48 



GEOLOGIC GUIDEBOOK ALONG HIGHWAY 49 



[Bull. 141 



along the Mother Lode which have been forced to shut down because of 

 wartime limitations on gold mining and postwar high operating costs. 

 The Eagle-Shawmut has many miles of workings and has produced huge 

 tonnages of low grade ore. The total vertical depth of the mine via shaft 

 and winze is about 3550 feet from the surface outcrop. The main shaft 

 dips at an angle which averages 65 and has been entered for many 

 years from adit number two. Ores in the mine consist of auriferous 

 quartz, gold bearing massive iron sulfides, and gold bearing ankeritized 

 country rock. The mine was opened at a very early date, being a con- 

 solidation of the old Eagle and Shawmut claims. However, most of the 

 extensive workings were driven since the turn of the century. The Eagle- 

 Shawmut has a recorded production of approximately $7,500,000. After 

 passing the Eagle-Shawmut mine, Highway 49 winds up the narrow 

 Shawmut grade to Chinese Camp. The summit of the grade affords an 

 excellent northwesterly view of the flat surface of Table Mountain lava 

 flow. The road-cuts along the Shawmut grade expose Mariposa slates 

 and green Logtown Ridge agglomerates. After passing the divide, the 

 highway crosses gently rolling country in which there are few exposures 



of basement rocks. 







Chinese Camp was a placer-mining center first settled by Chinese 

 laborers in 1849. Ruins of structures built in the fifties still remain 

 together with a few more modern dwellings. Piles of soil and gravel 

 which were turned over in frantic search for gold can still be seen in 

 every gulch. These gravels are reworked remnants of pre-existing Eocene 

 and later Tertiary deposits most of which have since been stripped off 

 by erosion. 



The prominent flat surface of Tuolumne Table Mountain lies 2^ miles 

 northwest of Chinese Camp. Though appearing as a table when viewed 

 from the road, it looks more like a sinuous river when seen from the air ; 

 and that is what it is a fossil river an ancient stream canyon cut in a 

 broad flat sheet of volcanic ash but filled to the brim by a viscous flow 

 of dark colored lava extending for 60 miles from the high Sierra Nevada 

 to its terminus against a bluff of sedimentary andesitic beds near Knights 

 Ferry. As the Sierra Nevada mass was raised and tilted slightly westward, 

 the softer ash beds were largely washed away by erosion, leaving the very 

 resistant lava flow high and dry as a dominating table. In places the 

 stream which originally cut the channel later occupied by the lava, cut 

 clear through the ash beds and even into the tops of some underlying 

 buried hill tops of bed rock. In other places where the ash beds lay deep 

 beneath the lava stream, the pre-volcanic topographic surface was pre- 

 served. It is in the pre-volcanic stream courses under this older ash cover 

 that the older gold placers were discovered and mined, as in the old New 

 York tunnel. The younger channel of Table Mountain was largely barren 

 of gold. 



The resistant latite lava flow of Table Mountain, which is probably 

 of late Pliocene age, has thus protected some of the earlier softer Tertiary 

 deposits from obliteration by erosion. The latite lava capping is flat- 

 topped, with vertical cliffs along the edge, showing columnar joining of 

 the rock. Tuffs and mudflows are exposed under or close to the base of the 

 cliffs. The latite closely resembles a porphyritic olivine basalt, the potash 

 feldspars being entirely confined to the groundmass. Some plagioclase 

 crystals, however, are more than an inch long. 



Andesitic tuffs, gravels and boulders can be seen lying directly 

 beneath the latite at Mountain Pass, where the ancient stream channel, 

 now occupied by the lava, cut a V-shaped gorge in the andesite cobble 

 deposit. At this point is a watering place known as Mountain Springs, 

 located less than half a mile from the junction of Highways 49 and 108 

 or 4$ miles from Chinese Camp. An area suitable for picnicking adjoins 

 the spring and water fountain. The water flows from the ancient ash- 

 covered bed of the Tertiary channel. 



The bouldery andesite is known as the Mehrten formation and is 

 well exposed on the road southeast of Mountain Pass. It lies strati- 

 graphically above the Eocene quartz gravels which are present lying on 

 bedrock. Some of the earlier quartz gravels, however, a mile northeast 

 of Mountain Pass occur intermixed with the andesite cobble. These 

 deposits represent post-Eocene channels which cut through the earlier 

 Eocene deposits and robbed them of their gravels and also their gold. 



The earliest gold-bearing stream channels, Eocene in age, often 

 followed the strike of bedrock, cutting gutters and depositing gold 

 nuggets. The andesite cobble was spread far and wide, covering both 

 the bedrock and the Eocene channels. The much later latite lava flow 

 took the course of a prominent post-andesite stream gorge and filled it 

 to the brim. Where drift mining discovered the deep Eocene quartz 

 gravel channels, rich placers were found ; but where the mines 

 encountered post-Eocene channels cut only through the barren volcanic 

 ash, disappointment was the result. For this reason only a few of the drift 

 mines which pass under Table Mountain were successful ; many others 

 were a disappointment. 



CHINESE CAMP TO ALTAVILLE MAP 3 



The highway, from Mountain Springs and on into Jamestown, more 

 or less parallels the trend of Table Mountain. Bedrock exposures are 

 poor because of the low relief, there being few good outcrops until Woods 

 Crossing is reached. The Mariposa slates appear briefly from beneath the 

 Tertiary gravel and andesite series, and then are faulted off against a 

 thin belt of Logtown Ridge metavolcanics. This contact is close to the 

 junction of the Montezuma cutoff and Highway 49 about 2 miles north- 

 east of Mountain Springs. From that point to Woods Crossing the base- 

 ment rocks are principally serpentine and related gabbro-diorite. 



