594 PROCEEDINGS OP THE AMERICAN ACADEMY. 



The groundmass under the microscope is seen to be composed of a filz 

 of violet tinged augite microlites and feldspar laths, presenting a very 

 homogeneous structure which can be designated as pilotaxitic. The 

 small augite prisms have rounded ends, and seem fully equal in amount 

 to the feldspar. 



The rock lacks phenocrysts of any size. The only mineral present 

 which is plainly of an older crystallization is olivine. This is well dis- 

 seminated ill the rock as small rounded sections whose original crystal 

 boundaries have been lost by corrosion. These sections are not much 

 larger than those of the groundmass. They have the common yellowish 

 to yellowish brown color from the iron staining, and only a few show 

 colorless centres. 



A small flake of biotite, which is nearly colorless normal to and deep 

 brown parallel to the polarizer, can be detected here and there in the 

 groundmass. 



Augite Andesite Lava from Komo. 



Komo is another island of the Eastern group, lying northeast of Kam- 

 bara. It is very small, one and a half miles long by half a mile wide. 

 The specimen was collected from the shore blutf, and is a black vesicular 

 rock. 



The groundmass shows a multitude of short rectangular sections with 

 a less number of augite grains in a brownish amorphous base which is 

 quite dusty from magnetite grains. There is a consideral)le inqji-egna- 

 tion of this base by brownish limonitic stains, and the same banded lilliug 

 of op-il in the cavities is noted. 



The phenocrysts are in the main large automorphic crystals of labra- 

 dorite. Many have been corroded by the magma, and most are filled 

 with dark glass inclusions. Some of the sections show alternate zones 

 of clear and cloudy layers. 



Auo^ite occurs, but is much inferior to the feldspar in amount. One 

 automorphic crystal has a pleochroic core, yellowish green to brown, 

 which resembles hypersthene, but no other indication of hypersthene was 

 seen in the slide. 



Olivine Andesite from Yanu Yanu. 



This is a very small island, two hundred feet high, of the Exploring 

 Isles. , 



The specimen presents a dark gray compact holocrystalline rock. It 

 lacks any definite porphyritic structure, although a few of its constituents 



