650 OREGON AND NORTHERN CALIFORNIA. 
tainous centre. The belt, which is nothing but the lower slopes of a 
former peak, consists of a series of sloping layers, and is about a mile 
in width; the angle of ascent without is very gradual, not exceeding 
five degrees, while within, it faces the interior with a bluff front, one 
to three hundred feet high. ‘There are several passages, one to three 
hundred yards wide, opening into the interior through the belt; and 
during freshets, the annular valley within is partly flooded, as well 
as the prairie around, so that both are a continuous level, except- 
ing some undulations of fifty or sixty feet that rise from this inner 
plain. Our party entered by a passage on the south side, travelled 
in the valley for seven miles, and left the crater again by a passage 
opening eastward. 
The central peaks are steep and rugged, and two or three are about 
eighteen hundred feet high above the plain; their summits are, in 
general, bold crests of rocks. On the ascent of one of the peaks, the 
rock proved to be a laminated trachyte, the lamination changing fre- 
quently in direction from horizontal to vertical. ‘The annexed cut 
shows one of the castellated peaks on the way up, and exhibits the 
vertical lamination. 
The rocks are all of trachyte or trachytic porphyry, and contain, 
besides glassy feldspar, disseminated crystals of hornblende and hexa- 
gonal scales of mica. In some places the feldspar and hornblende give 
it very much the appearance of a coarse syenitic gneiss. The usual 
colours are gray, reddish or purplish, and white. A common variety 
