MAGMATIC DIFFERENTIATION IN HAWAII 297 



domagnesic subrang, in the permiric section and permirlic rang 

 of the dofemane order, hungarare. If a name for this type in the 

 Norm classification is desired, the subrang may be called hilose, 

 from the name of the chief port of the island, Hilo. The corre- 

 sponding names for the rang-section and rang would be hiliase 

 and hilase. The hitherto unnamed section of the order may be 

 called hawaiiare. 



According to the Mode classification the rock is an ultra-femic 

 olivine basalt of an extreme type. In both chemical and min- 

 eralogical analyses it approaches the still more abnormal type 

 represented in the Uwekahuna laccolith. 



Andesitic basalt, upper slope of Mauna Kea. — On the eastern 

 side of Mauna Kea, from the 6,000-foot contour to about the 

 12,000-foot contour, the abundant lava flows seem to be very 

 uniformly composed of a rock species which is intermediate between 

 typical olivine basalt and a true augite andesite. These lavas 

 are almost entirely of the aa or blocky type; pahoehoe surfaces 

 are only locally developed and, within the area described, seldom, 

 if ever, show the perfection so often illustrated in Mauna Loa. 

 Among the specimens collected, one taken at the 1 1,000-foot con- 

 tour (see locality "3" in Fig. 1), 4,500 meters S 75 E of the sum- 

 mit of Mauna Kea, was selected for chemical analysis. Its descrip- 

 tion would doubtless apply, with but unimportant change, to the 

 average lava of all this part of the great volcano. 



The rock is dark gray, fresh, and strongly vesicular, again 

 showing the great irregularity in the size and distribution of the 

 vesicles, which is usual with aa lava. The only minerals micro- 

 scopically visible in the dense ground-mass are a few phenocrysts 

 of yellowish-green olivine and tabular plagioclase, with maximum 

 diameters of 2 mm. and 3 mm. respectively. In thin section a 

 few idiomorphic phenocrysts of augite, reaching 1 mm. in length, 

 are to be seen. Estimates made by the Rosiwal method show that 

 the olivine phenocrysts form no more, or little more, than one 

 per cent of the rock by weight, and that the augite phenocrysts 

 occur in about the same proportion. The very abundant plagio- 

 clase phenocrysts have cores averaging about Ab x An r in composi- 

 tion. They are often surrounded by a very thin shell of oligoclase 



