9(5 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
the former, like the latter, were produced by lateral pressure in the 
surface portion of a lava flow. The ridges on the old flows are large 
because they were formed in lava that had flowed far over a nearly 
level plain, and had formed thick crusts before they were forced up 
into ridges. As is shown by the small corrugations, hollow ridges, 
etc., on the surface of the recent lava flows, there is a direct relation 
between the thickness of the stiffened crust that forms as lava cools 
and the size of the arches that may be produced in it by lateral pres- 
sure. In the case of the old lava flows referred to the crust had become 
10 feet or more thick before the arching occurred, and correspond- 
ingly broad ridges were produced. When the lava at the surface was 
still sufficiently yielding to be stretched by a force slowly applied, it 
was not ruptured, but by far the greater number of the large ridges 
are cracked, showing that the surface had become too rigid to yield 
by stretching to the force applied. The extreme case, where the crust 
had become solid, is illustrated by the flat-sided or roof-like ridges. 
The large ridges and dome-shaped elevations on the surface of the 
older sheets of the Snake River lava have attracted the attention of 
previous observers. They are referred to as follows by F. H. Brad- 
ley : a " Whatever the source [referring to the lava of the Snake River 
Plains], the material had evidently become quite viscid; for, at some 
points, where it ran over small inequalities of the surface beneath, it 
now stands in low mounds, which could not have been the case if it 
had been very fluid. That these mounds were not all formed by an 
undermining and sinking of the surrounding masses, to which some of 
them have very properly been referred, is proven by the tapering 
shapes of the closely fitting blocks which form the arch. But there 
is still room for study on all these points." 
That the explanation suggested in the above quotation is not admis- 
sible, at least in a large proportion, and, I believe, in all the instances 
observed by me, will appear, I think, from the comparison with the 
more recent forms of a similar nature presented above. Moreover, 
characteristic domes and "cracked ridges" occur near Shoshone Falls, 
etc., on the immediate border of Snake River Canyon, where the lava 
is from 500 to 700 feet thick. In the walls of the canyon, as already 
stated, sections of the ridges are exposed, which show that they are 
either hollow arches above flat lava sheets, or else the folds die out 
gradually below. No clearer evidence could be presented to show 
that they were not produced by viscid lava being drawn over emi- 
nences, even if such an occurrence was known to be possible. 
In the production of wrinkles, corrugations, hollow ridges, etc., as 
just described, we have examples of the changes produced in a still- 
yielding crust under the influence of a force slowly applied, arising 
a Sixth Ann. Rept. U. S. Geol. and Geog. Surv. Terr.. Washington, 1873, pp. 204-205. 
