NORTH OF MOUNT DIABLO 287 



length, or even more, are i'oLind associated with the well rounded (|uart- 

 zitic and igneous boulders, and with these are found numerous smaller 

 limestone and sandstone pebbles, which were derived either from the 

 Martinez or the Chico, thus showing conclusively that these beds may ])e 

 considered as a true basal conglomerate. 



The shales of division C of the Meganos are especially noticeable in that 

 they are so different from anything found in the Tejon on either the north 

 or south side of Mount Diablo. The dark color, calcareous lenses and 

 nodules, mode of slaking on the surface into small fragments, presence of 

 carbonaceous material and layers of coarse sandstone which separate the 

 different shale members are all very similar to lithological characters of 

 the Knoxville shales (Lower Cretaceous or Upper Jurassic), as seen in 

 certain sections of this general area. These sediments probably represent 

 shallow-water conditions of deposition, and were perhaps laid down in 

 land-locked or partially land-locked basins. 



In the basal chocolate-colored shales of the Tejon great quantities of 

 impressions of leaves, rushes, and fossil wood are found. These beds, 

 which may be traced for many miles to the east and beyond the eastern 

 boundary of the quadrangle, contain everywhere the leaf impressions in 

 abundance. They will undoubtedly yield a large and well preserved flora 

 for the paleobotanists who wish to study them in the future. These leaf 

 shales were apparently laid down in marginal marine swamps. The pres- 

 ence of shells, belonging to the genus Corbicula, in a layer of stand stone 

 in the shales testifies as to the brackish or fresh-water conditions during 

 deposition. 



The most important of the coal beds of this region, and one which was 

 mined throughout most of the area, is found near the top of these lower 

 Tejon shales. In the vicinity of N'ortonville this bed is knowm as the 

 ^'Black Diamond Vein." It is reported to have a maximum thickness of 

 about 4 feet. Above this coal layer at N"ortonville is a sandy, conglom- 

 eratic bed varying from 1 to 3 feet in thickness, which is very highly 

 impregnated with iron, so much so in places that it may be called an iron 

 ore. Eush and leaf impressions were found also in this layer. The close 

 association of this bed with the leaf shales and coal, together with the 

 fact that the iron deposit is limited to a definite layer over a considerable 

 area, suggests a primary rather than a secondary origin. 



The coarse, cross-bedded, light-colored sandstones immediately above 

 the shale may well have been deposited under somewhat similar condi- 

 tions. Two of the important coal-layers, the ^^Little" and "Clark" veins, 

 mined for many years at Nortonville and Somerville, are found in these 

 sandstones. The coal of the Clark vein, which is about 21^ to 3 feet in 



