U.S. Washington — Basalts of Kula. 117 



in one case being 23°. Zonal arrangement of the color is very 

 common in the larger brown crystals, a dark core being sur- 

 rounded by a light zone, or rarely several zones alternately 

 light and dark. The dark core occasionally shows a corroded 

 outline. Inclusions are not common, being chiefly glass, 

 which are frequently arranged parallel to the outer planes of 

 the crystal, or make up a central core of shape similar to that 

 of the host. Augite is sometimes seen. 



The alterations undergone by the hornblende are of great 

 interest and will be described at some length. Of cases of 

 mechanical distortion or simple corrosion nothing need here 

 be said. The chemical alterations — to be ascribed to mag- 

 matic action — fall under three heads. The first, observed 

 almost exclusively in specimens of scoriae, consists of a simple 

 darkening and reddening of the original crystal, without any 

 change of form. It need not detain us here. The other two 

 seem to be more or less connected with one another, and may 

 be separate stages of one process, or two similar processes. 

 The first consists of the replacement of the hornblende sub- 

 stance by a reddish brown mineral, in small elongated crystals, 

 accompanied by grains of colorless augite and black "opacite;" 

 in the other the outer part, or it may be the whole, of the 

 hornblende crystal is changed into a mass of augite and 

 opacite grains. This last is of very frequent occurrence in all 

 eruptive rocks, though more common in the basic ones, and 

 affects the biotite as well as the hornblende. The nature of 

 the black grains of the so-called " augite-opacite aggregate" 

 is not definitely known, but, with most observers, the writer is 

 inclined to think them magnetite. Though very frequently 

 the original form of the crystal is scarcely changed, yet in 

 some specimens from a second period stream, obtained from a 

 well-digging, at a depth of about 35 ra , the hornblende " pseu- 

 domorphs," as they have been called, have lost their original 

 shape, and are reduced to spheroidal or ovoidal masses, or as 

 one writer puts it, looks "as if they had melted and run" — a 

 natural consequence of the action of a moving lava stream on 

 the granular, and probably not very coherent, mass of altera- 

 tion products. 



The alteration to " brown mineral aggregate," on the con- 

 trary, is of quite rare occurrence, only thirteen references to 

 it having been found in the range of petrographical literature. 

 Its geographical range is large, though, as first noticed by 

 Zirkel,* in 1870, in some Eibel basalts, it has since been found 

 (always in hornblende basalts) in Saxony, the Rhone, Palma, 

 Syria, Madagascar, Kilimandjaro, and Cabo de Gata in Spain. 

 It has never been observed in biotite. 



* Basaltgesteine, p. 26. Cf. Rosenbusch Mik. Phys., 3" Aufl.. i, 5G0. 



