of thick limestones which must have been traversed by the conduits 
of these lavas. In the same region are nephelite basalts. As 
Cross states, it may be ‘‘certainly true that the alkali-rich lavas 
are not present about Honolulu,” but the first step in limestone 
syntexis is obviously not an alkali-rich magma. ‘That can originate 
only under conditions allowing drastic differentiation. According 
to the present writer’s view, the flows of melilitic and nephelitic 
basalts are quenched phases, erupted before much concentration 
of alkalies in the vent was possible. As Bowen agrees, melilite is 
possibly a direct sign of syntexis with limestone rather than of 
pronounced differentiation. Ten years ago Becker published the 
hypothesis that the melilite in the basalts of the Wartenberg and of 
Southwest Germany in general have resulted from the absorption 
of calcareous sediments.’ 
Tahitt—Marshall, by actual field work, has well supplemented 
Lacroix’s petrographic studies in Tahiti2 In the central pipe 
of the island he found an alkali-rich syenite associated with wehrlite 
and gabbro. ‘This stocklike or necklike body traverses the flows 
™ Cf. N. L. Bowen, op. cit., Suppl., XXIII (1915), 89; R. A. Daly, Igneous Rocks 
and Their Origin (New York, 1914), p. 436; E. Becker, Zeit. deut. geol. Gesell., Band 
LIX (1907), 273. 
2P. Marshall, Trans. New Zealand Inst., XLVII (1915), 361. 
