26 



University of California Publications. 



[Geology 



cleavages, one parallel to the twinning and the other more pro- 

 nounced at approximately the feldspar angle. The extinction 

 is nearly symmetrically 20° to the twinning lines, and the biaxial 

 figure is nearly central. A section possessing such characters 

 indicates it beyond doubt to be albite. Another crystal has char- 

 acters which suggest in a rock like this one the soda-lime feld- 

 spar, oligoclase. Sections from different portions of the rhyo- 

 lite vary somewhat as to ferro-magnesian content. Some speci- 

 mens contain hornblende with biotite, while other specimens, 

 from other localities, contain either alone. The biotite is often 

 bent, and fluidal structure is exhibited. It and the hornblende 

 show alteration to magnetite and hematite. Many of the feld- 

 spar phenocrysts show a marginal growth and a concluding 

 corrosive action. The quartz and feldspar usually show embay- 

 ments and inlets of the groundmass, and rounded corners. Sub- 

 sequently there was a fracturing of the phenocrysts and migra- 

 tion of sharp, angular pieces. The groundmass is red and cloudy 

 from inclusions. Spherulitic structure is shown to some extent. 

 From the opacity of the groundmass it is difficult to ascertain 

 if it is wholly crystalline or not. The phenocrysts make up about 

 one-third of the rock. The following analysis was made of this 



rock. Duplicate. Difference. Average. 



% % % % 



SiO, 69.49 69.46 0.03 69.47 



AL0 3 11.74 11.72 0.02 11.73 



FeA 5.11 5.06 0.05 5.08 



FeO 0.48 0.47 0.01 0.48 



MgO 1.02 0.96 0.06 0.99 



CaO 2.60 2.66 0.06 2.63 



K,0 5.40 5.29 0.11 5.34 



NaX) 3.45 3.47 0.02 3.46 



H,O<@105° C 0.31 0.32 0.01 0.31 



H,0@ig 1.28 1.28 0.00 1.28 



MnO present present not determined 



PA present present not determined 



100.88 100.69 100.77 

 The Basalt. — This rock occurs on the map PL 1 at the south 

 end of Sing-ats'-e Ridge and on Whirlwind Mountain, shown on 

 PI. 2. Two thin sections were made of these rocks, but as they 

 are not distinguishable from one another only one, No. 121, will 

 be described. This rock on the freshest fracture is black, but on 



