14 SOME PRINCIPLES AND METHODS OF ROCK ANALYSES, [bull. 176. 
was confined to the ordinary separations of the commoner ingredients 
occurring in appreciable quantities, wfth little regard to supposed 
traces and with still less attempt to find out if the tabulated list really 
comprised all that the mineral or rock contained. 
With the introduction of improved methods of examination by the 
petrographer, especially as applied to thin rock sections, and the use of 
heavy solutions, whereb}^ on the one hand, the qualitative mineral 
composition of a rock could be preliminarily ascertained with consid- 
erable certainty, and on the other, chemical examination of the more 
or less perfectly separated ingredients was rendered possible, a great 
help and incentive was afforded to the few chemists engaged in rock 
analysis. The microscope often obviated in part the necessity for 
tedious and time-wasting qualitative tests, and the heav}^ solutions, 
by permitting the concentration and separation of certain components, 
facilitated the detection of elements whose existence had long been 
overlooked. 
Meanwhile in the progress of chemistry new methods and reagents 
for qualitative detection and quantitative separation and estimation 
were gradually being discovered and devised. The supposed adequacy 
of some well-established methods was shown to be unwarranted; some 
had to be discarded altogether; others were still utilizable after modi- 
fication. In the light thus shed it became possible to explain many 
hitherto incomprehensible variations in the composition of some rock 
species or types, as shown in earlier analyses, and in not a few cases 
it appeared that the failure to report the presence of one or more 
elements had obscured relations and differences which more thorough 
examination showed to exist (see pp. 16-17). Consequently there 
arose a feeling of distrust of much of the older work in the minds of 
those chemists and petrographers best fitted to judge of its probable 
qualities. This, and the incompleteness of nearly all the earlier work 
(and much of that of to-day, unfortunately), as shown by the largely 
increased list of those elements now known to enter into the normal 
composition of rocks, is rendering the old material less and less avail- 
able to meet the increasing demands of the petrographer. 
And yet these demands on his part are, with few exceptions, by no 
means so exacting as they should be. Often the analysis is intrusted 
to the hands of a student without other experience than that gained 
by the analysis of two or three artificial salts and as many compara- 
tively simple natural minerals, and with a laboratory instructor as 
adviser whose experience in rock analysis ma} r be little superior to his 
own. In other words, one of the most difficult tasks in practical analy- 
sis is expected to be solved by a tyro, and his results are complacently 
accepted and -published broadcast without question. Even to those 
thoroughly familiar with the subject rock analysis is a complex and 
