ELEMENTS IN IGNEOUS ROCKS—WASHINGTON. 281 
The most important chemical constituents (stated as oxides, in 
accordance with the usual custom) are as follows: Silica, SiO,; 
alumina, Al,O,; ferric oxide, Fe,O,; ferrous oxide, FeO; magnesia, 
MgO; lime, CaO; soda, Na,O; potash, K,O; and water, H,O. Some 
or all of these major constituents, as they are termed, are invariably 
present, so far as known, and collectively they constitute about 98 
per cent of all known rocks. The chief oxides, from silica to potash 
inclusive, enter into the composition of the most important and 
most commonly occurring rock-forming minerals, as well as the 
glass of imperfectly crystallized rocks. 
The rédle of water is somewhat different. It would seem to be 
universally present in the magma, and its presence (along with that 
of other substances) lowers the freezing point and increases the 
tendency to crystallization of the liquid mass. Most of this water 
is lost if the magma reaches the surface and it appears in the enor- 
mous clouds which accompany voleanic eruptions and the steam of 
voleanic fumaroles; and much of it also escapes if the magma 
solidifies beneath the surface, giving rise to subterranean water sup- 
plies, which are held by many to be an important factor in the for- 
mation of many ore deposits. A small proportion of the water 
originally present may remain in the solidified rock in a combined 
form, as part of the more complex mineral molecules, those of 
pyroxenes, amphiboles, and micas, for instance; and some may also 
remain as inclusions of water in the minerals of intrusive rocks. 
There are almost invariably present in igneous rocks smaii amounts 
of titanium, phosphorus, and manganese, though these are often 
neglected and thus overlooked in the less complete analyses. Carbon 
dioxide is also met with, but its presence, as reported in analyses of 
igneous rocks, is almost invariably due to decomposition, and it can 
not be usually regarded as an essential or original constituent of 
rocks. 
In addition to these most important constituents the refinements 
and increasing completeness of modern rock analysis show that many 
others are frequently present, often in scarcely more than traces, but 
again in very appreciable quantities. The most important of these 
minor elements are zirconium, sulphur (as sulphides and as sulphur 
trioxide), chlorine, fluorine, vanadium, chromium, nickel, barium, 
strontium, and lithium. Exceptionally, others may be determined, as 
boron, cobalt, copper, gold, silver, molybdenum, the metals of the 
cerium and yttrium groups, nitrogen, and others. Indeed, as Doctor 
Hillebrand® says, “a sufficiently careful examination of these rocks 
would show them to contain all or nearly all the known elements, 
not necessarily all in a given rock, but many more than any one has 
@ Bulletin No. 305, U. 8S. Geological Survey, p. 20 (1907). 
