GEANULAE GOEE AND DIKES. 247 



The first two rocks of the series (1388, 1389) are yellowish gray and 

 crystalline, plainly composed of feldspar and ferromagnesian silicates, partly 

 biotite, with small phenocrysts of augite. In thin section they are medium 

 grained, grade 23 of Table XVII, Chapter III, and consist of lath-shaped, 

 and also short, rounded plagioclase crystals and some of orthoclase. The 

 feldspars are very fresh, but contain a crowd of minute crystals of magnetite, 

 biotite, pyroxene, and apatite. There is a large amount of ferromagnesian 

 minerals in the rock. They are augite and hypersthene in rounded and 

 irregular o T ains of variable size, besides biotite in very irregularly shaped 

 individuals, and much magnetite. There are a few phenocrysts of augite 

 crowded with magnetite grains in clouds and zones, and in some cases there 

 are characteristic rod-like inclusions. Magnetite, pyroxene, and biotite 

 frequently occur in irregular aggregations. Olivine is present in partly 

 serpentinized individuals. The resemblance of many of these character- 

 istics to those already described for some of the intrusive masses imme- 

 diately connected with the core will be recognized. The chemical 

 composition of this modification of the rock (analysis 1 on page 260) is like 

 that of basalt. • The next two varieties (1390, 1391) are coarser grained, 

 grade 27, with a slight modification of the previous structure, caused by 

 the presence of abundant phenocrysts of tabular labradorite in a ground- 

 mass with the structure just described. In places there is a poikilitic 

 structure, occasioned by small rounded plagioclases and pyroxenes being- 

 inclosed in a broad individual of unstriated feldspar, which is undoubtedly 

 orthoclase. These poikilitic feldspars act as the cement for the idiomorphic 

 plagioclases and often equal the feldspar phenocrysts in size. There are 

 traces of a graphic intergrowth of quartz in orthoclase. The rock is very 

 fresh and unaltered. There is much hypersthene, augite, and magnetite, 

 some biotite, and a little green hornblende. The labradorite is very clear 

 and fresh, with some transparent rectangular inclusions resembling glass, 

 and in some cases with many inclusions of microcrystals of pyroxene, 

 biotite, and magnetite. The augite has a purplish tone, is light colored, 

 with many rod- or needle-like inclusions in the larger individuals. Besides 

 the prismatic cleavage there is distinct pinacoidal cleavage. The hyper- 

 sthene is light colored, with the usual pleochroism. It is occasionally in 

 long prisms, with the color stronger along the margin. Some of the crystals 

 have needle-like inclusions. In both pyroxenes these inclusions exhibit no 



