Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 559 



clays are strikingly similar to the deposition going- on in 

 Tomales Bay at the present time. 



Dr. J. C. Merriam 24 in a paper entitled "Ground Sloths in 

 the California Quaternary" has described the humerus of a 

 large ground sloth from the northeast shore of Tomales Bay, 

 and gives the following note upon the occurrence of this 

 interesting form : 



The place pointed out to Mr. Calkins as that from which the humerus 

 was obtained is in a small run about three-quarters of a mile southeast 

 of Hamlet and about 100 feet above the level of the bay. The stream in 

 the run has cut down quite sharply for about 12 feet into a loose, sandy 

 clay at the spot where the specimen was obtained. Above this point it 

 flows through or over deposits similar to those just mentioned, and 

 over rocks of the Franciscan series, so that the specimen must be derived 

 from one or the other of these formations. As the Franciscan rocks 

 are of middle Mesozoic age and have suffered much disturbance, the 

 only possible source of such a specimen as that which we have under 

 consideration is the more recent deposit. 



Incoherent, yellowish, sandy clays, similar to those just mentioned 

 form the most prominent feature of the geology along the east side of 

 Tomales Bay between Point Reyes Station and Hamlet. In many places 

 they form prominent seacliffs up to 40 feet in height. They are every- 

 where unsolidified and frequently show horizontal stratification. Mr. 

 Calkins considers the beds in the stream cutting in which the humerus 

 was found as an extension of this deposit up the slope of the hill. 

 Judging from their incoherent nature and horizontal stratification, these 

 beds are certainly much younger than the latest Pliocene in the region. 

 Excepting the humerus, the only fossil obtained from them is a badly 

 worn Elephas tooth, which was picked up on the shores of Tomales 

 Bay near Point Reyes. This formation resembles the deposits along 

 the shores of San Pablo and Suisun bays, in which a Quaternary fauna, 

 both molluscan and mammalian, has been obtained by the writer. In 

 this connection the preservation of the specimen is a noteworthy char- 

 acter, as the bone is absolutely intact and the original material un- 

 changed. One might almost suppose it a product of the last half cen- 

 tury. 



Dr. Chester Stock has identified the specimen from Tomales 

 Bay as belonging to Mylodon harlani, a species commonly 

 represented in the Pleistocene of Rancho La Brea. 



This form probably occurred in the Tomales formation as 

 the Millerton formation is missing from this portion of 

 Tomales Bay, and the lithological description corresponds to 

 that of the Tomales formation. 



The Millerton formation was deposited in a graben along 

 the San Andreas Rift, very similar to the Tomales Bay of 

 the present day. Both the flora and fauna obtained from 

 these beds indicate a climate considerably warmer than that 



24 Merriam, J. C, Bull. Geol. Soc. America, Vol. II. pp. 612-614, 1899. 



