hillebrand.] OBJECT AND SCOPE OF PliESENT TKEATI! b L9 
connection with the long mystifying hick of agreement between results 
for ferrous iron obtained by the Mitgcherlich and the hydrofluoric- 
acid methods, might not have been suspected. (See p. 89.) 
While strongly upholding the necessity for more thorough work, 
necessarily somewhat at the expense of quantity, it is far from the 
writer's intention to demand that an amount of time altogether dispro- 
portionate to the immediate objects to be sought should be expended 
on every analysis. But it is maintained that in general the constitu- 
ents which are likely to be present in sufficient amount to admit of 
determination in the weight of sample usually taken for analysis — say 
1 gram for Si0 2 , A1 2 3 , etc., to 2 grams for certain other constitu- 
ents — should be sought for, qualitatively at least, in the ordinary 
course of quantitative work, and their presence or absence noted 
among the results. If present in little more than traces, that knowl- 
edge alone may suffice, for it is often more important to know whether 
or not an element is present than to be able to say that it is there in 
amount of exactly 0.02 or 0.06 per cent. In the tabulation of analyses 
a special note should be made in case of intentional or accidental neg- 
lect to look for substances which it is known are likely to be present. 
Failure to do this may subject the analyst to unfavorable criticism, 
when at some future time his work is reviewed and the omissions are 
discovered by new analyses. 
Finally, whenever possible, a thorough microscopical examination 
of the rock in thin section should precede the chemical analysis. This 
may be of the greatest aid to the chemist in indicating the presence of 
unusual constituents, or of more than customary amounts of certain 
constituents, whereby, possibly, necessary modifications in the analyt- 
ical procedure may be empk^ed without waste of time or labor. 1 
II. OBJECT AND SCOPE OF THE PRESENT TREATISE. 
The literature relating to analysis of silicates is extensive but scat- 
tered, and in no single article is there to be found a satisfactory 
exposition of the methods to be followed or the precautions to be 
observed, especially in the search for some of the rarer constituents 
or those which, without being rare, have been of late years recognized 
as occurring persistently in small amounts. It is not intended to 
■make this little volume a treatise on mineral analysis, but it is believed 
that the experience gained by the chemists of this Survey during the 
twenty years since the establishment of its first chemical laboratory 
'The foregoing tables and accompanying remarks, including several sentences preceding the 
tables, have been largely taken from the writer's papers entitled "A plea for greater completeness in 
chemical rock analysis," published in the Journal of the American Chemical Society, Vol. XVI. pp. 
90-93, 1894; also in the Chemical News Vol. LXIX, p. L63, L894. See also " Distribution and quantita- 
tive occurrence of vanadium and molybdenum in rocks of the United States," in the American 
Journal of Science, lth series, Vol. VI, p. 209, 1898, and Chemical News, Vol. LXXVIII, p. 216, 1898. 
