88 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
also occurs alone in good-sized individuals shoeing no resorption. Magnetite 
apatite, and titanite are notable accessory constituents. This rock, called nepheline- 
syenite in the earlier report, is regarded by Rosenbusch as a member of an inter¬ 
mediate series, the neplieline-syenite-essexite series.® It occurs not far from the 
locality of the latite-phonolite (I), which is compared with it on page 85, and it is 
probable that the respective masses of which these are specimens are connected 
underground. 
III. A rather light gray granular rock of moderately fine grain, holding well- 
formed prisms of pyroxene in a slightly pinkish aggregate of feldspar grains. Under 
the microscope the feldspar is found to range from orthoclase to oligoclase inclusive. 
A very little sodalite is present in tiny grains, and some larger areas, now converted 
into sericite, were possibly sodalite or nosean originally. The pyroxene is ver}" pale 
green and is practically isochroic, with an' extinction angle of about 40°. It is 
commonly accompanied by irregular grains of hornblende with pleochroism in light 
browns and greens, and by occasional small plates of strongly pleochroic biotite. 
Apatite is especially noticeable and is accompanied by magnetite and titanite. 
The texture is hypidiomorphic granular. This specimen and No. IV of the table 
on page 87 are from the Portland mine, and though from different masses are strik¬ 
ingly similar in chemical composition. 
TRACHYDOLERITE (BULL CLIFF TYPE). 
GENERAL DESCRIPTION. 
The summit of Bull Cliff is made up of a dense, nearly aphanitic rock, almost 
black in color, which after partial weathering gives a peculiar rough fracture as if it 
were made up of polyhedrons. It was thought by Cross to grade into normal 
phonolite to the south and to be a peculiar local facies of that rock. While later 
field study has not been able to wholly establish its geological relations, there are 
certain facts which lead to a rather different view. The summit mass is on the 
whole very fresh, while most of the true phonolite to the south is much decomposed. 
The freshest of the near-by phonolite shows the fissility, the greasy luster, the 
tablets of feldspar, and the flow structure of the type, while the rock from the 
summit possesses none of these characteristics, but occasionally shows a very small 
glistening phenocryst of mica. There seems to be no gradation in appearance near 
the boundary between the two rocks and while talus and low scrubby bushes con¬ 
ceal the actual contact it is beyond question that an intrusive contact is present. 
The Bull Cliff mass overlies and is later than the breccia. On the west side of the 
hill the contact is nearly horizontal. On the east side it dips into the hill at some¬ 
thing like 45°. Workings in the Isabella and Victor mines directly underneath 
the cliff do not encounter this rock, so far as known, but it is probable that it has 
issued from some small throat and spread out into the surrounding rocks as a 
laccolith-like mass. A rude vertical columnar jointing on the east side favors this 
view. The rock is younger than the latite-phonolite, for it is seen cutting off a 
dike-like body of that rock in the Pilgrim tunnel on the northwest slope of Bull 
Cliff. From such field evidence as can be obtained, which, it must be admitted, 
is not entirely convincing, it appears to be later than the phonolite. 
a Elemente tier Gesteinslehre, Stuttgart, 1901, pp. 177, 179. 
