AN ANAELCITE-BASALT FROM COLORADO 6gI 
their communication — ‘‘ Uber Monchiquit, ein camptonitisches 
Ganggestein aus der Gefolgschaft der Elaolithsyenite”—is of 
course in harmony with the well-known attempts of Professor 
Rosenbusch to make geological occurrence the foundation stone 
in the classification of igneous rocks. And whether one believes 
or does not believe that the ‘‘dike rocks’’ of Rosenbusch have 
individually or collectively that restricted geological occurrence 
and that constant association indicating their origin which are 
assumed in the system of that master, it is of great importance 
to the development of petrography to know the facts. An asso- 
ciation of rocks, the importance of which may be exaggerated 
from certain standpoints, should not on that account be slighted 
by those who occupy other points of view. 
The analcite-basalt of The Basin occurs in a region where 
there is a great series of volcanic rocks, mainly andesitic, with 
basalts, trachytes, and rhyolites, all more or less prominent within 
five miles of The Basin. Phonolites occur in abundance at and 
near Cripple Creek, but there seems to be no reason for assum- 
ing any relation whatever between the magma of this analcite- 
basaltand that of the Cripple Creek center. . As has been des- 
cribed' there are basic dikes at Cripple Creek, some of them 
plagioclase-basalts and some containing a scanty, colorless, 
residual material of indistinct character, which was interpreted 
as nepheline in large part, and hence these rocks were called 
nepheline-basalts. From the much decomposed condition of 
these dikes I am unable to say upon reéxamination that they 
may not originally have contained analcite or a glassy base, but 
still believe it more probable that they were nepheline rocks. 
Since it is evident that the monchiquite or analcite-basalt 
magma contains nothing peculiar to itself except water, it is diffi- 
cult to see why Rosenbusch should regard it as a differentiation 
product of different origin from other basaltic magmas. Nor is 
it plain why any significance, as to genetic relations, should be 
attached to the fact that the supposed glassy base of the mon- 
"Geology of the Cripple Creek District, Colorado, by WHITMAN Cross,  Six- 
teenth Ann. Rep. U. S. Geological Survey, 1895. 
