1885.] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 31 



ON HORNBLENDE ANDESITES FROM THE NE"W VOLCANO ON 

 BOGOSLOFF ISLAND IN BERING SEA. 



By aEOROf: p. JTlEBRIIiL.. 



The rocks desoribed below were received from Lieut. G. M. Stoney, by 

 whom they were collected and donated to the National Museum. On 

 account of the interest just now attached to the locality they seem 

 worthy of a special description. It is well to remark in the beginning 

 that none of the samples received show freshly fractured surfaces, but 

 are in the form of irregular blocks with their corners broken and rounded. 

 They were accompanied and covered with a fine sand and dust of the 

 same mineral nature as the rocks themselves, but stained by sulphur 

 and iron oxides. Even in the absence of definite information on the 

 subject it seems safe to infer that they are simply ejected volcanic blocks, 

 and not from recent lava flows, none of which have as yet been reported. 



Two varieties of the rock were received, one consisting of a light-gray 

 slightly purplish, fine grained and porous grouudmass, in which small 

 glassy feldspars and dark brown and green hornblende-like crystals are 

 readily distinguishable by the naked eye. The texture is quite uniform, 

 the brown hornblende being the more variable constituent, in one case 

 a single crystal nearly half an inch iu diameter being observed. The 

 rock is rough to the touch and somewhat friable. Under the micro- 

 scope it is found to consist of a light gray groundmass, in which are em- 

 bedded deep reddish brown, strongly dichroic hornblendes, light green 

 augites, and numerous crystals of a plagioclase feldspar, together with 

 scattering grains of iron ore. The hornblendes are usually in irregular 

 crystals, though an occasional quite perfect basal section was observed 

 which showed a preponderance of the prismatic faces. The crystals are 

 often elongated in the direction of their vertical axes, and a portion of 

 them show the dark borders so commonly seen in the hornblende of an- 

 desites. The augite is light green in color and at first glance might 

 readily be mistaken for a green variety of hornblende. Its cleavage and 

 optical properties are, however, unmistakably those of augite. 



In form the plagioclases are short and thick, showing but few twinning 

 striations, sometimes none at all. A portion of them are clear and pel- 

 lucid, while others are clouded through the presence of numerous cavi- 

 ties and impurities. In many cases the outer portion of a crystal is 

 clear, while the interior is clouded, or again, both outer and interior 

 portions may be clear while there exists an intermediate zone full of 

 cavities. In addition to these the plagioclases contain numerous inclos- 

 ures of a yellowish glass, which often bears a bubble, and hornblende 

 and augite particles. A number of short and thick, clear, glassy feld- 

 spars are present, which show no sign of twinning, and which appear 

 from their optical properties to be sanidin. Both sanidin (?) and plagio- 

 clase show at times a very evident zonary structure. 



