278 W. Lindgren — Auriferous Conglomerate of 



Although the Mariposa slates usually show an apparent con- 

 formity with the older rocks, there is, between Colfax and the 

 Middle Fork of the American river, some structural evidence 

 strongly tending to show an unconformity. Between these 

 two localities the Mariposa beds have about the same strike as 

 the enclosing slates of the Calaveras formation, but while the 

 latter nearly uniformly have a steep dip of from 75° to 90° to 

 the east, the former show throughout a considerably smaller 

 dip of from 50° to 60° in the same direction. Mr. H. "W. 

 Turner recognizes from evidence gathered in the southern 

 counties, the probable unconformity of the two formations 

 and brings the following points to support it :* First, the 

 lithologic difference between the formations ; second, the 

 occurrence of a conglomerate with phthanite pebbles in the 

 Mariposa beds near Texas ranch, Calaveras Co. ; third, the 

 occurrence of a narrow streak of the Mariposa beds cutting 

 obliquely across the Calaveras formation ; fourth, the large 

 paleontologic break between the formations. 



Coming up the ridge between the two forks of the Ameri- 

 can river, which here are not more than three miles apart, the 

 Calaveras formation ends a quarter of a mile to the east of 

 Sheridan's Ranch. Between this formation and the Mariposa 

 beds is here intercalated an elongated body of diabase por- 

 phyrite and volcanic breccias extending nearly across the 

 divide. The diabase porphyrite forms the westerly summit of 

 Mile Hill ; in the breccia accompanying it some well washed 

 pebbles of phthanite are found. A thousand feet west of 

 Mile Hill toll house the Jurassic sedimentary rocks begin and 

 continue for three-quarters of a mile eastward when basic 

 igneous rocks again are met with. The exposures are at first 

 poor along the road which runs in a flat, broad gulch with 

 much loose soil containing well washed pebbles derived from 

 the conglomerate. On the hill south of the United States 

 ranch (E. 2200') the conglomerate shows in abundant outcrops 

 both on the top and in the ravines. North of the road, on top 

 of Mile Hill there are more outcrops though deep laterite 

 mostly covers the ridge. At the United States ranch there is 

 a good outcrop, part of it consisting of a hard, dark tufaceous 

 sandstone and part of a dark, soft shale containing small but 

 well washed pebbles of phthanite. From the hill mentioned 

 south of the ranch there is a steep descent to the Middle Fork, 

 the sedimentary area narrows greatly between two igneous 

 masses and only black slates and sandstones are exposed in the 

 bottom of the canyon. On the north side of Mile Hill there 

 is also a steep descent of 1500 feet to the North Fork ; the 

 conglomerate from the top of Mile Hill can be traced down 



* Loc. cit., p. 245. 



