38 



GEOLOGIC GUIDEBOOK ALONG HIGHWAY 49 



[Bull. 141 



Highway 49 begins in meta-volcanic* rocks of Jurassic age of the 

 Penon Blanco or Logtown Ridge formation and passes successively 

 through meta-sedimentary rocks of the Calaveras formation, a serpentine 

 intrusive body, more meta-sediments of doubtful age, and finally into the 

 black slates and sandstones of the Mariposa formation. The narrow belt 

 of rocks to the west of the serpentine near Mount Bullion was originally 

 mapped as Calaveras but probably belongs to the Amador group and is 

 Jurassic rather than Paleozoic in age. From Mount Bullion to the divide 

 where the highway begins its drop into Merced River canyon, the rocks 

 on both sides of the road belong to the Mariposa formation. 



Mount Bullion, 4.7 miles northwest of Mariposa, was once a flourish- 

 ing mining center being the site of the Princeton mine and ttie supply 

 center for placer diggings close by. Evidences of hand placering are 

 manifest along every stream bed and gravel exposure. The Princeton 

 mine was opened in 1852 and up to 1933 was the largest producer in 

 Mariposa County. Gold in excess of $4,228,000 was recovered up to 1915 

 since when very little work has been done. The shaft is inclined at angles 

 varying between 45 and 60 and falls to an inclined depth of 1660 feet. 

 The vertical depth is slightly greater than 1350 feet. The Carlo Marre 

 store, Mariposa Chamber of Commerce Historical Marker, and the more 

 recent Frank Trabucco store, across the street, mark the center of present 

 day Mount Bullion. 



Four-tenths of a mile north of Mount Bullion a road from Hornitos 

 and Merced Falls joins Highway 49. Hornitos is the hub of a very old 

 mining district, the town having been founded by Spanish Californians 

 before gold rush days. The Mount Bullion-Hornitos road cuts successively 

 across the strike of Mariposa and Amador group rocks and then enters 

 exposures of Guadeloupe granodiorite near the town of Hornitos. The 

 Ruth Pierce mine is at the contact of the uppermost member of the 

 Amador group, the Agua Fria formation, and the Guadeloupe grano- 

 diorite. The Agua Fria formation consists of thin-bedded, hard, platy 

 sediments and interbedded volcanics. It lies, in different areas, both 

 unconformably and gradationally on the Penon Blanco or Logtown Ridge 

 member of the Amador group and may be a nonvolcanic member of that 

 formation. The latter formation is, typically, a green meta-andesite 

 agglomerate having prominent augite phenocrysts. This formation per- 

 sists in more or less parallel alignment to the Mariposa slate from Mari- 

 posa County well into El Dorado County. 



The Washington or Jenny Lind mine, 1 miles northeast of Hornitos 

 has been one of the best in the Hornitos district. It was located in 1850 

 and installed the first milling machinery of note in the district in 1851. 

 The stamp mill which operated in the 50 's is reputed to have turned out 

 $1,000 in gold per day. The vertical Jenny Lind shaft is 1540 feet deep. 

 The recorded production is $1,099,000 and the estimated production, 

 $2,247,000. 



* The prefix meta means metamorphosfd. Implying that the rocks so designated 

 have undergone profound change due to heat and pressure. 



The Mount Gaines mine, located five miles northeast of Hornitos on 

 the Hornitos-Bear Valley road, is the only major gold producer in Mari- 

 posa County. It has an inclined shaft with an average dip of 30 having 

 a total inclined depth of 1322 feet. The ore is of sulfide type and the 

 veins dip at 30 to 35 from horizontal. Steeply dipping veins are more 

 common along the Mother Lode, in this respect, therefore, the Mount 

 Gaines veins are unusual. The wall rock is meta-andesite and vein minerals 

 include pyrite, quartz, chalcopyrite, galena, and sphalerite as well as 

 disseminated gold. Bornite, proustite, arsenopyrite, and argentite have 

 been found in small quantities. Early production of the Mount Gaines 

 totaled $1,250,000 to 1911. No recent figures are available. 



Two and two-tenths miles south of the town of Bear Valley, prominent 

 outcrops of quartz can be seen to the west of the highway. These stand up 

 like white walls on the gently rolling land surface and mark the trace of 

 one branch of the Mother Lode. 



Bear Valley, originally called Simpsonville, was promoted by J. C. 

 Fremont during his administration of the Las Mariposas grant. The town 

 is now largely a ruin of small stone, adobe, and frame buildings marked 

 by Division of Highways Historical Marker 331. Another road from 

 Hornitos joins Highway 49 just north of Bear Valley. 



Two miles north of Bear Valley and one-tenth of a mile past the 

 divide marking the drop into Merced River canyon, a broad shoulder 

 in the highway affords a good parking place from which to observe the 

 panorama to the northwest. The river has cut a gorge which is over 

 1200 feet below the level of Bear Valley and more than 2000 feet 

 below the ridge tops on either side. Remnants of the old rolling land 

 surface can be seen on the ridge to the east of the river. Hell Hollow, 

 which is a narrow gorge tributary to the Merced, lies immediately below 

 the road to the northwest. The first fossils by which the age of the Mari- 

 posa slates were determined came from outcrops in Hell Hollow. To the 

 east of the Merced River and more or less parallelling its course, the 

 white outcrops of the main branch of the Mother Lode can be followed 

 for miles. In the vicinity of Whites Gulch, the quartz outcrops take an 

 abrupt turn and strike almost due north. It is possible that the Mother 

 Lode, at that point, is offset by a cross fault, but work on that area has 

 been insufficient to establish the fact conclusively. The abrupt change in 

 strike of the Mother Lode outcrops is very noticeable from several points 

 along Highway 49 as it drops from Bear Valley into the canyon of the 

 Merced. The Mother Lode thrust fault system has displaced many thou- 

 sands of feet of beds. More than 15,000 feet of strata may have been 

 cut out in some places. Steeply dipping linear outcrops of meta-volcanics 

 of the Penon Blanco or Logtown Ridge formation can be seen in the 

 hillside to the west of the Merced River. 



The mine buildings, cyanide tanks, and adits of the Pine Tree and 

 Josephine mines lie in a gulch between the two major switchbacks 

 between Bear Valley and the Merced River bridge. These mines were 



