CHAPTER IV. 
GEOLOGY OF REGION BETWEEN BILL WILLIAMS'S FORK AND RIO VIRGEN. 
INFUSORIAL STRATA OF WHITE ROCK.—CHEMEHUEVIS VALLEY.—MOJAVE MOUNTAINS.—MoJAvE CANON.—METAMORPHOSED TERTIARY 
ROCKS ty POMPEY EIS. AND TRACHYTES, .—MOJAVE VALLEY.—BLACK MOUNTAINS.—PREVALENCE OF ERUPTED ROCKS.—DEAD MOUN- 
DONY AND CRYSTALLIZED QUARTZ.—PLUTONIC CHARACTER AND SCENERY OF THE BLACK MOUNTAINS.—Rock SALT AND GYPSUM FROM 
RIO VIRGEN, 
For many miles above the mouth of Bill Williams’s Fork the Colorado is bordered on the 
west by Monument mountains. For the first five or six miles little but granite is seen. This 
is similar to that exposed.in the cafion, but contains fewer veins. Mount Whipple, which 
belongs to the Monument range, is apparently granitic throughout. Near where we left these 
mountains the spurs which approach the river exhibit, in addition to the granite, porphyries, 
trachytes, and tufas, of various colors, but principally red and purple. Much of the porphyry 
is exceedingly beautiful, very hard and fine grained, and of a deep purple specked with white. 
On the east side of the river, in the immediate vicinity of the stream, for several miles 
above the mouth of Bill Williams’s Fork, are low hills of Tertiary sandstone and conglomerate, 
covered with loose gravel. This gravel is called by Mr. Marcou ‘‘ quaternary drift,’’ but it is 
necessary that this and all similar deposits in the valley of the Colorado should be carefully 
distinguished from the Drift, technically so called, of the Mississippi valley, as true Drift is 
wholly wanting in all parts of the route of the expedition west of the Rio Grande. 
The beds of recent transported materials which we met with all belong to what has been 
called ‘‘ valley drift,’’ being alluvial rather than diluvial in character. 
At our Camp 35 the Tertiary strata of which I have spoken are very fully exposed, forming 
bluffs in some places one hundred and fifty feet high. They are here considerably disturbed, 
being inclined at a high angle, and exhibit many dislocations. The most conspicuous member 
of the series is a soft dark red sandstone; but at one point a series of about twenty-five feet in 
thickness is exposed, consisting almost entirely of white chalk-like infusorial rock. This occurs 
in layers from a few lines to a foot in thickness, interstratified with which are beds of rounded 
and frequently large boulders. The infusorial strata are inclined at an angle of 30°, and rest 
upon the red sandstone of which I have spoken. A bed of yellow clay horizontally stratified 
lies upon the upturned edges of the infusorial rocks. This is a Quaternary deposit which 
occurs in many localities in this vicinity and higher up the river, forming bluffs and terraces 
sometimes two hundred and fifty feet above the present bed of the stream. 
Several lines of upheaval cross the river in the vicinity of Camp 36, connected with a range 
of hills which crosses Bill Williams’s Fork a few miles east of the Colorado. The igneous 
axes of these lines of elevation are greenstone and granite. By them the Tertiary rocks are 
thrown up into hills of fantastic outlines and vivid colors. 
CHEMEHUEVIS VALLEY. 
This is a basin of limited extent, enclosed by the Monument and Mojave mountains, and 
the spurs given off from these chains. The Tertiary infusorial strata, which have been men- 
tioned, reappear in several localities in this valley, but much disturbed. They are covered by 
