TO YELLOWSTONE NATIONAL PARK. 105 
®. 
precipitous. The angle of the cone is about 40° or 41°; indeed, it is so steep, and the loose blocks 
of trachyte furnish so insecure a foot-hold, that, were it not for the trails made by the mountain- 
sheep ascending and descending, it would be no easy task to climb it from the west side. 
The mineralogical character of this trachyte deserves to be described a little in detail, as it 
may be taken as a type of the variety which occurs most widely in these mountains. It is in gen- 
eral of uniform texture, hard and firm, though occasionally showing minute cavities containing 
quartz crystals as a secondary product. It breaks on weathering into the large thin slabs which 
cover the sides of Cone Butte. The main constituent of the rock is a triclinic feldspar, as revealed 
by a thin section under the microscope, though occasional crystals of orthoclase of greater size may 
be observed. Hornblende follows next in order, the crystals being very distinct; and, further than 
this, magnetite plays an important rdle—this is distributed more generally than is common in 
similar rocks, and is seen by the microscope as extremely minute grains, whose metallic character 
is revealed only in reflected light. These particles of magnetite have suffered alteration to a 
considerable extent, and the feldspar is often stained red and yellow in a ring about them by the 
oxidized iron. It is to this alteration that the peculiar red color of the talus on the sides of the hills, 
as seen from a distance, is undoubtedly due. A critical examination shows that a little quartz is 
also present; but, as it was often otherwise noted in minute cavities, it may be questioned whether 
it is not merely a secondary product. 
The descent from Cone Butte was made by way of the deep ravine which separates it from the 
trachytic hills to the south. The white trachyte is carried down nearly to the gap, where (see 
fig. 7) we passed a transverse dike, east and west in direction, of a hard green trachyte, with a 
cubical fracture breaking into large angular blocks, in striking contrast with the loose slabs of the 
other trachyte which cover the slopes of Cone Butte. This is probably a later dike, subsequent to 
the formation of the other hills. This trachyte, as well as that of Cone Butte, was found in frag- 
ments abundantly over the prairie, even to a distance of fifteen miles from the mountains. It is 
characterized by large crystals of a glassy orthoclase, which give it a porphyritic structure. Under 
the microscope, these crystals are found to be more or less clouded, in consequence of incipient 
alteration: this is also shown by the indistinct colors obtained in polarized light. Accompanying 
the large crystals of orthoclase are smaller thin-bladed crystals in large numbers. The other essen- 
tial constituent is the hornblende, which is seen in simple distinct prisms: it has a deep-green 
color, and is strongly dichroic. No quartz was observed. The most interesting feature of the rock 
eg 
SSS SS ee 
——S EOS ss 
Sh ee ee 
= Se ae 
6S ees AS ey oe 
is the green base, which, under a low magnifying power, seems to be without structure, but, when 
magnified highly, is resolved into countless minute, acicular crystals, jumbled together in a con- 
14 Ww 
