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GEOLOGIC GUIDEBOOK ALONG HIGHWAY 49 



[Bull. 141 



of architecture prevails among the older frame buildings and the large 

 frame hotel is a delightful spot shaded by large trees. Red, pock-marked, 

 Eocene gravels outcrop on the hills to the northeast of town much as the 

 gold diggers left them. 



Half a mile below the Camptonville turnoff, a group of pegmatite 

 and aplite dikes stand out of the deeply weathered bedrock. The light 

 colored minerals of the dikes show little evidence of weathering whereas 

 the biotite-rich matrix is decomposed. Half a mile above the Campton- 

 ville road deeply weathered cleavable meta-volcanics can be seen lying 

 below the deep red soil mantle and below Tertiary gravel deposits. The 

 meta-volcanics are so decomposed that they disintegrate when tapped 

 with a hammer. Few places so clearly illustrate the relationship between 

 the deeply weathered rocks of the Eocene surface, the Eocene channel 

 gravels, and the present-day land surface. The road cuts are also an 

 excellent study in direct formation of soil mantle from parent bedrock 

 lying immediately below. 



Between Camptonville and the Frog Hollow public camp, several 

 small bodies of deeply weathered granite rock outcrop along the road- 

 side. These approximate a gabbro in composition and were apparently 

 too small to be mapped by the early members of the Geological Survey. 

 They mapped this entire area as amphibolite but this designation covers 

 a wide variety of rock types including fine grained basic and inter- 

 mediate intrusives, green meta-volcanics, mica schist, and the afore- 

 mentioned gabbro. 



Frog Hollow, three miles north of Camptonville, is the first of a 

 series of public camps scattered along or near the North Fork of the 

 Yuba River. Frog Hollow is on Willow Creek which is not a tributary 

 to the North Fork of the Yuba but runs into Bullards Bar reservoir to 

 the west of Camptonville. These public camps lie in a naturalist's and 

 fisherman's paradise and are likewise good base camps for the geo- 

 logically minded. The 36 miles between Frog Hollow and Sierra City lies 

 through excellent outcroppings of a wide range of igneous and meta- 

 morphic rocks of great interest to petrologists and rock lovers of all kinds. 



In the vicinity of the Joubert hydraulic diggings, the highway 

 crosses from Yuba to Sierra County. Sierra County is famous for the 

 many large gold nuggets it has produced. One from the Monumental 

 mine at Sierra Buttes weighed 1596 oz. troy, worth $17,654. The placers 

 of French Ravine produced four nuggets which weighed from 93 to 532 

 oz. each. Henry G. Hanks in his compilation of famous gold nuggets 

 published in 1882 lists no less than 13 Sierra County nuggets among 

 the 83 largest produced in the world to that date. The Ruby mine, pre- 

 viously mentioned in connection with Alleghany, is still producing size- 

 able nuggets. 



Highway 49 passes through the heart of the Joubert hydraulic dig- 

 gings and the Eocene gravels and clays can be studied there at close 

 range. The Joubert deposits contain a large percentage of white clay 

 which would be valuable if it could be separated from admixed gravels. 

 Some areas are of almost pure white clay. Other parts of the deposit 

 have off-colored clays ranging from deep red to lavender. Clay deposits 

 are present in association with many other Eocene gravel deposits of 

 Nevada, Yuba, and Sierra Counties, but to date, the only clays of the 

 Sierran foothills that are being worked are at the edge of the Sacramento 

 Valley, principally in the vicinity of lone, Amador County. 



A splendid panorama up the V-shaped canyon of the Yuba may 

 be seen 2.6 miles northeast of Joubert just before the highway descends 

 to the level of the river. Youthful stream profiles may be seen at other 

 points along the highway between Joubert and Downieville. 



A thick series of partially metamorphosed volcanic rocks is exposed 

 in the vicinity of the highway bridge over the North Fork of the Yuba. 

 Amygdaloidal flows, agglomerates and tuffs may be seen in various stages 

 of metamorphism some completely recrystallized to schist and others 

 partially altered. Metallic minerals such as magnetite are disseminated 

 in the lavas along the dirt road leading off to the north of the bridge. 

 Good parking places may be had along this road and exposures are 

 good along the river and in places along the road. 



Indian Valley public camp, the second of a series of excellent camp- 

 sites beside the river, is located two miles east of the bridge. Half a 

 mile before reaching the camp is the granodiorite-meta-volcanic contact. 

 The granodiorite, which is well exposed in a quarry east of Indian 

 Valley camp, is much like the average granodiorite south of Campton- 

 ville. The belt of serpentine which appears on the map is not evident 

 from the highway. The granodiorite intrusion is 1.7 miles wide along the 

 highway. One mile west of Ramshorn Creek bridge is a good exposure 

 of gray-green chert associated with phyllite and meta-volcanics. Another 

 public camp is located at the junction of Ramshorn Creek with the North 

 Fork of the Yuba. 



Midway between Ramshorn Creek and Goodyears Bar, good views 

 of the columnarly jointed basalt of Bald Top Mountain may be seen to 

 the north. The Bald Top and adjacent lavas represent the latest period 

 of Sierran volcanism with the possible exception of some flows on the 

 east side of the range. 



Goodyears Bar, located near the junction of Woodruff and Good- 

 year Creeks with the Yuba four miles west of Downieville, was settled 

 in the summer of 1849. It prospered during the 1850 's before its rich 

 placers were exhausted, but a fire which virtually destroyed the town 



