3°° 



REGINALD A. DALY 



nable to the unaided eye. There is merely a hint at flow-structure, 

 registered in a rude parallelism of these phenocrysts. 



Under the microscope it is seen that a few, small, anhedral 

 olivines, and somewhat more numerous augite crystals — none, 

 in either case, surpassing i mm. in greatest diameter — -are to be 

 added to the abundant plagioclase (averaging labradorite, Abj 

 Am) in the list of phenocrysts. 



The ground-mass shows a confused crystallization of augite, 

 magnetite, and apatite, in a dominant felt of feldspar. A few 

 grains of an allanite-like mineral, pleochroic in tones from deep 

 brown to pale greenish-brown, form the only other accessory 

 material. No sulphide is visible in thin section. Most of the 

 ground-mass feldspar is plagioclase — acid labradorite or basic 

 andesine — -twinned on the albite law. Another feldspar arranged 

 interstitially in relation to the plagioclase has the low double 

 refraction and lack of twinning characteristic of orthoclase. This 

 mineral occurs in such minute individuals that a full demonstra- 

 tion of its nature has not been possible. Many of the labradorite 

 phenocrysts are surrounded with shells of alkaline feldspar with 

 extinctions on (oio) ranging from +5 to +o° 30', suggesting 

 orthoclase and soda-orthoclase, and it is very probable that both 

 of these represent the last product of crystallization in the ground- 

 mass. The total alkaline feldspar does not form much more than 

 15 per cent of the rock by weight. 



Mr. Steiger's analysis of this rock gave the proportions shown 

 in col. 1 of Table IV. 



By the Norm classification the rock is to be referred to andose, 

 the same subrang as that calculated for the andesitic basalt just 

 described. According to the Mode classification, this rock, con- 

 taining an essential amount of alkaline feldspar, is best included 

 among the trachydolerites, as defined by Rosenbusch, though 

 near the basaltic end of that series. In column 2 of Table IV the 

 average of the 34 analyses of trachydolerites, named as such in the 

 last edition of Rosenbusch's Elemente der Gesteinslehre, is given 

 for comparison. Column 3 gives Lyons' analysis of a more alka- 

 line, less femic, trachydoleritic type from the neighboring volcanic 

 pile in Kohala. 1 



1 A. B. Lyons, Amer. Jour. Sci., CLII (1896), 424. 



