280 8: Powers — Notes on Haivaiian Petrology. 



Toward the close of the evolution of the volcanic edi- 

 fice another change appears to take place occasionally — 

 the appearance of the more unusual rock types. Trachy- 

 andesite, in contrast to basalt, is found on the summits of 

 Haleakala and Mauna Kea ; trachyte forms a veneer over 

 portions of West Maui and Mauna Kea (where it has 

 been overrun by Hualalai and Mauna Loa flows) ; neplie- 

 lite-bearing rocks appear in the young tuff cones and 

 flows on East and West Palm, in a young crater on Kauai, 

 and probably in the youngest tuff cones on other of the 

 older islands. These rarer types are almost completely 

 lacking, so far as known, in the older rocks of any of the 

 main volcanoes. The occurrences of these rarer types, 

 which must be differentiates of basalt, seem therefore to 

 indicate that such differentiation is characteristic of the 

 closing stages of the Hawaiian vulcanism and that this 

 differentiation proceeds separately in separate volcanoes 

 or possibly contemporaneously in pairs of connected vol- 

 canoes. Each volcano has arisen at an intersection in a 

 fracture system in the earth's crust, has been fed from the 

 same primal magma, and has finally lost connection with 

 this source. When this takes place differentiation may 

 proceed in the magma chambers of the larger volcanoes 

 and the extreme products of Hawaiian vulcanism, nephe- 

 lite basalt and trachyte, may appear either at the close 

 of the main vulcanism or in a later phase after extensive 

 erosion. 



