TRACHYTE ON THE ISLAND OF HAWAII 517 



more recent basaltic lavas. In those of Mount Hualalai I observed 

 only inclusions of dunite, olivine-gabbro, pyroxenite, and basic 

 augite-andesites. 



If further exposures of the trachytoid rocks are found, it seems 

 to me probable that they will be in the area of the Waimea plain 

 which extends practically from Puu Anahulu for twenty miles north- 

 easterly to the north base of Mauna Kea, or in the northern and 

 oldest basaltic section of the island, the Kohala Mountains. 



Trachytic cones like Puu Waawaa may exist near the bases of 

 Mauna Kea or Mount Hualalai. Their materials, if of the black 

 glass or aphanitic rock described, might naturally be considered as 

 basaltic, if not subjected to chemical examination. 



Puu Waawaa has the appearance of being a cone built up by 

 explosive eruption of one short period, like the scores of basaltic 

 cones which dot the slopes of Mauna Kea and Mount Hualalai, and 

 which seem to belong to a period of local volcanism following that of 

 lava outpourings. The materials of Puu Anahulu is agglomerate- 

 like and is certainly not a part of a simple flow, but its crystalline 

 texture shows also that it is not like the tuffs of Puu Waawaa. In all 

 probability, therefore, there was in the comparatively remote past of 

 the island of Hawaii a period of trachytic eruptions of a magnitude 

 not now to be ascertained, but possibly of much importance. 



A broader significance of this discovery is connected with the 

 history of the Hawaiian group. While no one of the larger Hawaiian 

 islands has been thoroughly investigated as to the range in com- 

 position of the lavas which have built it up, it is hardly probable 

 that trachytoid rocks occur upon them. It is, therefore, highly inter- 

 esting that the most recent island of the group, Hawaii, should 

 exhibit this unique rock. 



In discussion of this question, it must be understood that the 

 island of Hawaii is not only the largest, but is also, in the current 

 view, the youngest, and lies at the extreme southeastern end of a 

 zone of oceanic islands which geologically belong together, extending 

 for fifteen hundred miles or more to the west-northwest, toward the 

 coast of Japan. The larger islands, commonly referred to in speak- 

 ing of the Hawaiian group, are all located in the eastern end of this 

 zone. The smaller ones, stretching out for more than a thousand 



