THE BOOKS OF THE WASHOE DISTRICT. G7 



gether wanting-. The mica seems to be biotite in most cases, but in two or 

 three locaUties cleavage scales give an unmistakably biaxial interference 

 figure. It is as unifornil}- surrounded by a Ijorder of magnetite as the horn- 

 blende. The augite presents no peculiarities in structure. The amount of 

 this mineral is commonly inversely as the quantity of mica. Magnetite is 

 remarkable only for its abundance, and nothing which could be pronounced 

 titanic iron was noticed. Apatites ar« rarer than in the older volcanic rocks. 

 Feldspars. — Ahuost all the large porphyritic feldspars show abundant 

 striations, even under the lens, and few large crystals appear to lack them 

 under the microscope. Man)- feldspars which do not show polysynthetic 

 structure under an objective of low magnifying power, show stria; under 

 higher powers. Many of the feldspars show zonal structure comparable with 

 that discussed on page fil and illustrated in Fig. 13, Plate III. The large 

 feldspars are manifestly crystals of first consolidation, while thegroundmass 

 is in great part made up of microlitic feldspars. While the large crystals com- 

 monly give angles of extinction indicating labradorite, the microlites appear 

 to be chiefly oligoclase. There are also among the larger feldspars a 

 comparatively small number of Carlsbad twins, and simple crystals which 

 might be regarded as sanidin if no further test were applied; but none such 

 which were cut in the determinable zones, gave angles of extinction appro- 

 priate to orthoclase. As some of the possible sanidins were not so oriented 

 as to make optical determinations practicable, I submitted a specimen of the 

 most trachy tic-looking rock in the district to Dr. George W. Hawes,' curator 

 of the National Museum, for separation by Thoulet's method. The speci- 

 men sent was from a cpiarry 2,000 feet east of the Occidental mill, E ,5, and 

 was in all respects identical with that described by Professor Zirkel under 

 slide 283. The following details are taken from Dr. Hawes' report on this 

 rock : 



Feldspars determined by Thoulet's method. The SpecimCU WaS pulvCrizcd tO SUcll 



an extent that it would pass through a sieve containing three meshes to the 

 millimeter; and from this mass of grains the dust that would not settle was 

 separated by elutriation. As the special object in view was to determine 



• While this report wa.s going through the press Dr. Hawes died (June 22), leaving a vacancy in 

 the ranks of American geologists which it will be hard to fill, as well as a deep personal regret in the 

 hearts of all who knew him, however slightly. 



