110 SOME PRINCIPLES AND METHODS OF KOCK ANALYSIS, [bull. 176. 
the .silica separated by acid from one of several mineral constituents 
of a rock is to be estimated. Even when dilution is considerable, 
solution is almost immediate, and as soon as this is accomplished — 
the point being known by the change in appearance of the residue — 
the solution should be diluted with cold water and filtered at once. 
The difficulty met with in filtration may often be overcome by faintly 
acidifying, which has the added advantage of at once arresting any 
further action of the alkali. If the dilution is sufficient no separation 
of silica results from so doing. Very dilute acid should also be used 
for washing. Lunge, when using sodium carbonate, washes with hot 
carbonate solution to which alcohol has been added, thus obtaining 
clear filtrates. 
XXVIII. ESTIMATION OF MINUTE TRACES OF CERTAIN CONSTIT- 
UENTS. 
If, as sometimes may happen, the problem is presented of examin- 
ing rocks for traces of gold, silver., and other elements which are not 
ordinarily looked for, as in Sandberger's investigations bearing on the 
origin of the metalliferous contents of veins, large weights of material 
must be taken, up to 50 grams or more. This involves the use, also, 
of large quantities of reagents, the purity of which must then be 
looked to with the utmost care. Special directions to meet such cases 
can not now be given, nor even a complete reference list of the scanty 
and scattered literature on this subject. Sandberger's own writings 
deal but little with its analytical side, and from its inaccessibility in the 
Washington libraries the writer is as yet unacquainted with the report 
by Von Foullon " Ueber den Gang und die Ausfiihrung der chemi- 
schen Untersuchung," 1 following Sandberger's own paper in the general 
report " Untersuchungen der Nebengesteine der Pribramer Gauge." 2 
The present writer has published a few data as to gold, silver, lead, 
zinc, etc., 3 in Mr. S. F. Emmons's report on " The Geology and Mining 
Industry of Leadville;' 1 and Mr. J. S. Curtis,* in his report on "The 
Silver-Lead Deposits of Eureka, Nevada," has given his method of 
assaying rocks for traces of gold and silver. 
To Prof. F. W. Clarke and Dr. H. N. Stokes the writer's thanks are 
due for friendly suggestions in the preparation of the foregoing work. 
] Jahrbueh der Bergakademie, Leoben u. Pribram, 1887, p. 363. 
"From Sandberger's report it appears that the rocks were treated successively with water, acetic 
acid, boiling dilute hydrochloric acid for two days, and finally hydrofluoric acid, the several extracts 
and final residue of fluorides | and pyrite) being separately examined for heavy metals. The products 
of distillation were also examined. A striking fact observed in all cases was the complete insolubil- 
ity of the pyrite, even after the severe treatment mentioned. This speaks strongly in favor of the 
correctness of ferrous iron estimations in silicates by the hydrofluoric and sulphuric acid method 
when pyrite is present unaccompanied by other sulphides. (See p. 95.) 
SMon. U. S. Geol. Survey, Vol. XII, Appendix B, pp. 592-596, 1886. 
< Mon. U. S. Geol. Survey, Vol. VII, pp. 120-138, 1884. 
