MAGMATIC DIFFERENTIATION IN HAWAII 



3°5 



198 analyses of fresh basalts taken from Osann's great compi- 

 lation for the world (analyses published between 1884 and 1900). 



TABLE VI 



SiO, . 



Ti0 2 



A1 2 3 



Fe 2 3 



FeO . 



MnO 



MgO 



CaO 



Na 2 



K 2 . 



H 2 . 



PA 



Average Hawaiian Basalt 



49 



100.59 



Average World Basalt 



49 



The close correspondence of the two averages is obvious at a 

 glance. In fact, it has been found that the greater the number 

 of reliable analyses included, the nearer the Hawaiian average 

 approaches the world average. Though perfect averages might 

 show the former to be slightly the more femic of the two, it is 

 certain that the staple igneous type in the mid-oceanic Hawaii 

 are chemically very similar to the average basaltic magma poured 

 out on the continental plateaus. 



THEORETICAL CONSIDERATIONS 



Origin of the ultra-femic types. — In columns 1 and 2 of Table 

 VII the analyses of the Uwekahuna laccolith and of the 1852 lava 

 flow are respectively entered. Column 3 gives the mean of these 

 two analyses. Column 5 gives the average analyses of four typi- 

 cal lherzolites, calculated as water-free. Column 6 shows the 

 calculated composition of the average Hawaiian basalt, while 

 column 4 gives the mean of columns 5 and 6. 



A comparison of the markedly similar columns 3 and 4 suggests 

 that the ultra-femic magmas of the island are due to the mixture 

 of a large amount of the ferromagnesian and cafemic (calcium- 

 iron-magnesium) constituents of the basalt with the average basalt 

 itself, though, of course, not necessarily in absolutely equal pro- 



