70 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
reveal themselves principally in the presence of lime-bearing feldspars and of 
nepheline, analcite, sodalite, or nosean, minerals which contain a higher ratio of 
alumina and alkalies to silica than the alkali feldspars. The alkali-syenite porphy¬ 
ries of Rosenbusch show close relationships to this group. The trachyphonolites 
or oligoclase-sanidine phonolites of Boric,ky a are very similar and would be 
included in this type. The monzonite-porphyries differ, of course, in the same way 
as theh equivalents, the latites, having lower alumina and alkalies and notably 
higher magnesia. The trachydolerites (or essexite porphyries), which partake in 
part of the alkaline character of phonolite, though richer in bivalent bases than latite, 
present certain relations to this type. 
The latite-phonolites of the Cripple Creek district include the trachytic phono¬ 
lites, syenite porphyries, and andesites described in the earlier report. The recent 
study, with its greater advantages, has shown the impossibility of making such a 
separation. Individual rock masses frequently exhibit as wide variations as are 
found in the whole group. Although these rocks are decidedly variable in texture, 
and not exactly identical in composition, all the evidence—areal, structural, macro¬ 
scopic, microscopic, and chemical—goes to prove that they are intimately related 
and are variants of a well-defined rock type. The connection of /these latite- 
phonolites with the true phonolites of the district, as shown by the transition mem¬ 
bers and by the chemical composition (see p. 79), is an interesting feature, and since 
latites are absent in the district and it is desired to bring out the close connection 
between all these volcanic rocks, this relation to the phonolites rather than to the 
latites is emphasized in the following description. In the other direction these rocks 
grade texturally and constitutionally into the essexitic syenites, which are described 
on page 84. They are holocrystalline porphyries made up of monoclinic 
and alkali-rich triclinic feldspars, abundant pyroxene, and analcite, sodalite, or 
nosean, while ampliibole and biotite are often present. A biotite facies of trachytic 
composition is described on pages 77-78. 
Occurrence .—These rocks are less abundant than the phonolites in the Cripple 
Creek district and are found principally in the vicinity of the volcanic center. Like 
the phonolites, they are in part older and in part younger than the breccia. They 
rarely occur in the outlying granite, and then only as small dikes. Large intrusive 
masses, of which many are now reduced in size through brecciation, occur in and 
on the slopes of the amphitheater between Battle Mountain, Bull Cliff, and Bull 
Hill and extend over on the southwestern slope of the latter. Bodies of consider¬ 
able size also occur on the southeastern slope of Battle Mountain, on the northeast¬ 
ern sides of Bull Hill and of Bull Cliff, on the gentle slope north of Big Bull Moun¬ 
tain, and south of the summit of Gold Hill. Mine workings show that a number 
of these large masses are flat and that in some cases they have a greater extent 
than their surface exposures would indicate. In one case, that of the crescentic 
mass near the Isabella mine, the latite-phonolite body narrows rather rapidly to a 
dike about a thousand feet below the surface. Smaller masses or irregular dikes 
are found near Anaconda and near the Gold King mine in Poverty Gulch and are 
seen in several other places underground. 
a Archiv der Naturwiss , Landesdurchforschung von Bohmen, vol. 3, pt. 2 1884, pp. 42-49, 60. 
