AN OCCURRENCE OF TRACHYTE ON THE ISLAND OF 



HAWAII.^ 



The Hawai an Islands are commonly described as consisting almost 

 wholly of basaltic lavas, though in truth there is a known wide range 

 in the rocks thus broadly characterized. These lavas have issued 

 as flows from many centers, building up great volcanic mountains. 

 The eruptions have continued for a long time with gradually shifting 

 scene of action, and agencies of degradation have reduced some ancient 

 basaltic mountains to mere reefs, while other volcanic piles are now 

 like deeply dissected models, showing their constitution to the very 

 core. With all this opportunity to examine the products of Hawaiian 

 volcanoes of various epochs and at many centers, no one has, to the 

 writer's knowledge, found and described any other than basaltic or 

 related lavas as existing in the entire group. However, during the 

 summer of 1902, while the writer was engaged in a reconnaissance 

 of the Hawaiian Islands for the Geological Survey, he was fortunate 

 enough to discover an occurrence of trachyte which, from several 

 standpoints, seems to merit description. 



The locality at which this unique rock was found is on the island 

 of Hawaii, at the northern base of Mount Hualalai, one of the great 

 basaltic volcanoes of the island, the last eruption of which took place 

 in 1801, from a low-lying point near that at which the trachyte 

 occurs. The accompanying map shows the geographic position of 

 the trachyte locality. 



Mount Hualalai is a basaltic cone of the Mauna Kea type, rising 

 8,000 feet above the sea, consisting mainly of lava flows, but dotted 

 with numerous small cinder cones and punctured by perhaps as many 

 remarkable "pit craters." The cinder cones are most numerous 

 near the summit of the mountain, and, so far as the writer is aware, 

 the products of all these recent and local outbursts are basaltic in 

 character. The later lavas on the northern slope are olivine-rich 

 plagioclase basalts. 



I Published with the permission of the Director of the United States Geological 

 Survey. 



