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LITTLE GRASS VALLEY AND VICINITY. 



467 



map (Plate X, Fig. 1). This map is on the same scale as that on Plate R, and includes a piece 

 of territory lying to the west of the northern part of that map. The district is one of which 

 very little is known with accuracy. The United States surveys have been extended over it only 

 in part. There are but few inhabitants, and but few good roads or trails. The chief importance 

 of the district, from a mining point of view, lies in the possibility that somewhere in this quarter 

 the higher portions of the La Porte channel may ultimately be traced. Extensive mining claims 

 have been laid out, and small amounts of work have been done in several places. In the short 

 time that I had to spare, I could not make any satisfactory examination of the country ; I saw 

 only the outline of a geological problem, which promises interesting results to some future explorer. 

 The routes I followed were these : from La Porte to Davis Point and return, from Gibsonville 

 to the Monitor shaft and return, and along the stage-road which leads from Gibsonville to 



Quincy. 



One very interesting feature of the geological problem is presented by the distribution of the 

 volcanic rocks and their mutual relations. Above the Little Grass Valley Bald Mountain, which 

 is evidently capped with lava, the whole of the valley of the South Fork, as seen from a point on 

 the stage-road near the base of Pilot Peak, appears to be free from volcanic rock. On the road 

 itself bed-rock is seen shortly after crossing the first ridge above Gibsonville, the volcanic rock at 

 the same time rising with the ridge to the east. Frequent exposures of bed-rock at high altitudes 

 are also to be seen upon the upper trail from the Monitor shaft to Gibsonville. The serpentine 

 belt is crossed by the road several times between Gibsonville and Onion Valley, but no more lava 

 is encountered until the crest of the ridge north of Onion Valley is reached. The altitude of the 

 highest point on the road between Gibsonville and Onion Valley I made to be G,390 feet ; that of 

 the hotel at Onion Valley, 6,160 ; and that of the summit between the last-named place and Nelson 

 Point, 6,430. The dotted red lines on the map on Plate 11, from Pilot Peak to Bald Mountain, 

 are intended to indicate only a probable former connection. 



The crest of the ridge from La Porte to Gibsonville has already been spoken of as covered with 

 an andesitic lava, grayish in color. On the descent from the crest of this ridge to the Monitor 

 shaft I saw boulders of black, basaltic lava near the bottom of the canon, at an altitude some two 

 or three hundred feet above the shaft. The .shaft is about four miles above Little Grass Valley, 

 at a point opposite La Porte ; it was started a few rods from the bed of the stream and scarcely 

 five feet above the level of the water. The bed of the stream at this point is in gravel. The alti- 

 tude of the mouth of the shaft I made to be 5,282 feet. About an eighth of a mile down stream 

 from the shaft a basaltic dyke of at least twenty feet in width crosses the stream. Similar basaltic 

 dykes are frequently met with in running bed-rock tunnels in this vicinity, as I was told by Mr. 

 Winchell, of Gibsonville. 



Westerly from Bald Mountain, Blackrock Creek gets its name from the color of the rock through 

 which its channel has been eroded. Seen from a little distance, the black rock lias all the appear- 

 ance of basalt. Upon the divide between the South Fork and Fall Kiver, known as the Moore- 

 town ridge, both andesitic and basaltic lavas are to be seen. The trail which I followed crossed this 

 ridge from a point near the junction of Blackrock Creek and the South Fork to the head of Fall 

 Kiver and thence down that stream as far as Davis Point. The altitude of the bed of the South 

 Fork at the trail crossing I made to be 4,930 feet. The ascent is quite steep from the base of the 

 ridge. A little less than half-way up the slope, at an altitude of 5,240 feet, there is a narrow, lint 

 bench, above which a second slope rises, steeper than the first. The summit of the trail has an 

 altitude of 5,660 feet. Owing to the presence of soil and vegetation, it was not possible to tell 

 just where the lower limit of the volcanic capping was, nor whether there was more than one kind of 

 lava on the ridge at this point. The surface fragments seen at the summit resembled those on the 

 ridge above La Porto. On the Fall Eiver slope of the ridge there are good exposures of rock in the 

 ditch which has been dug to bring water to the gravel claim at Davis Point. The ditch is two 



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miles and a quarter long, and it follows very nearly the line of junction between the bed-rock 

 and the volcanic capping- From a point a half-mile to the northeast of the claim I got 



