PHYSICAL GEOGRAPHY— THJ». OCEAN. 



91 



Luristan, towards the Persian Gulf and the 

 Indian Ocean. Such a nearly rectangular in- 

 tersection of geodetical lines has exerted a 

 vast influence on the commercial relations of 

 Europe with Asia and the north-west of Africa, 

 as wejl as on the march of civilization along 

 the once more fortunate shores of the Mediter- 

 ranean Sea("»). 



If vast and lofty mountain chains appear to 

 our imagination as evidences of great revolu- 

 tions undergone by the surface of the earth, as 

 boundaries of climates, as dividers and deter- 

 miners of the courses of rivers, as bearers of 

 another vegetable world, it is the more neces- 

 sary, by accurate numerical estimates of their 

 volumes, to show how insignificant, on the 

 whole, is the quantity of the upheaved masses 

 in contrast with the areas of entire continents. 

 The mass of the Pyrenees, for example, a chain 

 the mean height of whose ridges, and the ex- 

 tent of surface of the base which it covers, 

 have been ascertained by accurate measure- 

 ments, if distributed evenly over the area of 

 France, would raise the surface of that coun- 

 try by no more than about 108 feet. The mass 

 of the eastern and western Alps, spread in the 

 same way over the area of Europe, would only 

 raise the land by about 20 feet. By a laborious 

 calculation("°), which from its nature can only 

 give an extreme superior limit, in other words, 

 a number which may be less, but cannot be 

 greater th^ the truth, I have found that the 

 centre of gravity of the volume of the coun- 

 tries which in Europe and North America rise 

 above the level of the sea, lies at a height of 

 630 and 702 feet, and in Asia and South Amer- 

 ica, at an elevation of 1062 and 1080 feet. 

 These estimates show the slight elevation of 

 the northern regions : the vast steppes of the 

 Siberian levels are compensated by the enor- 

 mous rise of the Asiatic soil from 28^ to 40° 

 N. lat. between the Himalaya, the north Thi- 

 betic Kuen-luen, and the Thianschan or Celes- 

 tial Mountains. We read, to a certain extent, 

 in the numbers found, where the Plutonic forces 

 of the interior of the earth have put forth their 

 greatest strength in uplifting continental masses. 



There is nothing to assure us that these Plu- 

 tonic powers may hot in the course of future 

 centuries add new members to the mountain 

 systems of different ages and having different 

 directions, which have been enumerated by Elie 

 de Beaumont. Wherefore should the crust of 

 the earth have lost the property of folding on 

 itself 1 Almost the last of the mountain sys- 

 tems that appeared, the Alps and the Andes, 

 have reared colossuses in Mont-Blanc and 

 Monte Rosa, in Sorata, lUimani and Chimbo- 

 razo, that do not allow us to infer any falling 

 off in the intensity of the subterranean forces. 

 Geological phenomena of all kinds indicate al- 

 ternating periods of activity and r^se("i). 

 The repose we now enjoy is only apparent. 

 The shocks which the surface experiences un- 

 der every variety of climate, and along with 

 every description of rock, Sweden rising in its 

 level, and the appearance of new eruptive isl- 

 ands, bear no testimony to quiescence in the 

 internal life of the globe. 



The two coverings of the solid crust of our 

 planet — the liquid and the gaseous, the ocean 

 and the atmosphere, besides the contrasts , 



which arise from the great diversities in theli 

 states of aggregation and elasticity — also pre- 

 sent numerous analogies by reason of the mo- 

 bility of their particles, of their currents, and 

 their relations to temperature. The depth of 

 the sea and of the aerial ocean are both of thera 

 unknown to us. In some places under thf 

 tropics no bottom has been found to the sea 

 with 25,300 feet of line (more than a [German] 

 geographical mile) ; and the atmosphere, sup- 

 posing it, as Wollaston will have it, to be lim- 

 ited and so subject to undulations, may be in- 

 ferred, from the phenomena of twilight, to have 

 a nine-times greater profundity. The aerial 

 ocean rests partly on the solid earth, whose 

 mountain chains and lofty table-lands, as al- 

 ready said, rise up like green and wood-crown- 

 ed shoals ; partly on the ocean, whose surface 

 forms the fluctuating bottom upon which the 

 inferior denser and moister strata repose. 



From the limits of both the atmosphere and 

 the ocean upwards and downwards, the aerial 

 and liquid strata are alike subjected to certain 

 laws of decrease of temperature. In the at- 

 mosphere this decrease is much slower than in 

 the ocean. Under every zone the tendency of 

 the sea is to preserve the temperature of its 

 surface in equilibrium with that of the stratum 

 of air which rests immediately upon it, inas- 

 much as the chilled particles [supposing the 

 temperature of the air to be the lower] sink, 

 [and the warmer particles, vice versa, keep their 

 place on the surface]. A vast series of care- 

 ful observations on temperature, teach us that 

 in the usual and mean state of its surface, the 

 ocean, from the equator to 58° of north and 

 south latitude, is somewhat warmer than the 

 stratum of air that rests immediately upon 

 it(332). Q^ account of the decrement of tem- 

 perature with the increasing depth, fishes and 

 the other inhabitants of the sea, which, by rea- 

 son perhaps of the nature of their branchial and 

 cutaneous respiratory systems, love deep wa- 

 ter, are able to find the lower temperatures, 

 that agree particularly with them in higher lat- 

 itudes, under the temperate and colder zones. 

 This circumstance, analogous to the temperate, 

 even to the cold alpine atmospheres of the lofty 

 plateaus of the torrid zone, exerts an essential 

 influence on the migrations and geographical 

 distribution of many marine animals. The 

 depths in which fishes live, by the increase of 

 pressure they occasion, modify in like measure 

 the cutaneous respiration and the contents in 

 oxygen and azote of the air in the swimming 

 bladder. 



As fresh and salt water do not attain their 

 maximum density at the same temperature, 

 and the saline contents of the sea cause the 

 thermometrical indication of greatest density 

 to descend, water was obtained from the abyss 

 of the ocean in the voyages of Kotzebue and 

 Dupetit-Thouars, which indicated the low de- 

 grees of 2 8° and 25° C. This icy temperature 

 of the water also prevails in the depths of the 

 tropical sea, and its discovery gave the first in- 

 formation of the existence of inferior polar cur- 

 rents, proceeding from either pole towards the 

 equator. Without such under-sea currents, the 

 abyss of the tropical ocean could only have a 

 temperature equal to the maximum of cold 

 which the particles of water descending locally 



