308 J. M. Blake on Measuring the angles of Crystals. 
Pine Mountain ridge behind him. This bold land-mark I named 
in my notes “Mount Brewer.” ‘Fortress Rock,” on Trout Creek, 
is another fine example of the same kind, but the valley north 
is filled with similar table mountains from a few hundred to 4 
thousand or twelve hundred feet above the surrounding country. 
Here the same horizontal and gently inclined beds of light 
colored tufas already noticed as occurring near Silver Creek, 
fifty miles or more west of this, recur and are capped in a like 
manner by basaltic columns, 
Enormous dikes or reefs of quartz and of coarse quartzose 
feldspathic granite cut through the reddish gneissoid granite 
which forms the basement rock over a large part of the Wau 
Yuma District, rising in one case 100 to 150 feet above the 
cafion which cuts the vein at a point where I examined it, and 
where it is 50 feet thick. I cou!d not discover in those giganti¢ 
veins much evidence of any metallic value, nor had there been 
any exploration upon them. : 
The Sacramento District, about 45 miles N.E. of Fort Mojave, 
I did not visit, but inspected a large collection of argentiferous 
galena from its veins, made chiefly by soldiers of the Post. 
These lead veins occur in metamorphic rocks, and are such in 
size and metallic value, so far as I could learn, as to lead to the 
belief that they will one day be worked when labor and supplies 
are cheaper and more abundant, and they may furnish a most 
important auxiliary to the treatment of the silver ores of adja- 
cent districts. 
The Irataba district, south of Fort Mojave, comprises a num- 
ber of veins carrying copper, but few of them, in the opinion 
of my assistant, Mr. Frank Sample, who visited them, are 
worthy of exploration. 
ts 
Arr. XXXVIL—A method of Giving and of Measuring the angles 
of Crystals, for the determination of putin by the use of the 
Reflecting Goniometer ; by Joun M, BLAKE. 
Ir seems desirable that more general use should be made of 
angular measurements of crystals for the purpose of determin: 
ing species, than the ordinary methods of measurement and com- 
parison will allow. 
The time and ny required to understand the various forms 
Now it is possible to describe ivi ts 
: . a crystal, giving measuremeD 
which will locate every plane occurring pou it without the use 
