Gilbert's Geology of the Henry Mountains. 19 
In Mt. Ellsworth “from all sides the strata rise, slowly at 
first, but with steadily increasing rate, until the angle of 45° is 
reached. Then the dip as steadily diminishes to the center, 
where it is nothing. A model to exhibit the form of the dome 
would resemble a round- 
topped hat; only the level 
rim would join the side by a = 
rapidly outward. (See fig. 8.) -"— 
The base of the arch is not Restoration of Mt. Ellsworth. 
circular, but is slightly oval, 
the long diameter being one-third greater than the short. The 
length of the uplift is a little more than four miles; the width 
a little more than three miles, and the height about 5,000 feet. 
The curvature fades away so gradually at its outer limit that it is 
be easy to tell where it ends, 4, 
and the horizontal dimen- - 
sions assigned to the dome a 
are Sy 
dikes, and the weathering Sea, ee, 
brings them so conspicuous! mas 
to the surface, that the softer  Ground-plan of trachyte-dykes on the 
sedimentaries are half con- estern flank of the mountain. 
of the mountain, where they are less complicated than in the 
central regions.” 
“The trachyte masses and the altered rocks in contact with 
them are so much more durable than the unaltered strata about 
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