TRACHYTE ON THE ISLAND OF HAWAII 513 



Thin sections of the obsidian show it to be a colorless glass contain- 

 ing streams of feldspar microlites in some parts and free from them 

 in others. The dull aphanitic streaks and masses are largely crystal- 

 line, with more or less of fine magnetite dust and ferritic globulites, 

 and a colorless glassy base of variable amount. 



To ascertain the chemical character of this rock an analysis was 

 made by Dr. Hillebrand of the black glassy portion, with the result 

 given in column i of the subjoined table. It is clearly an almost 

 anhydrous, trachytic obsidian of close relationship to many other 

 known rocks, of which analyses are also given in the tables, a resem- 

 blance which will be discussed in a later paragraph. 



There is every reason to suppose that the associated pumice is of 

 the same character as the obsidian, since no other rocks were observed 

 among the fragments of the tuff.^ 



Turning now to the rock occurring in fragments at Puu Anahulu, 

 microscopical examination shows it to be a felsitic trachyte, holo- 

 crystalline in the specimens examined, of typical trachytic texture. 

 The feldspar microlites forming the greater part of the rock are 

 mainly very small, averaging but 0.02-0.05*"™ i^i thickness, less 

 than i'"'" in length, and, while some are prismatic, many are more 

 or less tabular in shape, causing by their nearly parallel fluidal 

 arrangement the characteristic semi-schistose texture of the rock, 

 which has already been commented upon. 



The feldspars are fresh, and among them may be recognized 

 sanidine, anorthoclase, and probably albite. The sanidine forms 

 somewhat larger microlites than the anorthoclase and albite, but a 

 distinct porphyritic texture was not observed. 



The feldspars make up more than four-fifths of the trachyte. The 

 norm of the obsidian analyzed indicates a possible content in nephe- 

 lite for the perfectly fresh rock, but a careful search for this mineral 

 failed to reveal it in the sections of the somewhat altered Anahulu 

 trachyte. The remainder of the rock consists of magnetite and 

 apatite, in small amounts, and two or more undetermined minerals 

 which occur for the most part in the angular interstitial spaces between 



^ The writer's observations at Puu Waawaa were necessarily quite limited and 

 leave much to be desired. He did not have time to explore the cone thoroughly, but 

 from the facts noted on the north and east slopes, it seems probable that Puu Waawaa 

 is composed solely of tracyhte tuff. 



