&USSELL.J OPEN FISSUKES AND FAULTS. 121 
Other faults, belonging to the series briefly described above and 
connecting it in a general way with the fault and open fissures near 
Cleft, occur on the west side of Canyon Creek, 5 miles northwest of 
Mountain Home. At the locality referred to there is an irregular but 
rudely triangular hill, as it would appear on a map, measuring about 
a mile on each side, the surface of which is inclined to the northwest. 
It is bounded by precipitous escarpments from 300 to 400 feet high on 
its eastern and southern border. The form of this prominent eleva- 
tion and the character of its precipitous borders facing eastward and 
southward show that it is an elevated block bounded in part by faults. 
The elevated tract is composed of stratified basalt, as is shown particu- 
larly in its southern escarpment, which is more clearly due to faulting 
than its steep eastern face. The fault scarp on the south side of the 
hill dies out when traced westward, but is succeeded by other escarp- 
ments bordering another and lower hill, the structure of which is 
obscure. 
The fault scarps to the north of Mountain Home are associated with 
other topographic forms, due to movements in the lava beneath that 
portion of the Snake River Plains. 
South of the locality where Canyon Creek leaves the mountains of 
rhyolite, in which its two principal branches have excavated deep, 
flaring canyons, and enters a narrow gorge with vertical walls of col- 
umnar basalt (PI. X, _B), there has been an upward bending of the 
sheets of lava so as to form what may be described as half of a low, 
elongated dome or broad-pitching anticline. The longer axis of this 
elevation runs approximately northeast and south w T est, or, in a general 
way, at right angles to the great escarpment forming the border of 
the adjacent mountains, and also normal to the direction of the small 
faults in the basalt between Canyon and Rattlesnake creeks. The 
arching of the. basalt is most pronounced at the base of the mountains, 
where it is perhaps 3 miles broad, and dies out southward in a dis- 
tance of about 4: miles, where it meets the triangular hill referred to 
previously. Canyon Creek follows the longer axis of the elevation 
and has cut a canyon in it which is 150 feet deep adjacent to the moun- 
tains and which decreases southward. On leaving the short canyon 
cut in the dome the creek becomes a surface stream and is spreading 
out an alluvial fan. The nature of the pressure which arched the lava 
in the manner just described was not determined. Seemingly it had 
no clear relation to the movements which produced faulting in the 
adjacent portion of the plain. Relief from pressure in one instance 
was obtained by an arching of the strata, while in the other instance 
breaks occurred. One of these breaks, however, as noted above, 
cut the fold near its northern end and was certainly of later date 
than the upraising of the arch. Then, too, the longer axis of the 
fold is at right angles to the trend of the neighboring faults, which 
