PETROLOGY. 155 
type of teanal auruncose, it is generally observed that in rocks derived from identical 
or very similar magmas, while some contain biotite the others do not contain this 
mineral, but leucite and olivine replacing it. That is, the alferric biotite molecule 
has been split up actually into the simpler salic and femic ones. 
By far the most important set of readjustments is that involved in the formation 
of leucite from magmas whose norms do not show this mineral. This topic will 
be discussed at some length on a later page, but it may be pointed out here that 
in these rocks the presence of leucite in such magmas involves the taking away of 
silica from normative orthoclase, which goes to form albite molecules from the 
normative nephelite, which albite enters modally into soda-lime feldspar. In a 
few cases some of the normative olivine seems to be involved, this becoming hyper- 
sthene which enters into modal augite. But this is exceptional, and in the majority 
of instances the readjustments take place between normative orthoclase and nephel- 
ite, forming leucite and albite in the mode, the normative minerals disappearing 
either in whole or in part. 
The generally close correspondence between norm and mode among the non- 
leucitic rocks of the region, and the simple molecular readjustments involved in 
the formation of leucite, are of interest as bearing on the propriety of the choice 
of the so-called "standard minerals" in the quantitative classification. It has 
already been pointed out* that in the majority of igneous rocks there is, on the 
whole, a close correspondence between the norm and the mode in the majority of 
rocks; that is, that the readjustments of the norm needed for the mode are mostly 
of small amount and the modes normative. In his recent monumental work on 
Mont Pele*e Prof. A. Lacroixf points out that the standard minerals are "among 
those which are formed by (simple) igneous fusion followed by slow cooling, at 
the expense of those, such as the amphiboles, micas, and garnets, which are not 
so formed." 
The practical absence of amphiboles, biotites, and the more complicated 
pyroxenes is certainly a striking feature of the region, and it would seem that for 
this igneous complex, at least, the choice of standard minerals is appropriate and 
expresses the modal facts as closely as any such selections could do. 
Mineralogical Characters. 
Introduction. That the mineralogical composition or mode of igneous rocks 
is determined, within limits, by the chemical composition, subject to the physical 
conditions obtaining during solidification, is so well known that the statement is 
almost a truism. But it is found, furthermore, that in a comagmatic region the 
minerals present, or at least some of them, show certain more or less common 
peculiarities, which may be regarded as characteristic of the region, and which 
are usually looked upon as evidences of "consanguinity" only second to the chemical 
* Cross, Iddings, Pirsson, and Washington, op. cit., p. 151; and Washington, Prof. Paper U. S. Geol. Surv. 
No. 14, 1003, p. 69. 
t A. Lacrobc, La Montagne Pelte el ses Eruptions, Paris, 1004, p. 529 
