40 



GEOLOGIC GUIDEBOOK ALONG HIGHWAY 49 



[Bull. 141 



opened about 1850 and were among the earliest of the lode mines. As the 

 veins outcrop on the flanks of a ridge of considerable height, they have 

 been worked largely by tunnels and winzes rather than deep shafts. None 

 of the workings extend lower than 800 feet from the tunnel levels. 

 Some of the veins have been traced for 1200 feet along their strike. 

 Production records on these mines are incomplete, but the total figure 

 for both mines is in excess of $2,679,000. Prominent exposures of veins 

 of the Mother Lode system can be seen in roadcuts in the vicinity of the 

 Pine Tree and Josephine mines. A narrow belt of serpentine discon- 

 tinuously parallels the Mother Lode fault east of the vein system. Between 

 the Pine Tree and Josephine mines and the bed of the Merced River, 

 the highway drops down rather abruptly across a typical section of the 

 Mariposa formation. The rocks are mainly black slate with interbedded 

 lenses of black, arkosic sandstone. The sandstone contains fragments of 

 slate which are probably re-worked Mariposa material. The slates weather 

 to a tan or grayish buff as they approach the soil mantle. Just before the 

 road crosses the Merced River, the Mariposa is exposed in fault contact 

 with serpentine. Serpentine has been sheared into the Mariposa at some 

 points along this contact. Calaveras meta-sediments lie to the east of 

 the serpentine belt. 



The site of the hamlet of Benton Mills, now known as Bagby, lies 

 just across the Merced River. A stamp mill which processed gold ore 

 from Fremont's mines above was situated there. None of the old build- 

 ings remain although foundations of some can still be located. The 

 Yosemite Valley Railroad once passed through Bagby but only the road- 

 bed now remains. The tracks were removed during World War II. 



North from Bagby for a distance of over six miles, Highway 49 

 winds up a steep slope through a barren area of serpentine and related 

 basic and ultra-basic rocks. Serpentine ordinarily is not a favorable 

 medium for supporting plant life although some species grow luxuriantly 

 on it. Vegetation here is exceedingly sparse and consists principally of 

 California holly and chemise with an occasional digger pine. The ser- 

 pentine series of rocks invaded the basement sedimentary series in late 

 Jurassic time intruding Paleozoic and Mesozoic rocks alike. The body of 

 serpentine north of Bagby is one of the largest areas of this type of rock 

 to be found along Highway 49. 



Slightly less than two miles beyond the summit of the Bagby grade, 

 the highway crosses the eastern border of serpentine and continues along 

 the contact of serpentine and Calaveras rocks for a distance of about 

 a mile. The first good exposures of the Calaveras to be seen northbound 

 along Highway 49 are exposed there. These outcrops consist mainly of 

 weathered tan mica schist and phyllite both of which somewhat resemble 

 weathered Mariposa slate. They have, however, suffered a much greater 

 degree of metamorphism, being schistose rather than slaty. 



A short distance north of the summit of the Bagby grade, 3.8 miles 

 from Bagby, a narrow dirt road from the Virginia mine leads off to the 

 west. The Virginia mine was one of the earliest patented claims in 

 Mariposa County and has been worked on a small scale over a very long 

 period. The mine is 1300 feet deep on the incline and the wall rocks are 

 Calaveras schist, Logtown Ridge greenstone, and serpentine. Records 

 are incomplete but known production is in excess of $660,000. 



A distance of 1.7 miles north of the Bagby summit, the highway 

 crosses several dikes of an unusual grayish white rock known variously 

 as albitite, soda syenite, and albite granophyre. Intrusive bodies of this 

 rock are also found in many places to the east of Moccasin Creek. The 

 rock is medium to fine grained with aplitic and porphyritic phases. It is 

 composed mainly of the soda feldspar albite plus variable amounts of 

 soda-rich minerals such as riebeckite and aegirite. The latter two are not 

 present in the dikes crossing Highway 49 but are present in some of the 

 Moccasin Creek outcrops. 



A red brick furnace and chimney and a concrete foundation are all 

 that remain of the boiler house and other buildings of the Mary Harrison 

 mine. These are located on a knoll to the west of Highway 49, 1.8 miles 

 south of Coulterville. The Mary Harrison was discovered sometime before 

 1867 and was operated for a considerable period prior to 1895 by the 

 Cook Estate. In 1895, the mines of the Cook Estate passed into the 

 ownership of the Merced Gold Mining Company which is responsible 

 for most of the recorded production. This was well in excess of $330,000. 

 The mine was worked to a depth of 1200 feet by shaft and winze. Most 

 of the workings were in the dolomite-ankerite-mariposite-quartz rock 

 which forms a very broad zone along this part of the Mother Lode. The 

 mine has not been operated since 1903. 



Immediately north of the ruins of the Mary Harrison, the highway 

 has been cut through the main part of the Mother Lode vein system. 

 This locality is easily accessible and is an excellent place to observe the 

 minerals which are associated with gold ores. Beautifully banded rocks 

 composed of mariposite (chrome mica), white quartz, and carbonate 

 minerals, such as dolomite, ankerite, and calcite, can be collected there. 

 Talc schist is present in some places probably derived by the shearing 

 of serpentine. The gold ore itself consists of gold bearing iron pyrites, 

 usually somewhat oxidized. 



Another excellent exposure of the veins of the Mother Lode is in the 

 vicinity of Maxwell Creek bridge two tenths of a mile south of Coulter- 

 ville. Many invasions of vein material along the same line of weakness 

 can readily be seen in these exposures. The white silica-carbonate vein 

 rocks are accentuated, in many places by rust-colored zones which mark 

 the positions of sulfide-bearing gold horizons. Other yellowish-brown 

 zones are made up of a leached, pulverized vein material which represents 



