176 POPULATION'S OF THE SEA 



1951, p. 1017) has provided useful reference data in the form of 

 paleotemperature equations, as well as his own estimated and 

 calculated mean temperatures for various intervals from the 

 Pre-Cambrian onward — computations soon to be outmoded, we 

 may hope, by isotopic paleotemperature data. 



Although the subject is not exactly marine or biogeographic, 

 physical evidence taken as indicative of pre-Pleistocene ice sheets 

 is also fundamental to the problem of polar location. This is too 

 important to be left to the experts and too prevalent at low 

 latitudes to be accepted as representing latitudinal changes without 

 seeking other explanations both for the deposits and for how those 

 demonstrably glacial came to be so. Crowell (1957) has aptly 

 underscored the similarities between tillites and pebbly mudstones 

 due to other forms of gravity mass movement, specitically marine, 

 and the point needs no repetition to one who has seen both. Even 

 striated pavements are not sacred; they also can be produced by 

 other and more rapid gravity movements than the creep of frozen 

 water (e.g., rock or gravel slides, nuees ardentes). Paleoglacial 

 events that survi\'e the callcd-for critical reexamination and are 

 too big to be the product of mountain glaciers may still be ex- 

 plained by the same adiabatic cooling of rising moist winds that 

 explains the mountain glaciers, but on a larger scale, as Brooks 

 (1949, pp. 252-257) suggested may be the case for inferred 

 Gondwana glaciation. 



Finally, we have to consider an ingenious and purely biologic 

 scheme elaborated by Ma (1952, and series of papers there cited) 

 for determining paleolatitudes on the basis of interrupted growth 

 in corals, and his interpretations in support of theories invoking a 

 blend of polar and crustal drift. Ma's data themselves, however, 

 are open to other interpretations. The formation of growth zones 

 in a coral (or a mollusk) are a function of metabolic variations 

 that could be and probably are related to a number of enxiron- 

 mental factors other than temperature. They are not really 

 comparable, for instance, to those of deciduous woods, which 

 annually pass through a temperature-controlled interval during 

 which growth can proceed only from already stored nutrients, an 

 effect that overshadows all other growth variations of the indi- 

 vidual tree. 



