8 ANALYSES OF ROCKS AND MINERALS, 1880-1908. 
Now, omitting the minor constituents and considering only those 
which appear in Washington’s calculation, his average and mine may 
be compared as follows: 
. Reduced to 100 per 
As given. cent. 
Clarke. | Washing- | Clarke, | Washins- 
60. 97 58. 239 60. 57 57.78 
15. 29 15. 796 15.18 15. 67 
2. 63 3. 334 2.61 3.31 
3. 40 3. 874 3.38 3. 84 
3.96 3. 843 3.93 3.81 
4.89 5. 221 4. 86 5.18 
3.47 3.912 3.45 3. 88 
3.00 3.161 2.98 3.13 
47 . 363 47 36 
1.48 1. 428 1.47 1,42 
74 1.039 74 1.03 
26 373 26 37 
10 219 10 22 
100. 66 100. 802 100. 00 100. 00 
One reason for the difference between my estimate and that of 
Washington is not far to seek. The two columns of reduced averages 
are not strictly comparable, for one contains many partial analyses, 
and the other relates only to those which are nominally complete. 
For the interpretation of a femic® rock, which is mineralogically 
complex, a full analysis is necessary. For the simpler salic? rocks, 
determinations of silica, lime, and alkalies are usually sufficient for 
petrographic purposes. The partial analyses, therefore, represent 
mainly salic varieties, and their inclusion in the average tends to raise 
the percentage of silica and to lower that of magnesia or iron. But 
the salic rocks are undoubtedly more abundant than those of the 
other class, and so I am inclined to regard the higher figure for silica 
as more probable than the lower. If, however, we include all the 
minor constituents of rocks as given in my computation, the final 
result assumes the form given on the following page. 
a The terms “‘salic’” and ‘‘femic,” proposed by Cross, Iddings, Pirsson, and Washington, are here 
used instead of the conventional ‘‘acid” and ‘‘basic.”” The latter terms have precise chemical meaning, 
and their use in another sense by petrographers is inexact and misleading. The terms persilicic, me- 
diosilicic, and subsilicie are even better. 
