166 
IDDINGS. 
Acmite. Na 2 0 : Fe 2 0 3 : Si0 2 =1:1:4 
Melilite. (Ca, F^, Mg) 0 : A1 2 0 3 : Si0 2 = 12 : 2 : 9 
Sphene. CaO : Ti0 2 : Si0 2 = 1:1:1 
Perofskite. CaO : Ti0 2 = 1:1 
Ilmenite. FeO : Tio 2 = 1:1 
Magnetite. FeO : Fe 2 0 3 = 1:1 
Picotite. (Mg, Fe)0 : (Al, Cr) 2 0 3 = 1:1 
Zircon. Zr0 2 : Si0 2 = 1:1 
Apatite. CaO : P 2 0 5 = 3:1 + (Ca, FI, Cl.) 
The table of molecular ratios just given shows that in the 
feldspars and feldspathie minerals the ratio of alkalies and 
lime to alumina is 1:1 in every case, and that the silica is 
variable ; hence the controlling factor in the production of 
any one of the alkali feldspathie minerals is the amount of 
silica available. It is not the presence of large amounts of 
alkali in rocks that causes the crystallization of leucite and 
nepheline, though these minerals usually occur in very 
alkaline rocks, but it is the relatively low percentage of 
silica, for leucite occurs in rocks in the Yellowstone Park 
region which contain but little alkalies, the silica being in¬ 
sufficient to form orthoclase. For the same reason the feld- 
spathic minerals low in silica: leucite, nepheline, sodalite, 
and analcite, do not occur in rocks in which quartz is an 
essential ingredient. A similar relation between the amount 
of silica and the development of olivine or hypersthene is 
noticeable, the essential difference between the two being 
the ratio of silica to the protoxides. 
The chemically intermediate position of biotite between 
feldspar and olivine is important. It consists of a molecule 
with the form of that of feldspar, except that it would be a pot¬ 
ash nepheline with the ratio of K 2 0 : A1 2 0 3 : Si0 2 = l: 1: 2, 
in which part of the potash is replaced by hydroxyl, and 
part of the alumina by ferric oxide. With this is combined 
an olivine molecule, (Mg, Fe) 0 : Si0 2 = 2:1. The develop¬ 
ment of biotite, under certain conditions, in place of olivine 
is discussed in connection with the granular rocks of Elec¬ 
tric peak and with that of Crandall basin. Biotite should 
be expected in rocks with potash in which the alumina is 
