466 EDSON S. BASTIN 
percentages for slates and schists given in the table on p. 456, we find, 
as shown below, that there is no indication of any tendency toward an 
= 
2 
increase of the ratio. 
increas Na,O 
K,0 Na.2O 
Shales sy ceeaecatee co eesercriset. 3.42 percent. | 1.38 per cent. 
Slates and phyllites.......... 3.44 Te 3i7 
Pelite schists and gneisses....| 3.47 1.93 
The comparisons already made between igneous and meta-igneous 
rocks also reveal no important changes in the potash-soda ratio dur- 
ing dynamic metamorphism. ‘The process is believed therefore to go 
on largely under conditions of katamorphism and particularly in the 
belt of weathering. 
If we compare the potash-soda relationships in the available 
analyses of slates and pelite schists with those in the igneous rocks, 
as was done for the lime-magnesia ratio, the results recorded in the 
second column of the table on p. 463 are obtained. For reasons 
already stated (p. 463) a fair comparison includes only classes I and II 
among the igneous rocks. Among the 74 slate analyses whose 
average is given in the table on p. 456, in which the alkalies were 
separately determined, K,O >Na,O in 92 per cent., while among the 
30 pelite schist analyses considered this relation holds in 83 per cent. 
Among the 1,481 igneous rocks of classes I and II tabulated in Wash- 
ington’s tables, K,O >Na,O in 364 per cent., while among the 4or 
rocks of the more basic classes III, IV, and V, the relation holds in 
about 14 per cent. 
The difference in the rates of removal of potash and soda during 
rock weathering, though less marked than in the case of lime and 
magnesia, are nevertheless of sufficient magnitude to be of value as a 
criterion of genesis. Its value is even greater in the case of basic 
rocks than of acid. 
CRITICAL VALUE OF THE DOUBLE RELATIONSHIP MgO >Cao 
AND K,0>Na,0 
If we examine the slate and pelite schist analyses which enter 
into the averages of the table on p. 456, we find that there is a domi- 
nance of MgO over CaO and also of K,O over Na,O in 78 per cent. 
