218 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



NATURE OF TERTIARY AND MODERN MARINE FAUNAL BARRIERS AND 



CURRENTS 1 



BY WILLIAM H. BALL 



Were the earth evenly covered with water of a uniform depth and 

 density, or the dry land confined to circular zones of which the center 

 coincided with the axis of the earth's rotation — in other words, with the 

 poles — it would be mathematically possible to compute the extent and 

 course of the marine currents. An approximation to such a computation 

 for the earth's atmosphere has been made by Ferrel and subsequent 

 writers on meteorology, but the smooth theoretical atmospheric circula- 

 tion of theory is much interfered with by the actual asperities of the 

 earth's surface and by differences in temperature and density, in a vertical 

 sense, due to the influence of the sun's rays. 



To such an extent as the emerging continents became physical bar- 

 riers to the oceanic circulation demanded by theory, the course of currents 

 and incidentally their densitv f^rtcl iamr^f^vQh^■rf^ "Kot^^ Uo^..---. --rT'fi.-.T 

 The north and south 

 tions taken by their 

 whole problem that a 

 for limited areas of c 



broad outlines of disi i)i iiu.:< .. ■ aiiviit;:: jau .h^ ^ .l_l-j^ 



are fairly well known. We have learned that the temperature of the sea 

 in which they live, within certain narrow limits, controls the distribution 

 of marine invertebrates. Whatever the tolerance they may exhibit, and 

 different species differ in this respect, there are distinct boundaries set to 

 their distribution in the differences of temperature brought about by the 

 circulation of oceanic waters. Whether these differences limit the range 

 of faunas by direct action on the individuals or by their effect on the 

 development of their progeny or by limiting their normal food supply has 

 not yet been demonstrated; but it seems quite certain that in the larval 

 stages the young invertebrates are markedly more susceptible to differ- 

 ences of temperature than adult animals, and, in the case of the oyster. 

 Brooks found that a difference of two or three degrees Fahrenheit in the 

 temperature of the water was sufficient to kill the whole larval brood. 

 By inhibiting natural increase, therefore, a species may be as sharply 

 limited in its permanent range as if material barriers interposed. 



Manuscript received by the Secretary of the Society May 23, 1911. 



