400 University of California Publications in Geology [Vol. 9 



by the Teredo were found filled in Avith sandstone. The whole mass was 

 highly charged with Peroxide of iron, which, indeed seemed to be the 

 chief constitutent. 



We had now progressed so far toward the mountains that we were 

 no longer upon the broad plains of the great valley of the San Joaquin, 

 but were traveling along the foot-hills of the Sierra Nevada. Our route 

 from this point, southward to Fort Miller, lay nearly on the border of the 

 lower granitic ranges, which presented occasional opportunities for ex- 

 amining them, in connexion with the sedimentary foot-hills. 



The first outcrop of any of the rocks of the granitic and metamorphic 

 series met on the survey was near Howard's ferry on the Merced. At 

 that place extended outcrops of dark-colored clay slate were observed, 

 having a trend N 55° W, and an inclination of 70° toward the east. 

 In some places they appeared chloritic; and in others had the general 

 character of roofing slate. They were traversed in the direction of the 

 bedding, by quartz veins of great thickness, the white quartz contrasting 

 finely with the black slate. This place is only nine miles west from 

 Quartzburg, where several mining companies have erected crushers and 

 stamps for pulverizing the quartz and extracting the gold it bears. The 

 unconformability of the sedimentary sandstone formation with these roof- 

 ing and chlorite slates is strikingly shown in this vicinity, where streams 

 have cut so deeply as to expose both formations. The horizontal strata 

 rest upon the upturned edges of the slates, as is shown in the section. 



The outcropping layers of hard sandstone were visible in the hiTls 

 on each side, the intermediate portion (between the hills) having been 

 removed by denudation. At the highest part of the outcrop of slates, a 

 quartz vein, five feet thick, stands out above the general surface, and forms 

 a wall-like mass of fragments. These being milk-white, contrast strongly 

 with the black slate on each side. This quartz has preserved the sur- 

 rounding slate from abrasion by currents of water, and thus it stands at 

 the summit of a little eminence. 



Burns' Creek. — A remnant of the former elevated plain of sandstone 

 formed a picturesque object near our route, and was sketched by Mr. 

 Koppel. It consists of a nearly circular disk of compact sandstone, ten 

 or fifteen feet thick, capping the summit of a round mound. Hills of this 

 character, of greater extent and elevation, are numerous for several miles 

 south of this point. 



Bear Creek. — At Bear Creek twelve miles north of the Mariposa 

 river, a good natural section or vertical exposure of the strata composing 

 one of these flat-topped hills was presented. It consisted of a series of 

 beds of coarse and fine sandstone and strata of gravel and conglomerate, 

 which are represented in section, Chapter XIII. The whole elevation of 

 the hill was estimated to be about 150 feet. The following is the suc- 

 cession of the strata as observed from the top downwards to the level 

 of the creek. The thicknesses are given as they were estimated, and are 

 therefore, merely approximate. The letters refer to the section. 



Section of the Strata at Bear Creek 



k. Conglomerate 

 j. Sandstone 



.50 

 22 

 . 1 



6 feet 



i. Conglomerate... 

 h. Sandstone, fine 



