PETROGRAPHY OF BASALTS 311 



The feldspar occurs in laths, which are in general somewhat coarser 

 than those in either of the summit rocks. A maximum extinction angle 

 of 33 degrees was observed, which corresponds to a labradorite of about 

 the same composition. With the larger laths the polysynthetic character 

 of the twinning is much more marked, eight or ten alternations being 

 not uncommon, and zonal structure is also more frequent. Small feld- 

 spars also occur and there are all intermediate sizes. In other words, 

 two distinct generations can not be made out. 



The olivine is pale green and partly automorphic. It is stained yellow 

 to reddish brown, especially along cracks through the grains. The grains 

 occasionally show the pinacoidal cleavage with axial plane perpendicu- 

 lar to both. The maximum birefringence is about .035, and the re- 

 fractive index is high. 



The pyroxene is in larger grains than in the summit rocks, but it has 

 the same peculiar pale grayish brown color. It has a good pyroxene 

 prismatic cleavage and a high refractive index. The extinction angles 

 up to 34 degrees were measured, and a maximum birefringence of about 

 .021. The grains are almost entirely xenomorphic. It is evidently an 

 augite very similar to the summit augites. 



Magnetite occurs in cubes, grains, or elongated aggregates, and is evi- 

 dently the oldest separation from the magma, as it is included by the 

 other minerals. In structure the rock is noncrystalline, but, unlike the 

 summit rocks, it shows only an imperfect fluidal orientation of the feld- 

 spar laths. The augite occurs in distinct angular wedges between the 

 feldspar laths, and in many cases larger augite masses were seen to 

 wrap about the ends of the feldspar in true ophitic fashion. The rock 

 is an olivine dolerite with a diabasic structure only slightly affected by 

 flow. 



Another West Base rock. — About half a mile south of the mouth of 

 Muttleberry canj'on is a basalt hill. On the lower slopes the rock is 

 brown and has a different appearance in the field from the basalts in 

 general ; but microscopic analysis shows that it is made up of the same 

 minerals in practically the same proportions and with the same structures, 

 the coarseness of grain and appearance of flow being intermediate be- 

 tween the above described summit and base specimens. The olivines, 

 however, are all weathered and stained reddish brown throughout, which 

 probably accounts for the difference in general megascopic appearance. 



Resume of petrographical study. — Microscopical petrographical study 

 shows that the summit and base rocks are made up of the same minerals, 

 showing in detail the same optical peculiarities and with essentially the 

 same structures, the only difference being a slight one in the size of the 

 grain and the greater or less influence of fluxional movement, both 

 non-essential variations. 



