

468 



SUPPLEMENTARY INVESTIGATIONS IN THE GRAVEL REGION. 





good specimens of basalt. The microscopic examination by Mr. Wadsworth lias confirmed the Held 

 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. I). 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 tacts. 

 According to Mr. Post's observations, the ridge between Eock 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 How, 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 volcanic 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 foot, 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 whioh support the hypothesis of the existence of such a 

 channel will be given further on. 



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, taleose 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 hod-rock seen in the ditch, near the point 

 from which the specimens of basalt were taken, is of a granitic 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 

 loading 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 Paver. The altitude of the bed-rock at the mouth of the tunnel I made to be 4,800 feet. 





























