130 PROCEEDINGS OF THE PALEONTOLOGICAL SOCIETY 



SAN PABLO FORMATION ON THE NORTH SIDE OF MOUNT DIABLO. 



CALIFORNIA 



BY BRUCE L. CLARK 



(Abstract) 



The San Pablo formation, on the north side of Mount Diablo, has a thick- 

 ness of about 600 feet. It lies unconformably on the Monterey series and is 

 overlain unconformably by the Pinole tuff. The formation consists of con- 

 glomerates, coarse sandstone, and a minor amount of shale, all representing 

 very close inshore deposits. In the middle of the San Pablo there are evi- 

 dences of land conditions. A fauna of 20 species of invertebrates has been 

 collected from the Monterey series just below the San Pablo and about 60 

 species from the San Pablo. 



FAUNAL ZONES OF THE SAX PABLO FORMATION EAST OF WALNUT CREEK, 

 NEAR MOUNT DIABLO, CALIFORNIA 



BY JOHN P. BUWALDA 



(Abstract) 



Discussion of a measured section of the San Pablo formation on the western 

 side of the Mount Diablo anticline, with the results of a study of the faunal 

 zones. 



PRELIMINARY INQUIRY INTO THE GEOLOGICAL SIGNIFICANCE OF ROCK- 

 BORING SHELLS 



BY ALBERT L. BARROW 



(Abstract) 



This inquiry is undertaken in order to learn what facts the habits of living 

 rock-boring shells may contribute to explain the conditions of deposition at 

 unconformities where fossil rock-boring shells are now found. 



While animals which bore into rock are found among nearly all the large 

 groups of invertebrates, the Pelecypod borers are far more numerous along 

 the Pacific coast than borers of any other class. The many genera of Pelecy- 

 poda which burrow into mud or sand are of little value in this inquiry. 

 On account of their peculiar habit of boring into fine-grained rocks along the 

 exposed portions of the shore, in a zone extending only a few fathoms below 

 the low-tide level, the several rock-boring genera of the Pholadidw and the 

 genera Adula, Lithodomus, Petricola, Saxicava, and Platyodon have a very 

 definite significance. The nestling shells of the genera Tapes, Saxidomus, and 

 Panopea, associated with the borers, are of importance. 



The remains of marine animals which are known to habitually bore into 

 rock is evidence of an unconformity, because it presupposes the existence of 

 a firm bed in an earlier formation, hard enough to attract the borers, before 

 the overlying beds were laid down. Subsequent crustal movements were up- 

 ward if the holes are found to have been eroded or filled with terrestrial 

 deposits, or downward if filled with marine deposits. The nature of the over- 



