116 II S. Washington — Basalts of Kida. 



The component minerals may be now described and we 

 begin with 



Feldspar. — This is constantly a plagioclase, and is sparingly 

 present. In the second period Kulaites it is found in lepto- 

 morphic grains forming, with the glass basis, the groundmass 

 of the rock. In the latest flows, on the contrary, it is con- 

 stantly in the form of lath shaped microlites in the glass basis, 

 generally very minute, but in some cases attaining a length of 

 0'l mm . They show fluctuation structure and seem to have been 

 among the last products of crystallization, being very spar- 

 ingly present in the tachylitic forms. This plagioclase, but 

 very rarely porphyritic, is shown by optical examination to be 

 bytownite. 



Augiie. — The largest, megascopically visible crystals are 

 light greenish yellow, and generally fragmentary. The greater 

 part though is in well formed crystals, from O'l— 0'4 mm long, 

 generally elongated parallel to <?, and showing the usual planes. 

 They are either colorless or of a very pale fawn gray, sometimes 

 with a core of greenish gray. The extinction is normal, 

 though in one case as high as 43°. The prismatic cleavage is 

 not well developed. As inclusions were seen glass and ground- 

 mass, rarely hornblende, and still more rarely magnetite, the 

 latter in a few cases arranged in small grains parallel and close 

 to the plane 5(111). Twins are not common. The well 

 known " hourglass " structure was well developed and some 

 interesting modifications were noticed. A zonal structure is 

 very frequent. Augite is also present as a microlitic con- 

 stituent of the groundmass, scattered thickly through the 

 glass basis, and showing fluctuation structure. 



Hornblende. — This, of all the constituent minerals, is the 

 most constant and characteristic, and offers the most points of 

 interest. It is present in relatively large quantity in all the 

 lavas examined, in the leucitic, as well as in the non-lencitic, in 

 the pure taehylytes and in the basalts of a normal structure — 

 it is in fact the stamp and seal of the Kula basalts. Mega- 

 scopically it is more frequent and prominent than either the 

 augite or olivine, the crystals sometimes reaching a length of 

 8 mm , showing all the usual characters of basaltic hornblende — 

 dark brown color, rounded edges and pitted surfaces. 



Under the microscope it is seen that it is always porphyritic, 

 never forming part of the groundmass. It is well formed, in 

 short stout crystals, showing the usual planes, and with the 

 cleavage well marked. The color is usually yellowish brown, 

 but occasionally, and this as a rule in the more glassy speci- 

 mens, it is greenish yellow. Pleochroism strong; (c) = dark 

 yellowish brown ; (b) = yellowish brown, (a) very pale gray, 

 (c) > {b) > (a). The angle of extinction varies from 4° to 14° 30', 



