66 california academy of sciences. 



2. Pleistocene. 



The Lower San Pedro Series. — The lower San Pedro strata of Deadraan 

 Island and the San Pedro bluffs were dejjosited in water shallower than that in which 

 most of the underlying Pliocene strata were laid down. The sediments and fauna of 

 the lower San Pedro deposits show this. The sediments consist for the most part of 

 medium grained gray sands such as are being deposited at the jiresent time in about 

 ten to twenty fathoms of water off the San Pedro shore-line. The occurrence in the 

 lower San Pedro deposits of many species which live only between tides also offers 

 evidence of their close in-shore deposition. The fauna of the lower San Pedro series 

 is one of transition between the boreal or subboreal fauna of the later Pliocene and 

 the semitropical fauna of the upjjer San Pedro series. Two causes account for this. 

 First, the deposits being laid down in shallower water than that in which the Pliocene 

 sediments were deposited would necessarily contain fewer of the colder, deeper water 

 forms of the Pliocene; and second, the cold climatic conditions prevalent during the 

 later Pliocene epoch were giving place to a warmer climate, which had its effect 

 upon the boreal species which inhabited the water off San Pedro during Pliocene times. 



The species found in the lower San Pedro deposits and now found living only 

 north of San Pedro comprise 17.4 per cent, of the lower San Pedro fauna. This 

 percentage is only a little lower than that of the northern species found in the Plio- 

 cene (the latter being 18.5 per cent.), and shows, considering the fact that the lower 

 San Pedro beds were deposited in shallower water than that in which the Pliocene 

 was laid down, that the climatic conditions had changed but little during the period 

 intervening between the deposition of the Pliocene and lower San Pedro series. The 

 evidence offered by several species (3.4 per cent, of the entire lower San Pedro 

 fauna) which are found in the lower San Pedro series, but are now found living only 

 south of San Pedro, shows, however, that the conditions of temperature were changing. 



Besides the distinctly northern forms which either disappear or become scarce 

 in the period between the Pliocene and lower San Pedro series, there are many 

 species best suited for northern conditions which become rare in the lower San Pedro 

 series, while species best suited for southern conditions begin to increase in num- 

 bers. All of the evidence shows that climatic conditions were changing from boreal 

 towards tropical during the time of the deposition of the lower San Pedro series. 

 That the boreal conditions still preponderated during this period, however, is shown 

 by the fauna of the lower San Pedro. 



The Upper Saii Pedro Series. — The deposits of the upper San Pedro series 

 consist of coarse gravels, and sands which .show alternating dune and water bedding; 

 and the rapidly changing conditions under which the deposits were laid down is evi- 

 denced by their variation. The fauna of the upper San Pedro series is southern in 

 character, and, as would be expected, approaches more nearly the present living 

 fauna of the San Pedro region. Being deposited in shallower water tlian either the 

 Pliocene or lower San J'edro deposits, one would expect it to contain fewer of the 

 cold water species and more of the species found only between tides and in the warm 



