104 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ogeneous to an extent not matched in the usual plutonic mass. Stand- 

 ard examples have been described at Electric Peak and Sepulchre 

 Mountain in the Yellowstone Park ; at the Lipari Island vents ; at 

 Tonopah, Nevada ; at the Kaiserstuhl in Baden, etc. 



The extinct Mauna Kea in Hawaii (alt. 4213 meters) affords a good 

 illustration of a lava column which has not contacted with important 

 sedimentary masses. In this case magmatic differentiation seems to 

 have played the most important role, though its heavy winter snow- 

 cap may have furnished a special condition for explosiveness in the 

 last stage of the cone's activity. (Figure 11.) The broad base of 

 this volcano, visible in the long sea-clififonthe north side of the island, 

 is essentially composed of olivine basalt of the fluent pahoehoe type. 

 From about the 1500-meter contour to the 3500-meter contour, the 

 slopes are chiefly underlain by flows of a much less femic basalt, with 

 some interbedded ash and breccia. Above the 3500-meter contour the 

 mass is very largely pyroclastic, and the leading petrographic types 

 are trachydolerite and andesitic basalt or basic augite andesite. (Plate 

 IV, B.) The following analyses show the march of the differentiation : 



