from (rough's Island, South Atlantic. 381 



clase, the last two in two generations. The augite is of a pale 

 brownish, color, non-pleochroic and shows a marked dispersion 

 of the optic axes emerging from extinction, on one side bluish, 

 on the other yellowish in color. In the second generation it 

 is similar and scattered through the feldspar staves in rounded, 

 irregular granules and often attached to iron-ore grains. The 

 feldspar phenocrysts are tabular on 2 - 4(0L0), and from their 

 high extinction angles in the zone 100 a 001 and on the clino- 

 pinacoid (010) it is seen that they are to be referred to labra- 

 dorite. On the other hand the lath-like feldspars of the 

 groundmass are to be referred to andesite. The usual laws of 

 twinning abound. The other minerals present nothing worthy 

 of mention. In structure the rock is an interwoven mesh of 

 feldspar laths in which the other minerals lie imbedded. It is 

 rather of an andesitic habit owing to the prevalence of the 

 feldspar but from the abundance of olivine and the basicity of 

 the rock, a determination of silica having given 48 - 61 per cent, 

 it should undoubtedly be referred to basalt. Its specific grav- 

 ity is 2-890. 



The second type of this rock is macroscopically of a dark 

 grayish brown color, weathering to a brown, porous in texture 

 and thickly dotted with white broad lath-like phenocrysts of 

 feldspar which frequently attain a length of from 5-6 mm and 

 often group themselves into radial starlike clusters. In thin 

 section iron ores, apatite, olivine, augite and feldspars are dis- 

 closed. The olivine exists in two forms, in large irregular 

 phenocrysts attaining l mm in diameter and in small crystals 

 averaging about 05 mm . The larger phenocrysts are undergo- 

 ing a process of alteration into an orange-red colored mineral. 

 This proceeds inwardly along the edges and cracks and seems 

 to be in two stages which are best obser.ved under crossed 

 nicols. The first stage seems fibrous and gives a rolling ex- 

 tinction, until the second stage is reached where the mineral 

 again becomes apparently homogeneous and extinguishes as 

 such. In this product the orientation of the axes of elasticity 

 are different from the original. In the smaller olivines men- 

 tioned the process has been entirely completed and they all 

 appear of this orange-red color but appear otherwise homo- 

 geneous and extinguish parallel. A somewhat similar case is 

 mentioned by Michel Levy* as occurring in the basalts of 

 Mont Dore. It is probably conditioned by a change of ferrous 

 to ferric oxide and is somewhat like the coloration in the oli- 

 vines in the limburgite of Sasbach.f 



* La Chaine des Puys et le Mont Dore, Bull. G-eolog. Soc. France, 3d serie, 

 xviii, 1890. 



f Rosenbusch, N. J., 1872, pp. 59; also Physiog. d. Min., 1892, pp. 472. 



