ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 299 
to be little doubt that it is most abundant in the more basic rocks, 
especially those which carry pyroxene and hornblende rather than 
olivine, but no evidence seems to exist as to its relations to the 
chemical constituents other than silica. 
Silver and gold have both been detected analytically in igneous 
rocks, while metallic gold has also been observed as an apparently 
primary ingredient of some rhyolites and granites. Both of these 
metals are “ cosmopolitan in their relations,’ as Kemp puts it, and 
they are known to occur in such highly siliceous rocks as granite, 
rhyolite, and quartz porphyry, and, on the other hand, in diabase 
and gabbro. There is, however, good reason for the belief that gold, 
and probably silver as well, are most apt to occur in rocks high in 
silica, but their relations to the other elements are still quite unknown. 
Zine and cadmium (which latter is found only in connection with 
the former) are also very uncertain. There is, however, some reason 
for thinking that zine is more apt to be present in acid rocks, as 
granites, this opinion being based on a few analytical data and the 
facts of some of its occurrences. The common occurrence of zinc in 
limestones, due presumably, at least in part, to the precipitating 
effect of the sedimentary rock, has no apparent bearing on its rela- 
tions to igneous magmas. 
Mercury is considered by G. F. Becker to be associated with 
granites, but his evidence is not very convincing. Its usual occur- 
rence in sedimentary rocks tends to obscure its true relations, and, 
to the best of my knowledge, it has never been looked for or reported 
in an analysis of an igneous rock. 
Tin, as the oxide cassiterite, almost invariably occurs as the re- 
sult of pneumatolytic processes in pegmatites, granites, and other 
rocks high in silica, and the mineral has been found in some rhyolites. 
Judging from the common association of cassiterite with lithium 
and beryllium minerals, and the presence of small amounts of tin in 
certain feldspars, micas, zircons, and in the rare mineral stokesite, it is 
probable that tin is associated rather with distinctly sodic than with 
potassic or calcie magmas, but much more chemical study of the 
rocks in which it occurs is needed to elucidate its relations. 
Lead can often be found in rocks by using large amounts of material, 
and is occasionally reported, as in the analyses of rocks from British 
Guiana by J. B. Harrison and in those of rocks from New South 
Wales. No generalization in regard to it is possible as yet, but I 
am inclined to think that, like zinc, it favors the acid rather than 
the basic rocks, though it has been observed in both. The remarks 
in regard to the-occurrence of zinc in limestones apply as well to 
lead. 
Platinum and the other metals of this group are, as is well known, 
most commonly found in connection with peridotites, rocks low in 
