CHEMICAL COMPOSITION IN SEDIMENTS 463 
table on p. 456, this relationship holds in 77 per cent. It has already 
been shown (see table on p. 457) that all of the pelite schists when 
classified according to the quantitative system fall in the more acid 
classes I and IJ and usually near the border between these two classes. 
A fair comparison of the composition of the pelitic foliates and the 
igneous rocks should include therefore only the more acid classes I 
and II among the latter. Among the igneous rock of classes I and 
II of Washington’s tables, MgO exceeds CaO in only 8 per cent., a 
Percentage Weights 
Percentage Weight of|/Percentage Weight of} of Both MgO > 
MgO>CaO K,0>Na.,0 CaO and K.0 > 
NazO 
(79) (74) (74) 
SIBLCS HM ee etree ee arelanemainae ears 84 per cent. Q2 per cent. 78 per cent. 
; : _ (39) (30) (30) 
Pelite schists and gneisses...... 77 per cent. 83 per cent. 74 per cent. 
Igneous rocks of classes I and (1481) (1481) (1481) 
II, Washington’s tables*..... 8 per cent. 364 per cent. 4% per cent. 
Igneous rocks of classes III, IV, (411) (401) (401) 
iil Waccosuvendoaon eno aoa nal! Sis ile come I4 per cent. 7 per cent. 
* H. S. Washington, ‘‘Chemical Analyses of Igneous Rocks,’’ Professional Paper No. 14, U. S. 
Geol. Survey, 1903. 
The figures in parenthesis denote the number of analyses considered in each case. 
figure markedly in contrast with the 84 per cent. and 77 per cent. 
observed in the slates and pelite schists, respectively. For the igneous 
rocks of the neutral and basic types of classes III, IV, and V in Wash- 
ington’s tables, the number of analyses in which MgO >CaO is 
about 35 per cent. The amouni of this dominance is also much 
greater in many of the basic rocks than in the acid. 
Dominance of magnesia over lime is therefore of very considerable 
value as a criterion of genesis. Its value is greater in the case of acid 
than of basic foliates. 
CRITICAL VALUE OF THE POTASH-SODA RATIO 
Attention is frequently called in geological literature to the fact 
that sodium salts are in general more soluble than the corresponding 
salts of potash, as an explanation of their more rapid removal from 
rocks in the processes of weathering. ‘Their rate of removal is not 
