236 BULLETIN OF THE 



Of the numbers given below, the first is that of the specimens as 

 now entered upon the Museum Catalogue, and the second, enclosed in 

 parentheses, is that by which the specimen is known in the records of 

 the Fish Commission. 



67526 (8110). Basalt. Wreck Bay, Chatham Island : near haci- 

 enda of Mr. Cobos. This is an ordinary basalt, consisting of a dark 

 gray vesicular mass, which in thin sections is seen to be made up of faint 

 wine-colored augites in very irregular forms, lath-shaped plagioclases, 

 small opaque particles of iron ore, and numerous large, clear, faintly 

 greenish blebs of olivine. 'No. 67527 (8111), from the same locality, 

 offers no difference worthy of note. 



67528 (8113). Basalt. Landing on northwest face of Malpelo Island. 

 This rock much resembles the last in mineral composition and structure, 

 with the exception that extensive alteration has badly obscured the 

 groundmass, and given rise to abundant chlorite, epidote, calcite, and 

 sundry ferruginous decomposition products. The presence or absence 

 of a glassy base cannot with certainty be determined. In the hand 

 specimen this is a finer grained, more dense rock than 67526 or 67527, 

 and shows small amygdaloidal cavities filled with a white zeolite. The 

 general aspect is that of an older rock than the others (perhaps a mela- 

 phyr) though obviously it will not do to speak too positively from an 

 examination of the section alone. 



67531 (8116). Andesite (1). Near Chatham Bay, on Cocos Island. 

 In the hand specimen this is a light brownish gray, fine-grained, in 

 some cases vesicular rock, studded with small (1-2 mm.) white specks 

 indicative of feldspar, but in which none of the constituents are of such 

 size as to be accurately determined by the eye alone, or even when 

 aided by the pocket lens. The thin section under the microscope 

 shows a dense aggregate of stout feldspars which are in part sanidins 

 and in part a soda-lime variety, obscurely striated, and occurring in 

 stout lath-shaped forms. The structure is indistinctly granular, and 

 occasionally slightlj' porphyritic through the development of the larger 

 feldspar above noted. So far as observed there is no interstitial glass. 

 Abundant red ferruginous and opaque hornblendes, granules of iron ore, 

 minute colorless apatites, and ferruginous decomposition products com- 

 })lete the list of determinable constituents. 



