16 TERTIARY AND QUATERNARY PECTENS OF CALIFORNIA. 



Within the last five years strata have been discovered in the Santa Cruz 

 (|uadrangle which contain a fauna representing a horizon probably lower than 

 any of the known lower Miocene horizons of California. The faunal relations 

 and stratig'raphic position of this new formation have led the writer to believe 

 that it belongs in the Oligocene. 



With the exceptions above noted, no marine Oligocene has heretofore been 

 recognized on the west coast north of that found in the Central American States. 



.SAX LORENZO FORMATION. 

 TYPE LOCALITY. 



The type locality is on San Lorenzo Kiver, '2^ miles above the town of Boulder 

 Creek, Santa Cruz County, Cal. 



DEFINITION. 



The San Lorenzo formation consists essentially of a series of grayish "muddy" 

 shales and tine sandstones, which are typically exposed along the bed of the San 

 Lorenzo River, about 3 miles above Boulder Creek, Santa Cruz County. The 

 formation extends westward from the type locality into the Big Basin, on the 

 north side of which it rests conformably against the older yellowish sandstones 

 (possibly Oligocene in age) of the Butano Ridge. To the southeast of the exposures 

 on San Lorenzo River the formation, in the shape of a broad anticlinal nose, 

 gradually dips conformably under the sandstones and conglomerates of the lower 

 Miocene. The approximate thickness of the formation in the vicinity of the 

 type locality is about 2,300 feet. 



LOCALITIES. 



San Lorcnzii River, Boulder Creek, Kings Creek, Twobur Creek, Bear Creek, and Love Creek, 

 all near the town of Boulder Creek, Santa Cruz County. 



FAUNA. 



The fauna of the San Lorenzo formation consists, for the most part, of forms 

 best suited by the conditions prevalent during the deposition of sandy shales. In 

 other words, it is a moderately deep-water fauna. It shows several species found 

 in the Monterey shale^ but it also contains many species which appear to be 

 closely related to Tejon (Eocene) forms. Bearing in mind these faunal relations 

 and the stratigraphic position of the formation, it appears pi'obable that it belongs 

 in the Oligocene. 



Pecten fauna. — In this horizon we meet with the common Miocene Pseu- 

 damuslum, P. peckhaiul, and characteristic representatives of Pecten s. s. and 

 Chla//n/.s, the groups which are so well represented in the later formations. P. 

 {Pecten) mnctivcrmensh is quite closely allied to /'. heJlns and P. heinjyhiUi, which 

 are found in the Pliocene, while in the top of the formation, associated with a 

 transitional Oligocene-Miocene fauna, is 1\ {Chlavtys) hranneri., the analogue and 

 probable precursor of P. idandicnK, and the hitter's related forms. P. cJallamensis 

 and I\ icaylandi from the Oligocene-Miocene series of Washington should also 

 probably be included in the Oligocene fauna. 



