520 WHITMAN CROSS 



Hawaiian volcanic province was characterized by basaltic or allied- 

 lavas for a very long geologic period, and that in comparatively 

 recent times the first highly siliceous and feldspathic magma appeared. 

 Even now the basaltic emissions continue with no known recurrence 

 of the trachytic magma. 



General significance of the occurrence. — The long sequence of 

 basaltic or allied lavas, which has probably occupied a long geologic 

 period, is in striking contrast with the succession of widely differing 

 lavas, ranging from highly siliceous and alkalic rocks like rhyolite 

 or trachyte, to basic basalts, produced during the same time in 

 various continental provinces adjacent to the Pacific Ocean. To be 

 sure, there is much greater variety in the dark Hawaiian lavas than 

 is implied in the common term given them, but this trachytic magma 

 is certainly highly exceptional, if indeed there has ever been another 

 eruption of equally salic material in the history of the group. 



To the modern petrographcr this occurrence must at once suggest 

 many interesting problems of petrogenesis. One of these will be the 

 query as to whether magmatic differentiation has produced this 

 unusual lava or not. The extreme advocates of this process, con- 

 cerning which so much has been assumed and so little proved, will 

 no doubt treat it as a matter of course that this magma, so exceptional 

 for the Hawaiian province, is one of the complementary products of 

 differentiation. This view has, however, not much to support it in 

 the history of the older centers, and until further examination of the 

 chemical characters of the "basalts" of this province have been made 

 elaborate discussion of this point has no good foundation. It remains 

 a matter for speculation as to why the process of differentiation has 

 not long ago produced a greater variety of lavas, or, assuming that 

 the differentiation has taken place, why the products have not been 

 emitted at the more recent centers of volcanic activity. Directly 

 connected with these questions arises also that of possible funda- 

 mental differences between the subjacent magmas of the Hawaiian 

 and the continental provinces. 



In all of these problems the relations of the Hawaiian trachyte to 

 similar rocks of other localities will be of use. The table already 

 presented gives a series of analyses of allied rocks, and in that follow- 

 ing are the norms calculated from those analyses. The latter table 



