ARNOLD — THE PALEONTOLOGY AND STRATIGKAPHY OF SAN PEDRO. 59 



San Diego Mesa — Pliocene. — The San Diego mesa is composed of Pliocene 

 strata. Outcrops of the soft yellow and brown Pliocene sandstone occur in many 

 places in the northern part of the city. A good exposure is found at the north end 

 of Tenth street, a short distance northwest of the Russ school, where a cut on both 

 sides of a small ravine reveals a section of the Pliocene about fifty feet thick. This 

 exposure is only about two hundred feet noi'th of the "San Diego well," which was 

 dug about twenty years ago, and which has become famous through the discussion 

 of the age of the fossils which were taken from it. Mr. Hemphill, who obtained the 

 fossils from this well during its excavation, informed the writer that fossiliferous strata 

 were penetrated to a depth of one hundred and forty-nine feet. As the exposure of 

 the San Diego formation above the mouth of the well is fifty feet thick, the fossil- 

 bearing strata of the Pliocene formation of the San Diego mesa are at least two hun- 

 dred feet in thickness. The dip of the strata above the well is south, or toward the 

 bay, at an angle of eight or ten degrees. 



Twenty-sixth Street — Pleistocene. — A bluff about eighteen feet high rises from 

 the edge of the bay at the foot of Twenty-sixth street, San Diego, and extends for 

 two or three blocks both toward the east and toward the west from Twenty-sixth 

 street, forming the shore line along this part of the bay. At the base of this bluff, 

 and covered by the water at liigh tide is a stratum six inches thick made up almost 

 entirely of the upper valves of Anomia limatula. No right valves were found in this 

 deposit, and this species seemed to be restricted to this layer. A stratum of fine, 

 yellow fossiliferous sand, four or five feet thick, rests upon the Anomia beds; and 

 above the fossiliferous bed is about twelve feet of fine brown sand, overlain by sandy 

 soil. Dosinia poiiderosa, Oallista newcombiana, Mactra californica, and Cardium pro- 

 ceruin are the predominating species in the yellow sand stratum. The fauna of this 

 locality is equivalent to that of the upper San Pedro series at San Pedro. 



Spanish Bight — Pleistocene. — The Coronado peninsula is a long, low, narrow 

 sand-spit lying between San Diego Bay and the ocean. Near its western end is a 

 small inlet on the bay side, known as Spanish Eight. The western shore-line of this 

 inlet is a bluff' varying in height from twelve to eighteen feet, while the stratum at 

 the base of the bluff forms the beach, and is covered by the water at high tide. This 

 lowest layer is composed of a firm, fine brown sand in which are imbedded numerous 

 large Amiantis callosa, which have the appearance of living shells, so naturally do 

 they lie on the sand. An attempt to remove them, however, dispels the delusion, for 

 in most cases they are quite firmly imbedded in the sand layer. 



There are three feet of fine, soft, unfossiliferous gray sand above tlie Ainiantis 

 layer, and this is overlain by a deposit, varying in thickness from three to five feet, of 

 soft gray sand, which is very fossiliferous near its base and gradually grades into 

 the almost unfossiliferous gray sand a few feet above. About twelve feet from the 

 base of the bluff is a layer from three to six inches thick containing numerous Donax 

 Icevigata cemented together. This Donax layer is the uppermost fossiliferous stratum 

 the bluff above this being composed of unfossiliferous sands. The fauna of the 

 Spanish Bight deposits is similar to that of the upper San Pedro series at Los Cerritos, 

 and the deposits are probably of contemporaneous origin. 



