J. W. Powell—Types of Orographic Structure. 407 
XI.— Henry Mountain Structure. 
Sometimes we find the sedimentary strata displaced. by a 
quaquaversal upheaval and the same fractured, and through 
these fractures floods of lava have poured, and these may lie in 
patches about the flanks of the mountains, or stand in dikes 
where the walls of the crevice have been swept away by denud- 
ation. In the Henry Mountains we have a fine illustration of 
this type of structure. These mountains have been studied by 
Mr. Gilbert during the past season, and in his preliminary re- 
port he says: “The eruptions of the Henry Mountains are of a 
character entirely novel to me, and they were studied with an 
Interest stimulated by surprise. A description of a single one, 
though it will not stand for all, will serve to illustrate the type. 
Mount Ellsworth is round, and its base is six or eight miles 
broad. The strata of the plain about it are horizontal on every 
side. Near the mountain the level strata become slightly in- 
clined, rising from all sides toward the mountain. At its base 
the dip steadily increases until on the steep flanks it reaches a 
maximum of forty-five degrees. Then it begins to diminish, 
and the strata arch over the crest in a complete dome. But the 
top of the dome has cracked vpen, and tapering fissures have 
Tun out to the flanks, and they have: been filled with molten 
tock, which has congealed and formed dikes. Moreover, the 
curving strata of sandstone and shale have in places cleaved 
part and admitted sheets of lava between them. So the moun- 
fan is a dome or bubble of sedimentary rocks with an eruptive 
core, with a system of radial dikes, and with a system of dikes 
Rnb with the strata. It isa mountain of uplifted strata, 
en i ed 
: ed and permeated by eruptive rock, . “ 
fi . i 
‘Oo r : : 101 { i e i a ie 
°r convenience, I subjoin the following SE ccoind dis- 
cuss! 
