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



the brachypinacoid and hence showing no other kind of twin- 

 ning. The form of one of these groups is shown in fig. 12. 

 The cleavage marks the position of the basal plane and the 

 angle of the section (about 80°) shows that it is bounded by the 

 planes c (001) and y (2OI). The extinction makes an angle of 

 a few degrees with the basal edge, varying + or — with a slight 

 change in the direction of the section. This optical character and 

 the further fact that the acute bisectrix is nearly normal to the 

 brachypinacoid would make the feldspar an oligoclase. Occa- 

 sional feldspar individuals are cut more nearly parallel to the ba- 

 sal plane and have the usual elongated form, and show the twin- 

 ning like the other specimen, but as a rule they all lie nearly 

 parallel to the brachypinacoid. The amount of silica present, as 

 determined b} T Mr. Wheeler, is 61*63 p. c, which corresponds 

 to the microscopic determination. This remarkable feldspathic 

 andesite is a totally different rock from any other which has 

 been as yet obtained from the islands, and the writer hopes 

 to be in the position later to give a more minute account of its 

 occurrence and composition. 



5. Lavas of Oahu. 



Of the specimens in hand from the island of Oahu, six 

 (33, 36, 40, 41, 44, 45) are from the Kaliuwaa valley, near Pu- 

 naluu on the north side of the island ; four (27, 38, 39, 43) are 

 from the Waialua plain ; one (42) from a point just north of 

 Kahuku Bluff ; another (37) from a gulch beyond Monolua, 4 

 miles west of Honolulu ; and, finally, there are a number of 

 specimens of the tufa from the Punchbowl near Honolulu. 

 Among these specimens, two are forms of highly chrysolitic 

 basalts ; these are the specimens from Kahuku Bluff and one of 

 those from near Waialua. In the first of these (42) the chrys- 

 olite makes up probably two-thirds of the mass of the rock ; it 

 is present in distinct isolated crystals, having the characteristic 

 form, each crystal having a rather broad, rusty border, though 

 the interior is for the most part clear and unchanged. The 

 chrysolite incloses grains of iron, but very little glass. The 

 ground mass is a fine-grained mixture of augite and plagioclase 

 with considerable iron, the augite being the more prominent 

 constituent. 



In the specimen from Waialua (43) the chrysolite is also 

 prominent ; its specific gravity is 3*06. With the chrysolite, the 

 augite and feldspar also occur in large individuals besides 

 being present in the base. The feldspar here contains dark- 

 colored glassy inclusions in large numbers arranged parallel to 

 the vertical axis. The base is a confused mixture of dirty 

 brown augite and feldspar, with iron in considerable amount. 

 The specimen (33) from a dike in the upper part of the Kal- 



