366 HENRY S. WASHINGTON 
are low in silica. Furthermore, the greater part of them contain 
large amounts of femic minerals, falling in salfemane and dofemane, 
with a few in dosalane, and none at all in persalane. Again, it will 
be observed that, with some exceptions, especially of melilite-basalts, 
they are all intrusive bodies, which have therefore solidified under 
conditions very different from those of the effusive flows, as are 
almost all of the rocks with both normative and modal leucite. 
Of these exceptional rocks some contain very considerable amounts 
of biotite, as Nos. 109, 140, 159, 164, and 193. These rocks are all 
intrusive, and the conditions of solidification were such that the potash, 
in combination with magnesia and ferrous oxide, and in the presence 
of mineralizers, formed the complex biotite molecule, instead of 
leucite and olivine, as would have been the case under other circum- 
stances. It may be noted, however, that in biotite the potash controls 
as much or more silica than it does in leucite, and thus conforms to 
the assumed order of affinity. Thus, if the molecule of biotite be 
written as (K,H),O.(Al,Fe),O,.25i0, +n[2(Mg,Fe)O.SiO,], the ratio 
of SiO, to K,O in the salic portion is that of leucite if K : H be 1: 1, 
while it is that of orthoclase if it be 1: 2, as is usually the case. Conse- 
quently, while the physico-chemical conditions have brought about 
the formation of the very complex mineral biotite instead of the 
more simple minerals leucite and olivine, the assumed order of 
affinity may be considered to hold good. 
Others of these exceptions, as Nos. 73, 125, 127, and 178, are rocks 
which consist very largely of hornblende or augite, and the norms of 
which contain very small amounts of leucite, only from 1 to 3 per 
cent. In these cases the amount of potash present is relatively so 
small that a great affinity for silica might be readily masked or over- 
come by the mass action’ of the complex molecules of the minerals 
which make up most of the rock, and which would incorporate the 
small amount of potash in their molecules. 
Some other cases are of ijolites and urtites, rich in nephelite and 
aegirite or aegirite-augite, as Nos. 100, IoI, 102, 103, 158, 170, and 
171. The formation of the sodic pyroxene in these is conditioned 
by the excess of alkalies over alumina and the presence of ferric oxide, 
and as the soda in this controls twice as much silica as it does in nephe- 
«Cf. S. L. Penfield, American Journal of Science, Vol. XXIII (1907), p. 25. 
