DETAILS OF VALLEYS. 55 
sheet and contains fragments of scoriaceous basalt immediately above 
each sheet. The basalt extends completely across the old debris- 
filled valley to the rock walls on either side. 
That the basalt burst upward through the sands and gravels in the 
midst of the old filled valley is proved by the occurrence of a large dike 
or neck, about 400 feet wide and several thousand feet long, exposed 
in the Williams Valley near the point where the section was made. 
The stratified deposits through which this dike broke are disturbed and 
burned to a brick red for about 300 feet from the dike. Several 
smaller dikes occur in the vicinity, and some of the eruptive masses to 
the north, seen only from a distance, appear to be volcanic necks 
standing as sharp pinnacles several hundred feet above the level of the 
surrounding plain. 
OTHER VALLEYS. 
Location and character.- — There are several intermontane plains 
south of Williams River, known as the Cactus, Posas, and Ranegras 
plains, McMullan Valley, and an extensive unnamed plain south of the 
Harquahala Mountains. These are parts of the great desert plains of 
southwestern Arizona. They are, in part at least, old debris-filled val- 
leys similar to those just described, but their original depth and the 
•character of the filling can be judged only by surface indications. 
These plains have a general altitude of about 2,000 feet in the eastern 
half of the area described, and decline gradually westward to Colorado 
River (altitude 345 feet) . 
Detrital filling. — Near Williams River the Cactus Plains have been 
somewhat dissected by erosion; detrital bluffs 500 feet or more in 
height occur, while the total depth of erosion into the detritus (the 
difference between the general surface altitude of 2,000 feet and the 
river elevation of 600 feet) is about 1,400 feet. The bluffs thus ex- 
posed are composed of clay, sand, gravel, and angular rock fragments, 
consisting of granites, andesites, etc., with sheets of embedded basalt. 
Near the mouth of Date Creek a sheet of basalt about 30 feet thick 
caps the bluffs, but passes into the detrital beds a short distance back 
from the face of the cliffs. West of Date Creek two basalt sheet s occur 
in the detrital beds. The lava was outpoured upon an old surface and 
metamorphosed the material beneath it to some extent. At the sur- 
face of each of the lava sheets the cavities contain zeolites, and the 
overlying sands contain fragments of the basalt, indicating that the 
gravels and the lavas are contemporaneous. 
The detrital material exposed in the bluffs is comparable in thick- 
ness and has the same general appearance as the oldest det lit us in the 
Big Sandy Valley and the Temple Bar conglomerate of the Detrital- 
Sacramento Valley. The basalt sheets embedded in it have the same 
