THE ORIGIN OF IGNEOUS ROCKS. 
165 
molten state, is the variability of the molecular proportions 
of the oxides in various rocks, and the consequent mineral 
composition of rocks. 
A consideration of the chemical composition of igneous 
rocks shows no fixed ratio between the oxides which are 
treated as bases and the silica with which they combine to 
form most of the rock-making minerals. With the excep¬ 
tion of iron, which is sometimes protoxide and sometimes 
sesquioxide, all of the elements appear in one state of oxida¬ 
tion, which seems to be fixed; but these oxides may be 
combined with silica in a number of stoichiometric propor¬ 
tions. 
While there appear to be limits beyond which differentia¬ 
tion does not pass, and the result in any case is a silicate 
rock, still the variation of the elementary constituents may 
be very great, and it is possible that greater extremes than 
those already recognized may yet be found. 
The dependence of the mineral development of rocks on 
the molecular proportions of the constituent oxides will be 
appreciated if we compare the theoretical molecular ratios 
of the commoner rock-making minerals with the chemical 
composition of the rocks in which they occur. 
Molecular Proportions of the Roch-Maldng Minerals. 
Orthoclase. K 2 0 : A1 2 0 3 : Si0 2 = 1:1:6 
Albite. Na 2 0 : A1 2 0 3 : Si0 2 = 1:1:6 
Leucite.K 2 0 : A1 2 0 3 : Si0 2 = 1:1:4 
Analcite. Na 2 0 : A1 2 0 3 : Si0 2 + 2aq = l:l:4 + 2aq 
Nepheline. Na 2 0 : A1 2 0 3 : Si0 2 = 1:1:2 
Sodalite. Na 2 0 : A1 2 0 3 : Si0 2 + Na Cl = 1 : 1 : 2 + Na Cl 
Anorthite. CaO : A1 2 0 3 : Si0 2 = 1:1:2 
"Riotifp / ( K > H) a 0 : (Al, Fe) 2 0 3 : Si0 2 = 3 : 3 : 6 = 3 (1 : 1 : 2) 
i310tlte . \ (Mg, Fe) 0 : Si0 2 = 12 : 6 = 6 (2 : 1) 
Olivine. (Mg, Fe) 0 : Si0 2 = 2:1 
Hypersthene... (Fe, Mg) 0: Si0 2 =1:1 
Auo-ite I Ca0 : Fel 0 : Si0 * = 1 : 1 : 2 
Auglte . X (Mg, Fe) 0 : (Fe, Al) 2 0 3 : Si0 2 = 1:1:1 
Hornblende l Ca0 : Ye) 0 : Si0 * = 1 : 3 : 4 
Hornblende... j (Mg> Fe)0 . (Fe> M) ^ . x x 
Riebeckite. Na 2 0 : Fe 2 0 3 : Si0 2 = 1:1:4 
