TERTIARY AND MODERN FAUNAL BARRIERS 219 



There are two principal ways in which the sea temperature of a given 

 region may be decided, apart from the normal amount of direct heat it 

 may receive daily from the sun. One of these is due to the invasion of a 

 region by an oceanic current, properly so called (that is, a body of water 

 with motion in a definite direction usually differing from the sea about it 

 in temperature, and more or less distinctly laterally limited), analogous 

 to a river on a land surface in its relation to the adjacent sea. Such a 

 current may carry cool water into a warmer region, or warm water into a 

 cooler region, and by the temperature and evaporated moisture it gives 

 off may also alter the aerial and terrestrial climates of the region invaded. 

 Such examples as the Gulf Stream or the Equatorial current will occur 

 to every one reflecting on the subject. 



The rate and direction of such currents are determined, first of all, by 

 the friction of the trade winds on the surface of the sea ; secondly, by the 

 land barriers encountered, and to a less extent by barometric pressure, 

 differences of density due to concentration of saline matter and other 

 minor factors. 



The other way in which sea temperatures are affected is due to oceanic 

 circulation independent of the friction of the winds, and which would 

 occur if there were no winds at the surface of the sea. The rotation of 

 the earth causes a lagging of the surface waters and a welling up on the 

 western shores of continents of colder bottom waters when the contour of 

 the sea bottom is favorable. The evaporation from surface waters in the 

 tropics increases the salinity and density of the water affected, and there 

 is a constant interchange of less dense cold polar waters with those of the 

 tropics. The waters of the deeps are nearly always of polar temperature. 

 The movements of the tides impinging on continental shores aid in this 

 system of circulation. 



It does not seem possible, under conditions of atmosphere approximat- 

 ing those of the present time, that there should ever have been a time 

 when the tropic seas were not perceptibly warmer than the polar waters, 

 though the latter may have been much warmer than at present. As soon 

 as marine animals developed to a stage where temperature became a 

 factor in their physiological history, it was inevitable that faunas should 

 develop, and the more susceptible the inhabitants of the sea became the 

 more distinctly faunas would become limited. 



Of course, the development of the food supply, itself dependent on the 



sea temperature, the presence of large bodies of fresh water at the mouths 



of great rivers, the evolution of destructive gases arising from the sea 



bottom, or the invasion of limited areas of sea by noxious salts derived 



XVI — Bull. Geol. Soc. Am„ Vol. 22, 1910 



