1916] 



Dickersoii : Tejon Eocene of California 



395 



white clay, is well exposed around lone, whence the formation takes its 

 name. Farther south the white clays are overlain by sandstone, above 

 which is a fine-grained clay rock. The lower, white clay is in places 

 quite free from grit and is used in making pottery. Other portions are 

 sandy. The formation contains iron-ore and coal seams. The sandstone 

 Is used for building purposes. It is usually white, but at one quarry 

 a brick-red variety, colored by finely disseminated hematite, is obtained. 

 At other localities it is rusty and contains pebbles of white quartz, passing 

 into a conglomerate. A peculiar hydrous silicate of alumina occurs 

 abundantly in the sandstone in the form of cream-colored, pearly scales. 



The clay rock occurring above the sandstone is light-gray — but usually 

 more or less discolored. The fracture is, as a rule, irregular, and the rock 

 frequently contains minute, tubular passages. Under the microscope it is 

 seen to be composed of fine particles of feldspar and fine discolored sedi- 

 ment, with occasional quartz grains. Analyses of two specimens gave 59 

 and 72 per cent silica and 4.8 and 1.6 per cent of alkali. 



The succession of white clay, sandstone and clay rock may not be con- 

 stant throughout the entire area mapped as belonging to the lone forma- 

 tion. It has been suggested that the white clay of the lower beds was 

 formed from rhyolitic tuffs, in which case eruptions of rhyolite must have 

 occurred at the beginning of the lone epoch. 



The thickness of the lone formation is known partly by natural ex- 

 posures, partly by boring. In Jones Butte the strata, protected from ero- 

 sion by a lava cap, are 200 feet thick above Coal Mine No. 3. A boring 

 at the mine is said to have penetrated sandy clay to a depth of 800 feet 

 below the coal seam, which is 60 to 70 feet below the surface. Thus the 

 lone beds appear to be more than 1000 feet thick at this point (see fig. 5). 



To the east of Buena Vista Peak the series has a visible thickness of 

 600 feet. The table-land south and southwest of Buena Vista is chiefly 

 composed of the lone formation, overlain by rhyolitic and andesitic tuff 

 and Neocene shore gravels. The lower clay occurs at the east base of 

 the table-land, and a patch of lone sandstone caps Waters Peak, a little 

 farther east, which has an elevation of about 900 feet. 



The relation of the sandstone to the clay rock is finely exposed on the 

 south side of the Mokelumne River, by the bridge north of Camanche. 

 Here the sandstone forms the lower part of the bank of the river. The 

 upper surface of the sandstone has a gentle westerly dip, and a little 

 west of the bridge reaches the level of the river, which at this point is 

 about 175 feet above sea-level. East of the bridge it rises at an angle of 

 about 1°, reaching an altitude of 1000 feet on the flat ridge just north 

 of Valley Springs Peak. Along the banks of the Mokelumne west of 

 Lancha Plana this sandstone attains a thickness of more than 100 feet. 



Turner, in describing the Neocene shore gravels, states their re- 

 lationship to the lone as follows : 



The most striking evidence of nonconformity, however, may be seen at 

 the red sandstone quarry three miles southeast of Buena Vista. Here the 

 Neocene shore gravels rest unconformably on the smooth waterworn sur- 

 face of the sandstone, which is red where quarried, but white at the 

 northern end of the exposure. Waterworn bowlders of the white sand- 

 stone may be seen in the gravel. Southwest of the quarry the ridge is 



