310 G. D. LOUDERBACK STRUCTURE OF THE HUMBOLDT REGION 



The feldspar occurs in moderately small laths, .1 to .5 millimeter, 

 rarety .6 to .7 millimeter, and generally shows simple albite twinning, 

 although not infrequently crystals with four or more alternations can 

 be found. Careful examination by the statistical method show T ed a 

 maximum extinction angle, in symmetrically extinguishing twins, of 35 

 degrees (another 34 degrees), corresponding to a medium labradorite. 

 Zonal structure is rare, and shows only three or four la}' , ers. Two Carls- 

 bad twins were seen with halves twinned according to the albite law. 



Olivine is rather abundant and occurs in grains of varying size from 

 .2 to 1 millimeter, with intermediate gradations. Some of the grains 

 show a partial development of crystal planes. When fresh it is a very 

 pale green, almost colorless, but is universally stained yellow or reddish 

 brown on the edges, or along cracks, frequently throughout the whole 

 mass. In general, the grains are traversed by irregular cracks, but a few 

 show both pinacoidal cleavages — (010) and (100). Extinction is straight, 

 and the optic axial plane is perpendicular to the cleavages. The maxi- 

 mum birefringence is about 035 and refractive index high. In general, 

 it appears to have formed earlier than the feldspar, though occasionally 

 laths of the latter were found penetrating the olivine. 



The pyroxene is of a peculiar pale grayish brown color (quite unlike 

 the ochreous or reddish brown of the olivine) and shows a good cleavage- 

 Its grains are rather small and are interstitial as regards the feldspar, 

 and therefore xenomorphic. It is not pleochroic, has a high refractive 

 index, and a maximum birefringence of about .020. High extinction 

 angles were measured up to about 40 degrees. It is evidently augite. 



Magnetite in cubes and grains or in crystalline aggregates in the form 

 of strings or masses is quite abundant. It does not occur in a fine dust. 

 It is included in such a way by the feldspars, etcetera, that it appears 

 to have been the first mineral separated from the magma. 



With the exception of the stained olivines, there is practically no de- 

 composition of the rock except on the very surface. 



There is a rather distinct flow arrangement, especially of the feldspars, 

 and these latter include between them the augite in small, irregular, grain- 

 like, or elongated anhedrons. The rock, in short, is an olivine dolerite, 

 with a fine grained diabasic structure showing a fluxional arrangement 

 of the particles. 



Another Summit rock. — About a mile north of the above described rock 

 specimens were taken which answer the same description throughout 

 except that larger laths, which show a zonal structure, are considerably 

 more abundant. 



West Base basalt. — In megascopic characters this rock, which occurs a 

 mile or more north of the mouth of Muttleberry canyon, is very like the 

 summit rocks, except that the feldspars are more easily discernible. 



