22 SOME PRINCIPLES AND METHODS OF BOCK ANALYSIS, [bill. 176. 
Of late petrographers have begun to demand, with considerable 
reason, an arrangement "which shall bring the essential chemical 
features — both the percentage figures and the molecular ratios — prom- 
inently and compactly before the eye, so that the general chemical 
character and the relations of the various constituents may be seen at 
a glance." 1 
In accordance with this demand it is now our practice to follow 
pretty closely the arrangement proposed by Pirsson and very recently 
strongly advocated by Washington (loc. cit.), namely: 
Si0 2 , A1 2 0.„ Fe 2 3 , FeO, MgO, CaO, Na 2 0, K 3 0, H 2 (above 105- 
110° C), H 2 (below 105-110° C), C0 2 , Ti0 2 , Zr0 2 , P 2 5 , S0 3 , CI, 
Fl, S (FeS 2 ), OA, V 2 3 , NiO, CoO, CuO, MnO, SrO, BaO, Li 2 0, C, 
NH, 
By this arrangement the nine constituents which in the great major- 
ity of cases determine the character of the rock are placed at the head 
of the list, thus greatly facilitating the comparison of different analyses 
similarly arranged, especially when, as Washington recommends, the 
molecular ratios are calculated for these leading constituents and 
placed immediately after the corresponding oxides. The order of the 
remaining members is determined somewhat by the following consid- 
erations: C0 2 is placed next after H 2 0, since these two are generally 
a measure of the alteration the rock may have undergone. Ti0 2 and 
Zr0 2 naturally follow C0 2 on chemical grounds, and S0 3 and CI, being 
common constituents of the sodalite group, are conveniently placed 
together. 
IV. TIME NEEDED FOR MAKING AN ANALYSIS. 
The question has often been put, ' ' How long does it take to com- 
plete an analysis of this kind?" This will depend, of course, on the 
mineral complexity of the sample and on the personal factor of the 
individual worker. If there is a competent assistant to do the grind- 
ing, and specific-gravity determinations are not required, it is quite 
possible after long experience for a quick worker to learn to so econo- 
mize every moment of time in a working day of seven hours, w T ith an 
abundance of platinum utensils and continuous use of air and water 
baths through the night, as to finish every three days, after the com- 
pletion of the first analysis, barring accidents and delays, one of a 
series of rocks of generally similar character, each containing from 
eighteen to twenty quantitatively determinable constituents, exclud- 
ing, for instance, fluorine, carbon as such, nitrogen, metals of the 
hydrogen sulphide group, and cobalt. On one occasion a series of 
fourteen rocks, of comparatively simple composition, was completed 
in one month, with the help of an assistant who made the phosphorus 
1 II. S. Washington, The statement of rock analyses: Am. Jour. Sci., 4thseries, Vol. X, p. 61,1900. 
