TERTIARY VOLCANIC ROCKS-PHONOLITE. 57 
belonged to the same general period of Tertiary time as the rocks of the Cripple 
Creek volcano. 
To one acquainted with the work of Cross it is at once apparent that the pres¬ 
ent nomenclature of this group differs from that adopted by him. His names for 
the rocks of the Cripple Creek volcano were phonolite, trachytic phonolite, syenite 
porphyry, biotite andesite, pyroxene andesite, nepheline syenite, and basalt, the 
latter including plagioclase basalt and nepheline basalt. This difference arises 
from different conceptions of the characters of the rocks rather than from unlike 
definitions of the rock names. While this difference of views will be dealt with 
under each rock type recognized in the present report, a general comparison of the 
names used in the two papers is given at this place. 
The phonolite of Cross corresponds exactly to the phonolite of this report, with 
the one exception that the mass occurring on the summit of Bull Cliff was thought 
by him to be a peculiar phase of phonolite, while further studj^ shows it to be a 
trachydolerite. The trachytic phonolite, syenite porphyry, and pyroxene andesite 
established by Cross have been found impossible of separation, and are here named 
latite-phonolite. Microscopic and chemical examination of the rock formerly 
named biotite andesite show that it is a biotite trachyte, but is closety related 
to latite-phonolite. Nepheline has not been found in the granular rocks called 
nepheline syenite by Cross, and they are now described simply as sj^enite. Study 
of fresher representatives of the basic dike rocks than those collected by Cross indi¬ 
cates that they fall into three groups, namely, a second type of trachydolerite 
corresponding to his plagioclase basalt, monchiquite corresponding to his nepheline 
basalt, and vogesite, a rock probably not seen by him. The breccia, of course, is 
the same in both cases, though, as will be shown, there is a divergence of opinion 
concerning its composition. 
PHONOLITE. 
GENERAL DESCRIPTION.® 
Definition .—The phonolites are in general dense, fine-grained, porphyritic rocks 
of medium light to dark color, characterized by a large amount of alkalies, particu¬ 
larly soda, and considerable alumina, together with low bivalent bases and insuffi¬ 
cient silica to form free quartz or to combine with all of the abundant alumina and 
alkalies in the ratio of feldspar. The essential mineral composition of phonolite is 
in consequence alkali feldspar, nepheline, and an alkali pyroxene which usually con¬ 
tains ferric iron. Phonolite is a rare type of rock in the United States. 
Occurrence .—Rocks of this description occur in the Cripple Creek district as 
large and small bodies in and about the central volcanic area. The larger masses 
represent irregular intrusions from isolated local vents, and now, because of their 
greater resistance to degradation, form the summits of several of the Cripple Creek 
hills. As indicated on the map, phonolite occurs on Trachyte, Rhyolite, and Cop¬ 
per mountains, to the north, and on Big Bull and Straub mountains and Grouse 
Hill, in the southern part of the district. It also forms the crests of the hill east of 
Cameron, the hill east of Victor Pass, Beacon Hill, and the ridge west of the town 
a A comprehensive and detailed description of these phonolites was given by Cross, General geology of the Cripple 
Creek district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1S95, pp. 25-41. 
