ISOSTATIC THEORY 
101 
to represent. The scarp proved to be in all cases the face of a 
recently cut rock-shelf fashioned out of solid rock much after 
the manner of a wave-cut cliff on an exposed coast of the sea. 
Nor was the scarp confined to the raised side of the tilted mountain 
block. In some desert ranges it marked both sides. In others 
all sides; in fact some were completely girdled. There was clearly 
no genetic relation between steep face and structure. Much 
prospecting had evidently been uselessly undertaken. 
That among all the mountain masses of the Great Basin and the 
Mexican Tableland there should not appear a single instance of 
major faulting of recent date was surprising. Instead of being 
orographic blocks lately outlined, dislocated and tilted, the desert 
ranges proved to be mainly mountains of erosion. Longitudinal 
fault-lines very generally failed to characterize the piedmonts. 
The tectonics were manifestly chiefly relatively ancient. Wherever 
longitudinal faulting developed it was usually far out on the 
plains. Isostatic compensation surely did not obtain in the case 
of narrow, single mountains. Along the so-called fault-scarp ore- 
bodies were not to be sought. All geological conditions pre¬ 
cluded ore localization in such situations. Small wonder, then, 
that mining camps did not display themselves along lines most ex¬ 
pected. 
Mine exploration has another fundamental bearing upon the 
verity or falsity of isostasy as usually postulated. On a scale 
very much larger than any desert range the cordillera of the 
Rocky Mountains is recently the theme of isostatic measurement 
by means strictly physiographic, supported by practical expression 
in mine development. The crustal span here is so long that if the 
floating-block idea be true mineralization should go on extensively 
at the foot of the limbs on either side. This clearly does not 
obtain. ^ From earliest times this great mountain tract seems to 
have undergone continuous oscillation. The region is one which 
is repeatedly uplifted. It is one which suffers again and again 
tremendous planation. 
Profoundest planation characterize several relatively late 
geologic periods. In this region especially Comanchan, Laramian, 
Miocene, and Recent times are conspicuously marked. Peneplana- 
tion during the first of these periods was particularly wide spread. 
Singularly the Jurassic ancestral Rockies do not appear to have 
grown higher as their summital substance wastes away, as the 
