OROGENY AND EARTH’S ROTATION 
61 
earth’s external form which has come into prominence mainly 
through the labors of that illustrious group of American geologists 
to whom the science is so deeply indebted for certain broad views 
and far-sighted generalizations which in spirit and expression 
recall the wide regions and clear atmosphere in which the authors 
worked. 
Prior to, and during, the surveys of the Fortieth Parallel, 
Clarence King,^® his colleagues, and others, guided mainly by the 
then recent conclusions regarding the origin of the Appalachians, 
were inclined to find for the mountain ranges of the Great Basin 
an analogous explanation. When, then, G. K. Gilbert sud¬ 
denly and dogmatically advanced his opinion that these desert 
ranges were not of the folded type of orogeny, but were formed 
through simple normal faulting on a gigantic scale, intense and 
world-wide interest was instantly aroused in them. Surrounded 
as were the mountain bases by long smooth slopes extending out¬ 
ward in all directions until they merged into the central flats of 
the intermontane plains the inference was that these slopes were 
composed of the debris washed down from the neighboring high¬ 
lands, and that the general plains area was a normal, construc¬ 
tional feature of vast proportions. In many instances these so- 
called wash-aprons extended up the mountain sides until the 
rugged, bare rocks seemed to be all but completely buried. 
It was then a short stride to that most brilliant of tectonic con¬ 
ceptions — that the desert mountain-blocks were floating and 
tumbling about upon the liquid interior of the earth, and that the 
movements did not cease until the blocks attained a state of float¬ 
ing equilibrium.^^ Accordingly, the Great Basin mountains were 
conceived to be still rising as the unloading of their summits took 
place through erosion. 
Later, the mountain profile proved not to be the result of geo¬ 
logic structure; the major tectonics was found to be very ancient; 
the so-called scarp-face bordering the piedmont resolved itself 
into a wind-cut shelf, girdling the mountains in the same way that 
a sea-cliff marks an exposed coast; the vast debris-apron, appar¬ 
ently burying the mountains up to their shoulders, was discovered 
to be often a sloping rock-floor rendered excessively smooth by the 
19 U. S. Geol. Surv. Fortieth Par., Vcl. I, p. 715, 1878. 
20 Geog. and Geol. Surv. W. lOO Merid., Kept. Prog, for 1872, p. 48. 
21 Bull. Philosophical Soc., Washington, Vol. XI, p. 52, 1889. 
