AN ANALCITE-BASALT PROMPEOLONAD OF 
THE discovery that analcite plays the role of an important 
primary constituent of certain igneous rocks must be regarded 
as one of the most interesting developments of recent petro- 
graphical investigations; and I, for one, am inclined to believe 
that Pirsson has not gone too far in his general conclusions, pub- 
lished in this JoURNAL a year ago,’ that analcite is an essential, 
primary component of many rocks now assigned to the mon- 
chiquites, a rock group described some years ago by Hunter and 
Rosenbusch. As each definitely proven instance of primary 
analcite in igneous rocks must for some time to come be of value 
in establishing its true rank as a rock constituent, the following 
description is offered, although some important details of occur- 
rence cannot be given. | 
The rock in question was found by myself in 1893, while 
engaged in the geological survey of the Pike’s Peak quadrangle. 
The exact locality, which may be identified by reference to the 
published map of that area,3 is in the small park called ‘‘ The 
Basin,’ twelve miles nearly west of Cripple Creek. Near the 
southern end of The Basin, and on its western side, at the end 
of a little ridge between two branches of High Creek, there is 
an outcrop of black basaltic rock directly on the line where a 
great complex of andesitic and basaltic breccia and agglomerate 
rests on the Dakota Cretaceous sandstone. This volcanic series 
extends far to the westward, between South Park and the Arkan- 
sas River, but only a few tongues and remnants now exist to 
the eastward of The Basin. The outcrop mentioned was regarded 
t Published with the permission of the Director of the U. S. Geological Survey. 
2“ The Monchiquites or Analcite Group of Igneous Rocks,” by L. V. PIRSSON. 
Jour. GEOL., Vol. 1V, 1896, pp. 679-690. 
3 Geological Atlas of the United States, Pike’s Peak Folio (No. 7), Washington, 
1894. 
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