468 SUPPLEMENTARY INVESTIGATIONS IN THE GRAVEL REGION. 
good specimens of basalt. The microscopic examination by Mr. Wadsworth has confirmed the field 
observation. Near the point from which these specimens were taken, there were also good expo- 
sures of bed-rock. On the crest of the ridge, just above the mining claim, on the Oroville road, at 
an altitude of 5,060 feet, there is again a gray, andesitic lava. If these observations are correct, 
this part of the Mooretown ridge shows basalt at the base and andesite above. This, I believe, is 
‘an order of arrangement just the reverse of what has been supposed to be the rule in the Sierra 
Nevada. In further support of the observations here described, I will give the substance of the 
information I obtained upon this point from Mr. D. Post, a miner and prospector whom I came 
across by accident upon the ridge. Mr. Post showed himself to be an unusually intelligent 
observer, and was evidently very familiar with the principal topographical and geological features 
of the vicinity. I took the time to go to one or two places in regard to which Mr, Post’s descrip- 
tions had excited my curiosity, and found that his statements were fully in accord with the facts. 
According to Mr. Post’s observations, the ridge between Rock Creek, a creek lying to the south- 
west of the limits of the map on Plate X (Fig. 1) and the South Fork, is capped with “ black 
lava”; the Mooretown ridge is in the main capped with black lava, similar in character to the 
specimens taken from the ditch, but it is crossed by a flow of “ gray lava” at an angle of between 
forty and sixty degrees. This flow is about a mile in width, and can be traced in a similar way 
across other ridges, in a direction from southeast to northwest. 
There is also evidence in the bed of Fall River of a deeper flow, or of a broad dyke, of gray 
lava. Under the guidance of Mr. Post, I went to examine the locality where the deep channel 
was reported to cross the river, which is about a half-mile below the mouth of the east branch of 
the river, the branch on which the Davis Point gravel lies. Near the junction of the branch with 
the main stream a dam has been built to turn the water of the stream into the China Gulch Com- 
pany’s ditch. The altitude of the dam I made to be 4,615 feet. From the dam I followed the 
line of the ditch down the stream. The bed-rock exposed is granite. About a half-mile below 
the dam the granite suddenly gave place to what looked like a gray volcanic tufa. Between the 
granite and the tufa there was a layer of clayey material, six feet in thickness. This belt of vol- 
canic rock is said to extend for nearly half a mile along the ditch, after which the clayey streak 
and the granite come again in the reverse order. I did not. have time to verify this statement by 
personal observation. When first excavated, the voleanic rock was hard and required blasting. 
The specimen which I brought from this point has been examined microscopically by Mr. Wads- 
worth, who calls it “ trachyte (?) or an allied somewhat altered andesite.” In the bed of the 
stream, below the ditch, volcanic rock takes the place of granite for a distance estimated at from 
ten to twelve hundred feet. A specimen taken directly from a good exposure of this rock, where ~ 
it was kept clean by running water, has been called andesite by Mr. Wadsworth after a microscopic 
examination. ‘The altitude of the river-bed at this point I made to be 4,540 feet. Upon the map 
(Plate X, Fig. 1) I have marked the approximate position of my point of observation only, without 
connecting the lava with that of the ridge. Nothing is known as to the depth to which this lava 
extends. Mr. Post’s examination of the country leads him to regard it as the filling of an old chan- 
nel eroded at some time subsequent to the formation of the basalt, and to estimate its depth at the 
river crossing to be 300 feet. A few facts which support the hypothesis of the existence of such a 
channel will be given further cn. 
It has been stated already that granite bed-rock is found below the dam across Fall River. At 
the Davis Point mine the bed-rock is a very soft, green, talcose or chloritic slate. It strikes N. 
40° W. (magnetic), and is nearly vertical in dip. This slate-belt, however, cannot be more than 
200 feet in width. To the northeast of the mine the bed-rock seen in the ditch, near the point 
from which the specimens of basalt were taken, is of a granitie or dioritic character. A similar 
rock is seen near Post’s cabin in Wilson’s ravine, a ravine on the southeastern slope of the ridge 
leading to the South Fork of the Feather River. 
The gravel of Davis Point lies on the northwestern slope of the ridge, a short distance above © 
Fall River. The altitude of the bed-rock at the mouth of the tunnel I made to be 4,800 feet. 
