1922] Trask: The Briones Formation of Middle California 137 



coarse-grained reef-like beds, so prominent in other sections, gradually 

 merge into soft fine-grained sandstones. 



In the San Pablo Bay region, the Briones is composed of three 

 lithologic members, two very similar sandstones and an intermediate 

 shale. As a rule the sandstones are fine-grained, soft, yellow, more or 

 less fossiliferous, and in places grade into sandy shales. The maximum 

 thicknesses of the lower and upper sandstones are 1000 and 800 feet, 

 respectively. 



The middle member, which is a shale, attains a maximum thickness 

 of 600 feet near San Pablo Bay. Lawson 7 has named it the Hercules 

 shale member. It thins to the southwest, and six miles from the bay, 

 it has a thickness of less than 200 feet. At this point the continuity 

 of the section is broken by a fault, and the Hercules shale member 

 has not been definitely recognized to the south. 



The Hercules shale is composed of a yellow, unfossiliferous, bitumi- 

 nous shale, which, lithologically, appears similar to the shales of the 

 Monterey group below. 



As yet no differences in dip and strike have been noted between 

 the Briones and the formations above and below. There is, however, 

 a rather sharp change in lithology between it and the adjacent for- 

 mations. As a general rule the upper part of the Monterey consists 

 of a yellow shale (though in a few localities it is composed of a soft, 

 yellow, fine-grained sandstone), while the base of the Briones is char- 

 acterized by a gray, coarse-grained sandstone. The upper part of 

 the Briones is usually composed of a soft, yellow, fine-grained sand- 

 stone or sandy shale, while the base of the San Pablo consists of a 

 hard, gray, coarse-grained to finely conglomeratic sandstone. 



On the southwest side of Mt. Diablo in Wall Point Canon there 

 is an irregular contact and a sharp change in lithology between the 

 Monterey and the Briones. The upper Monterey is composed of a 

 yellow shale, while the base of the Briones consists of a hard, gray, 

 coarse-grained sandstone. Astrodapsis brewerianus (Bemond) has 

 been found in the sandstone five centimeters above the contact, thus 

 indicating the identity of these beds with the Briones. The contact 

 is quite distinct and somewhat irregular. Erosional gutters, ten to 

 fifteen centimeters in depth, are observed in the upper part of the 

 Monterey shales. Cracks in the Monterey, a foot or more in depth, 

 are found filled with the sandstone matrix from above. A layer of 

 isolated pebbles, about two centimeters in diameter, occurs just above 



7 Lawson, A. C, San Francisco Folio, U. S. G. S. No. 193, p. 11, 1914. 



