ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 293 
addition to these two groups proposes two others, the “ mineralizing 
agents ” and the vein metals. 
Evidence, however, is accumulating that the relations of the rare 
elements to the igneous rocks can not be expressed so simply as is 
done by Vogt and De Launay. Their relative abundance is not 
dependent on the silica alone, and hence referable only to the “ acid- 
ity ” or “basicity ” of the rock. The relations are more complex 
and dependent, not so much on the amount of silica, as on the rela- 
tive amounts of other constituents, notably soda, potash, iron, mag- 
nesia, or lime. They conform, on the whole, to the general relations 
of the most abundant constituents, some of the rarer elements being 
characteristically at home in the rock groups which show high 
alumina and alkalies, and which include those high in silica; while 
others again are most abundant in the rocks high in iron, magnesia, 
or lime, and which consequently most often show low silica percent- 
ages. Further than this, on the one hand, certain rare elements are 
not equally at home in the alkalic rocks in general, but are most 
abundant either in those high in soda or in those high in potash. 
On the other hand, some of the elements segregated in the basic rocks 
seem to be most at home in those which are highly ecalcic, others in 
those which are high in iron or in magnesia, though here the evidence 
is not so clear and the distinctions apparently not so well marked as 
in the preceding case. 
We may consider first those minor constituents of rocks which are 
determined in the most modern and complete analyses, and next those 
which exist in rocks in such small amount as almost to defy determi- 
nation by the usual analytical methods, but whose presence is made 
known either mineralogically or by their segregation in ore deposits. 
The second group includes almost all of the commercially important 
metals (except iron and manganese), while the former includes many 
elements which are assuming an increased practical importance as 
their economic possibilities and uses become better known. In a 
general way the elements will be taken up in the order of their posi- 
tions in the periodic classification. No references will be given, as an 
attempt to render them complete would unduly lengthen the paper. 
This course seems the more advisable, in spite of the apparent injustice 
to those whose invaluable work and contributions must thus be ig- 
nored, since the present paper may be considered as merely a prelimi- 
nary one to a more exhaustive and monographic treatment which it 
is hoped to publish later. 
Lithium is very widely distributed among igneous rocks, but always 
in very small amounts. While it frequently is to be detected by the 
spectroscope, it seldom occurs in weighable quantities, and the diffi- 
culty of its exact separation from the other alkali metals and its com- 
parative unimportance cause it to be but seldom estimated quantita- 
