TERTIARY VOLCANIC ROCKS—LATITE-PHONOLITE. 
83 
This calculation is based oil the assumption, suggested by an examination of 
the thin sections, that the pyroxene is about double the amount of the hornblende. 
It also assumes that the ration of segirine to augite is 1:10,° and that half the water 
given off above 100° represents primary water, and rests on the following arbitrary 
but more or less probable compositions assigned to the augite and hornblende: 
Augite = Ca(), MgO, Fe0(Al,Fe) 2 0 3 , 3Si0 2 , the ratio of A1 : Fe"' being 4 : 1. 
This is the same formula as that used for the phonolites (p. 67). 
Hornblende = CaO, 2MgO, 2FeO, 3(Al,Fe) 2 0 3 ,6SiO ? , the ratio of A1 : Fe'" being 13:1. 
Since the aegirine is united with the augite to form a single pyroxene mineral, 
and since the albite and anorthite form an intermediate plagioclase, oligoclase- 
albite, the following table expresses as accurately as possible the kind and relative 
amounts of the mineral components of these rocks: 
Mineral coniposition of latite-phonolite. 
Orthoclase b . 29.75 
Oligoclase-albite (approximately Ab s A n i) _- 29. 18 
Sodalite. 2. 74 
Nosean. 3. 53 
Analcite. 4. 60 
Soda-augite.-. 11.25 
Hornblende. 5. 63 
Apatite. 2. 37 
Titanite. 2. 36 
Magnetite. 1. 96 
Other minerals, hygroscopic water, water of 
hydration, etc. 6. 63 
100.00 
TRANSITIONS TO PHONOLITE. 
In the course of the mapping and study of the Cripple Creek rocks, three varie¬ 
ties have been found which show, each in a somewhat different way, a mineralog- 
ical gradation of latite-phonolite into phonolite, or vice versa. This is what one 
would expect from the chemical relations just given. These three varieties will be 
briefly described, most emphasis being placed on the points which show the relation 
of the one group to the other. 
On the upper southern slope of Bull Cliff occurs a fine-grained rock, holding an 
occasional small feldspar phenocryst and showing, by slight decomposition, the 
fluidal arrangement of its groundmass feldspars. It strongly resembles the phono¬ 
lites in appearance and seems in the field to grade directly into that type. The 
microscope confirms this similarity, and shows a few narrow feldspar phenocrysts, 
many grains of jegirine, and small phenocrysts of segirine-augite, a few decomposing 
nosean crystals, numerous minute grains of sodalite, but only an occasional small 
crystal of nepheline near or in the irregular analcite grains. 
In a railroad cut on the east side of Battle Mountain, near the Rigi mine, is 
exposed a rock, a little of which is fairly fresh, which appears to be a contact facies, 
or rather marginal facies, of the large latite-phonolite area occurring east and south¬ 
east of the Portland mine. It is brownish gray, considerably mottled, nearly or 
a It happens that the molecular weights of oegirine and of the augite chosen are identical. 
* It is probable that a small amount of the soda calculated as plagioclase enters into the composition of orthoclase. leav¬ 
ing the plagioclase a little more calcic than AbjAni. 
