464 E. S. Dana — Petrography of the Sandwich Islands. 



the presence of minute scales of a dark brown mineral, prob- 

 ably biotite, which, however, is only present very sparingly. 



Another interesting specimen (30) which was obtained from 

 the top of Haleakala is a thin, almost schistose rock, light gray 

 in color and presenting the same sort of an aggregation of feld- 

 spar and augite under the microscope. Chrysolite, however, 

 is a prominent constituent especially in the hand specimen. 

 There are also large elongated but usually ill-defined aggregates 

 of magnetite grains marking the presence of original large indi- 

 viduals, biotite or hornblende, which have been re-absorbed 

 into the magma. Occasional remnants of the original mineral 

 are noted but in very small amount. Another curious feature 

 of this rock is the presence of a zone of augite about the grains 

 of chrysolite. One case of this is illustrated by fig. 4, n. 

 The chrysolite crystal though separated into different parts has 

 throughout the same optical orientation as indicated by the 

 shading, while that of the augite varies from grain to grain. 

 The mantle of magnetite grains about the upper end of the 

 chrysolite seems to represent the remains of the augite which 

 has disappeared, This re-absorption of augite is not commonly 

 observed, but this case, and still more another one where of a 

 single augite crystal alone a large part has disappeared in this 

 way, places the matter above doubt. This zonal arrangement 

 of the augite about the chrysolite has been noted by other 

 observers in a number of cases.* 



The structure and composition of both these last mentioned 

 rocks suggest that they should perhaps be classed among the 

 augite-andesites rather than the basalts. To decide this point 

 we have the silica determinations, for which I am indebted 

 to Mr. Henry L. Wheeler of the Sheffield Scientific School. 

 He found in the first (No. 29) 48*42 p. c. Si0 2 , and in the 

 other 50*44 p. c, which conform to that of normal basalt. 



The remaining specimen from the top of Haleakala is a dark 

 gray, almost black, rock of the finest grain, very compact and 

 breaking with a conchoidal fracture. It is characterized by 

 the large amount of iron in minute grains very thickly distrib- 

 uted, so as to make the section nearly opaque unless extremely 

 thin. The feldspar microlites are the most prominent constit- 

 uent, and these show a rather distinct fluidal arrangement. 

 The two specimens from Paia on Maui are much like those from 

 Haleakala just mentioned, especially ]STo. 29, and like it they 

 bear the same resemblance to andesite. A curious point about 

 them, is their readiness to alter, the exposed surfaces passing 

 into a soft earthy mass of a light brown color. 



The specimens from Western Maui, collected by Rev. S. E. 

 Bishop, are rocks of peculiar and interesting character. Mr. 



*See F. D. Adams, Amer. Naturalist, 1087, 1885, G-. H. Williams, this Journal, 

 xxxi, 35, 1886. 



