PALEONTOLOGY ZORRITOS FORMATION 25 



This postulate demands a distribution of marine currents 

 and temperatures, and thus marine animals, somewhat differ- 

 ent from that existing today. In a study of the recent 

 littoral molluscan fauna of the coast of Peru Dall found 

 that the Magellanian element in the fauna is the most con- 

 spicuous, despite the fact that many of the species are 

 common to the Panamic Province, and that accordingly the 

 fauna seems to be largely of southern origin. Since the 

 beginning of Tertiary time conditions have probably vacil- 

 lated somewhat, with the north Peruvian coast as the scene 

 of change from colder-water, Magellanic faunas to tropical, 

 Antillean-Panamic faunas as changes in the coast line varied 

 the point of divergence of the southern, now called the Hum- 

 boldt current. Temperature conditions seem to be the chief 

 governing factor in the constituency of a littoral molluscan 

 fauna. 



It is possible that during Variegated time there was a 

 reversion from the purely tropical conditions of the Lower 

 Zorritos to conditions under which the colder currents again 

 had access to the north Peruvian coast, and the Chilean 

 element noted by Grzybowski may thus be accounted for. 

 Two things, however, alter the clearness of such a postu- 

 late : first, the existence of the Chilean forms in the Varie- 

 gated has not been checked, and second, it is the opinion of 

 some observers of South American geology that a consider- 

 able land mass existed during Miocene time to the west of 

 the present coast and in the general region south of central 

 Peru; a slight change in the course of ocean currents such 

 as that called upon to explain the Chilean element in the 

 Variegated depends on coastal conditions essentially similar 

 to those now existing, and the presence of a considerable 

 projecting land mass to the south would have eliminated the 

 colder water entirely from the Peruvian region. The exis- 

 tence of such a land mass, however, is far from proven, 

 and the presence of Chilean types in the Variegated beds 

 needs to be explained. A shifting of the point of land which 



