288 The American Geologist. November, 1903. 
river proper and Black river, from the confluence with the lat- 
ter stream, flow along the downthrow side of the fault or down 
the fault itself. 
Since the faulting and tilting took place, the fault escarp- 
ment, has so far retreated that its front now presents a series 
of transverse valleys and mountains extending toward the fault 
line. 
The White Mountains. — These mountains were examined 
but little by the writer in the vicinity of the highest points, viz., 
Mts. Ord and Thomas; but wherever visited the structure was 
found to be practically the same as that of the Mogollon range. 
The mesas of the two unite at the northeast, and the two are 
probably identical in all their features, structural and otherwise, 
except in the Sierra Blanco part. To use Mr. Gilbert's descrip- 
tion : 
"Sierra Blanco has an imperfectly conical form, and, for a moun- 
tain of its magnitude, a remarkably low angle of slope. Standing 
alone upon a high plateau, it is a conspicuous peak, but the maximum 
depth of its lava is probably le>s than 3000 feet. Its summit has no 
crater, but h composed of massive eruptions of trachyte — a variety of 
trachyte affiliated with, and passing into sanidin-doloryte — and com- 
prises a variety of knobs. From it there stretches in every 
direction long slopes of sanidin-doloryte, that appear 10 have flowed 
from side fissures, and spread in successive sheets over the 
plain. To the east these sheets extend for ten to fifteen miles, and 
to the west thirty miles. To the southwest the same material is 
continuous for forty miles to the borders of the San Carlos and 
Bonito valleys, but these were probably independent eruptions 
of it in that direction. Scattered over these broad sheets are 
rounded cinder cones, not exceeding a few hundred feet in bight, 
and with some of them are associated '"coides" of basalt. North of 
the main peak is a dense cluster of these cones, associated with basalt 
flows, that have completely covered the sanidin-doloryte. if, as is prob- 
able, it extended there. The line of eruption of which this cluster is 
the culmination, runs northwestward toward the San Francisco moun- 
tains, and is the Sierra Mogollon."" 
The Xantan Mountains and Plateau. — The Nantan Plateau 
faces the South fork Of White river at the north and extends 
from Sierra Blanco as a lava-covered table land in a south- 
western direction for a distance of forty miles where it term- 
inates in a mountain-like escarpment due to a fault of not less 
than 2000 feet throw which has occurred since the lava erup- 
tion. The lavas are of the trachvte and sanidin-doloryte types, 
