176 W. N. THAVER 
physiographic history, to enable us to classify Pte as closely 
related provinces of a single major division. 
Topographically the Great Basin is a region of fault-block 
mountains and detritus-filled valleys or basins, with small areas of 
horizontal lava flows scattered over its surface. Its altitude is 
everywhere lower than that of the bordering provinces. Ransome 
says that “the impressive feature of the Great Basin . . . . to 
one . . . . who looks over it from the crest of the Sierras or from 
the edge of the Colorado plateaus in Arizona is that it is a collapsed 
region.”* The western boundary is characterized throughout 
almost its entire length by a fault-scarp that rises sharply to the 
crest of the Sierras. The eastern boundary is in some places a 
prominent scarp and in others a gentle slope from the adjacent 
highlands. 
The Columbia Plateau, although built of almost horizontal lava 
flows and lacking on the whole the detrital filling of the Great Basin, 
is not to be sharply separated from it. Fenneman’ shows a broad 
transition zone between the two divisions. This province, like 
the Great Basin, also lies between and beneath two mountain 
provinces. 
Prior to the extrusion of the lavas which now form its surface 
the Columbia Plateau was a region of rugged topography,’ prob- 
ably not unlike the Great Basin before the beginning of the period 
of basin-filling. This rugged surface was not entirely obscured by 
the lavas, as witness the Blue Mountains of Oregon. There is some 
evidence that there may have been a fault-scarp separating the 
plateau from the Cascades, but if such was the case it has been 
obscured by the lavas, and today the eastern margin of the pene- 
plain of the Cascades descends gradually to the plateau of the 
Columbia apparently without a break. 
The similarity between the Columbia Plateau and the Interior 
Plateaus of British Columbia is striking, although the two provinces 
are not contiguous, being separated a distance of something less 
than roo miles by the Colville Mountains, which stand like a bridge 
across the plateaus and connect the Rockies with the Cascades. 
1F. L. Ransome, op. cit., p. 343. 2.N. M. Fenneman, op. cit., pl. 2. 
37. C. Russell, U.S. Geol. Survey, Bull. 199, p. 61. 
