196 EDSON S. BASTIN 
mentary slates 78 per cent showed a dominance both of magnesia 
over lime and of potash over soda. In 70 analyses of pelitic 
schists and gneisses this relationship is almost equally characteristic, 
holding in 74 per cent of the analyses. In the more siliceous 
igneous rocks (Classes I and IJ), on the other hand, dominance of 
magnesia over lime and of potash over soda occurs in only 45 per 
cent of the 1,481 analyses considered, and in the case of all the 
other igneous rocks (Classes III, IV, and V) in only 7 per cent of 
the 401 analyses considered. The contrast between igneous rocks 
and sediments shown in the other columns is also striking. Such 
wide differences as are here exhibited can hardly be fortuitous, and 
the nature of data upon which they are based would seem to recom- 
mend them to the serious consideration of geologists. 
In criticism of the value of the potash-soda and magnesia-lime 
ratios as criteria of genesis, Mr. Trueman’ states that “‘in the case 
of many, possibly in the majority of, igneous rocks ezther the 
MgO is in excess over the CaO or the K,O over the Na,O... .” 
and points out with perfect correctness that “the tables of Washing- 
ton show that in the majority of igneous rocks containing over 70 
per cent of silica the K,O is in excess over Na,O. .. . . Accordingly 
while the double relationship is apparently significant, the single 
ratios have but little value.” 
To the present writer there would seem to be no valid reason 
for restricting the comparison to those igneous rocks which show 
more than 70 per cent of silica, especially as these include nearly 
all of the greatest group of potash-rich rocks, the granites and 
rhyolites, and also as the average percentage of silica in the meta- 
sedimentary rocks with which they are compared is only 60 to 
: K,0>Na,O 
65 per cent. While the double relationship and is 
MgO> CaO 
the most significant and the single relationship of K,O>Na,0 
or MgO>Ca0O is of little or no critical importance when standing 
alone, the writer believes that either of these relations, if supported 
by notable excess of alumina, has very considerable critical value. 
Each of the criteria suggested has its “limit of error”? which 
UOp. cit., p. 301. 
