.'>ti Walther — Origin and Peopling of the Deep Sea. 



noticeable changes in the same geographical latitude if the 

 land is raised into mountains. Kilimanjaro lies in the tropical 

 zone and still its peak is covered with eternal snow and "polar" 

 glaciers. 



Just as the climate of the continent with increasing topo- 

 graphical altitude becomes more like polar climate, so we 

 notice in the sea with increasing depth a constant lowering of 

 temperature. Even at 120 meters* the daity and yearly fluctu- 

 ations of the warmth of the water cease as a rule, and under 

 the surface water, with a warmth of 30° C. in the equatorial 

 region, we And even at 200 meters a temperature of 12° C. and 

 at 1200 meters one of 5° C. From here to the bottom reigns 

 an unvarying temperature of zero to 5° C, which in the south- 

 ern Atlantic sinks even to — 2° O. 



But while on the land the colder regions occupy only scanty 

 space, the contrary condition rules in the sea bottom. For 

 even in the equatorial regions, the warm water is restricted to 

 very narrow zones parallel to the coasts and the whole expanse 

 of the true deep sea bottom is covered with ice-cold water. 

 A gigantic but immeasurably slow stream of cold south polar 

 water flows toward the equatorial region in the depths and 

 projects the thermal characteristics of the southern ice seas 

 to the deep sea bottom. 



By examination of a world chart, we do not get the correct 

 impression of the relation of the sea to the continents, because 

 the border region of the continental mass is washed over by 

 the sea, and consequently around almost all coasts extends a 

 broad shallow-water zone, whose depth very slowly sinks to 

 200 to 300 meters. The whole North sea, the Irish sea and 

 the ocean for 300 kilometers west of Ireland belong to this 

 so-called continental shelf, and not until beyond them does the 

 sea bottom sink suddenly to 4000 meters. 



But even if we consider the continental shelf as the sub- 

 merged edges of the continents, still half the earth's surface 

 belongs to the area of the deep sea, with an average depth of 

 4000 meters and a maximum depth of 8 to 10 kilometers. 

 This vast region, embracing half of the globe, is so significant 

 in the natural history of the earth to-day that one can well 

 understand the important role it has also played in the geo- 

 logic past. But in order to discern the past history of the deep 

 sea, we must point out still another important characteristic of 

 the present deep sea. 



Waves and currents were produced by passing or periodical 

 winds and set in motion only the upper water strata. In a 

 depth of 1000 meters, even the Gulf Stream is scarcely notice- 

 able and farther down all measurable water movements cease. 



* For the sake of simplicity the figures are given in round numbers. 



