ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 297 
iron rather than with magnesium, and occurs in most abundance in 
some iron ores of magmatic origin, but no definite relations to the 
alkalies can be made out. Its common occurrence in ashes of coals 
and its abundance in certain carbonaceous deposits recently described 
are noteworthy, though outside the present discussion. 
Sulphur is, by far, more abundant in the basic than in the siliceous 
rocks. It may exist, in the oxidized condition, in the minerals hauy- 
nite and noselyte, in which case the rocks containing these minerals 
are almost invariably distinctly sodic; or it may form sulphides, as 
pyrite, pyrrhotite, and chalcopyrite, these being most common in 
rocks rather high in iron, magnesia, and lime. 
Chromium, like vanadium, is a constituent of the basic rocks, but, 
unlike this, is most abundant when magnesia and not iron is high, 
and when olivine, rather than pyroxene or hornblende, is abundant, 
in spite of the fact that it occurs as the sesquioxide, Cr,O,. Presum- 
ably this is because, instead of replacing alumina and ferric oxide in 
the ferromagnesian minerals, it is most commonly met with in the 
minerals chromite and picotite. It is reported to reach very high 
figures again in certain effusive rocks which are so high in lime and 
low in silica that the rare mineral melilite is present. 
Molybdenum is seldom looked for in rock analysis, and our knowl- 
edge of its magmatic relations is based almost wholly on an investi- 
gation of Hillebrand. He found that it is much less common and is 
present in smaller quantity than vanadium, and that, unlike the 
latter, it is present only in the more siliceous rocks, though in quanti- 
ties too small to permit of further discrimination. As molybdenite 
it occurs most often in quartzose rocks. ; 
Fluorine is almost universally present in very small amount as a 
constituent of most apatites, and is usually regarded as a “ mineral- 
izing agent,’ and, as such, is frequently present in pneumatolytic 
minerals. <As stated by Vogt, it seems to be more common in the 
acid rocks, but there seems to be a marked tendency on its part to 
favor especially rocks which are high in soda. This is seen in the 
fact that fluorite is frequently present as an original constituent of 
such highly sodic rocks as nephelite-syenite, phonolite, and tinguaite; 
the association of fluorine and sodium in certain rare minerals, as 
leucophanite, meliphanite, johnstrupite, rinkite, etc., which are al- 
most always found in sodic rocks; and by the recent discovery by 
Lacroix of sodium fluoride in nephelite-syenites of West Africa. 
Chlorine resembles fluorine in being a pneumatolytic constituent, 
and is present in igneous rocks chiefly in the minerals sodalite and 
noselite, which are almost wholly confined to sodic rocks and especially 
those which are low in silica, in this resembling the occurrence of 
BOn 
