280 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Some of the results along these lines obtained by study of the 
vast accumulation of analytical data now available are well known to 
petrologists, while others do not seem to be so generally understood. 
To the nonpetrologist they are, naturally, mostly unknown, and, as the 
general principles involved, and, indeed, some of the specific in- 
stances, have a more or less important bearing on the occurrence and 
characters of certain deposits of metallic ores and other economically 
important minerals, a discussion of the subject may be of interest to 
mining engineers. 
Indeed, to the observations and operations of mining engineers 
and mining interests generally, the petrologist is indebted and must 
look for some of his data. This is especially true of those relating 
to the precious metals and others of commercial importance, the 
amounts of which usually present in rocks are so small as almost or 
quite to defy detection by ordinary analytical methods, and whose 
presence is often revealed only through search for and the exploita- 
tion of localities where they have undergone concentration. It must 
be premised, however, that our knowledge is at present very uneven, 
allowing fairly safe and detailed generalizations as regards some of 
the elements, very rudimentary or general ones as regards others, and 
again allowing almost none at all. 
Il. GENERAL CHEMICAL COMPOSITION OF IGNEOUS ROCKS. 
The first and most important fact to be noted of igneous rocks is 
that, with the exception of some rare ore bodies due to the differentia- 
tion of igneous magmas, they are composed almost wholly of silica 
and silicates. The vast majority of igneous rocks are silicate rocks, 
in which silica forms the most prominent and the never-failing con- 
stituent. Most of the minerals which compose them are combina- 
tions of silica with various bases, and it is a striking fact that the 
number of minerals which go to make up the majority of igneous 
rocks, and which are most abundant and most often met with, is 
very small. 
The proportions in which these minerals may be present vary very 
widely. Some rocks are known which are composed wholly, or 
practically so, of but one mineral. Combinations of two are not 
infrequent, while most rocks contain at least three, and usually many 
more, minerals and in the most widely diverse proportions. It 
follows, therefore, that the chemical composition of igneous rocks 
may vary within very wide limits, as regards any or all of the 
chemical constituents; and that, furthermore, some rocks may be of 
very simple chemical composition while others may be very complex, 
with many constituents present, since the minerals themselves may 
be either very simple or highly complex in chemical composition. 
