68 
GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
good-sized individuals of imperfect crystal outline. But the mineral in question 
is abundant in sharp crystals of small size, and hence is probably not analcite. 
On this account, as well as because of the general character of the rock in which 
it occurs, the mineral is considered to be leucite. A number of small rectangles 
of nepheline, now somewhat altered, occur in groups here and there. A few grains 
now replaced by carbonate suggest the original presence of a sodalite mineral. 
The outlines of what were probably pyroxene phenocrysts are preserved in a few 
cases. Several good-sized grains of magnetite are present. The groundmass is 
quantitatively important, but now much decomposed. The only recognizable 
constituent is orthoclase in narrow laths. The interstices between the feldspar 
are turbid and stained deep brown, probably by iron, which may have resulted 
from the oxidation of minute grains of aegirine such as occur in the phonolites. 
The texture is trachytic, with fluidal structure imperfectly shown, and with a trace 
of intersertal texture suggests a relation to basalts. 
This rock could probably never have been present in great amount or it would 
have been detected elsewhere. The existence, however, of even a small amount 
of a potash-rich leucite rock as a product of a magma essentially sodic® is of more 
than passing interest. 
LATITE-PHONOLITE. 
GENERAL DESCRIPTION. 
Necessity for a new division .—There are present in the Cripple Creek dist rict 
numerous intrusive masses which give evidence of close relationship. They are 
composed of rocks, similar or identical in appearance, which study shows to be so 
closely related that they constitute a definite and distinct rock type. Although 
this type shows certain mineralogical and chemical relations to plionolite, trachyte, 
latite, and trachydolerite (or essexite), and although its extreme facies, if considered 
by themselves, might, perhaps, be placed in some of these well-known families, 
nevertheless when account is taken of the close interrelation of all the facies it 
becomes plain that even the extremes are bound more closely to one another than 
to somewhat similar rocks within the near-by divisions. This type, therefore, 
though w y ell defined, does not correspond with any of the rock groups generally 
recognized, but instead occupies an intermediate position. In view of the principal 
relationships which these rocks show and which it is desired to emphasize, they 
have been given the name latite-phonolite. 
Definition .—It is proposed to apply the name latite-phonolite to those por- 
phyritic rocks which have a composition intermediate between that of plionolite 
and that of latite. To make this term definite and specific it will be well to give 
exact definitions of both latite and plionolite. Plionolite is used, in the sense 
already employed, to designate a rock containing high alkalies and alumina with 
moderate silica and 1ow t bivalent bases, characterized by the presence as pheno¬ 
crysts of alkali feldspar, nepheline, and an alkali pyroxene. Latite, 6 a rock 
intermediate between trachyte and andesite, and the porphyritic equivalent of 
monzonite, contains moderate amounts pf silica, iron, magnesia, and lime, plentiful 
alumina, and notable alkalies of which potash is most prominent, and is commonly 
a See p. 113. 
b Ransome, F. L., Am. Jour. Sci., vol. 5, No. 29, 1898, p. 372. 
