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University of California Publications in Geology [ VoL - 13 



the contact. There is no evidence of differences in dip and strike, as 

 none of the beds of the Monterey appears to be truncated by the 

 Briones. The lithologic change, irregular erosion line, isolated layer 

 of pebbles above the contact, and the finding of Astrodapsis brewer- 

 ianus (Remond) just above the contact indicate that this is probably 

 the contact between the Briones and the Monterey. 



Similar irregular contacts have been found between the Monterey 

 and the Briones in two other localities. One of these is on the northern 

 end of Shell Ridge, some seven miles northwest along the strike of the 

 contact mentioned above. The other is located in the northern part 

 of the Concord Quadrangle, in a Santa Fe Railroad cut, one mile east 

 of Muir Station, on the north end of the Pacheco syncline. In each 

 of these two localities there is a sharp lithologic change, irregular 

 contact line, and in the one on the north end of the Pacheco syncline, 

 shale pebbles, three to five centimeters in diameter, very similar to the 

 underlying Monterey shale, are found in the sandstone above the 

 contact. Pholas borings are also found just beneath the contact. 

 This contact is somewhat more irregular than the others — erosional 

 gutters nearly a foot in depth being present. 



On the southwest side of Mt. Diablo on the northern side of Syca- 

 more Creek, there is an irregular contact between the Briones and the 

 Lower San Pablo (Cierbo). Here a road cuts across the contact three 

 times, and the break may be traced interruptedly for about one-half 

 mile. There is a sharp change in lithology. The beds below the 

 contact usually are composed of yellow, sandy shales, containing 

 Xassa whitneyi, n. sp. The beds above the contact consist of hard, 

 massive, finely conglomeratic sandstones. Numerous borings of the 

 Pholadid type are found in the beds immediately below the contact. 

 Cracks a foot or more in depth are found in the shale below, filled with 

 the sandstone matrix from above. There is no appreciable difference 

 in dip and strike. The sharp change in lithology, uneven contact, 

 presence of Pholadid borings, and Nassa whitneyi, n. sp., found just 

 below the contact, point to this being the line of demarcation between 

 the Briones and the San Pablo. 



Clark s has described another disconformity between the Briones 

 and the San Pablo, which occurs about two miles to the north in "Wall 

 Point Canon. This is very similar to the one mentioned above. There 

 is a sharp change in lithology, irregular contact line, and borings of 

 the Pholadid type are found in the beds just beneath the contact. 



s Clark, B. L., Fauna of the San Pablo group of Middle California, Univ. Calif. 

 Publ. Bull. Dep. Geol., vol. 8, p. 408, 1915. 



