330 R. A. DALY- — PETROGRAPHY OF THE PACIFIC ISLANDS 



These and allied facts have prompted the suggestion that the solution 

 of sedimentary material in subalkaline magma has led to the development 

 of many of the so-called alkaline rocks. 7 Limestone is here the most 

 significant of the sediments, on account of its fluxing power, its inoculat- 

 ing power, and its extremely high content of volatile matter. Yet other 

 kinds of sediment would in less degree affect the chemical equilibrium 

 of a subalkaline magma in which they might be dissolved. According to 

 the writer's hypothesis, the most potent "magmatic mineralizers" engaged 

 in concentrating the alkalies while forming alkali-rich submagmas are 

 largely "resurgent" rather than "juvenile" in origin. This recognition 

 of syntexis and sedimentary control in the development of such rocks as 

 soda trachyte, phonolite, foyaite, etcetera, seems to supply the lack noted 

 in Smyth's hypothesis. 



Cross and Marshall have rejected the writer's explanation of the alka- 

 line rocks of Hawaii and Tahiti respectively. Each is doubtful that the 

 volcanic vents supplying the alkaline magmas pass through limestone 

 which could be assimilated in the required quantity. 8 But the percentage 

 of limestone so dissolved in a large body of hot basalt may be very small, 

 in order to cause the development of a flow of phonolite or a flow of the 

 strongly contrasted melilite basalt. Can any one doubt the possibility 

 that much calcareous material is included in the submarine portions of 

 the Hawaiian and Tahitian volcanic masses ? Is it extreme to hold that 

 enough of this deep-lying sediment is present to match the relatively 

 minute volumes of alkaline rocks in Hawaii and Tahiti? Clearly the 

 origin of the alkaline rocks can not yet be demonstrated, but little prog- 

 ress toward that demonstration is made by implying or asserting that no 

 limestone beds have been cut by volcanic vents in islands like Hawaii 

 and Tahiti. 



Other rather remarkable arguments have been published against the 

 hypothesis of limestone control. One of them may be quoted: "If an 

 alkaline rock cuts a limestone in dikes or stocks, this fact proves that that 

 limestone did not have anything to do with the alkalic character of the 

 cross-cutting body." 9 To a believer in magmatic stoping and abyssal 

 assimilation this "proof" carries no conviction. 



However, this is not the place to discuss the objections in detail ; suffice 

 it to say that those so far announced have not shaken the grounds on 

 which the sedimentary-control hypothesis of the alkaline rocks has been 

 founded. 



7 R. A. Daly : Bull. Geol. Soc. Am., vol. 21, 1910, p. 87 ; Igneous rocks and their ori- 

 gin, New York, 1914, pp. 393-445. Cf. C. H. Smyth : American Journal of Science, vol. 

 36, 1913, p. 33. 



8 W. Cross : Professional Paper 88, U. S. Geol. Survey, 1915, p. 90 ; P. Marshall : 

 Trans. New Zealand Institute, vol. 47, 1915, p. 372. 



9 W. Cross : Op. cit., p. 90. 



