GEOGRAPHICAL DISTRIBUTION. 101 



frease of density from the upper surface to tfct center of tha 

 Earth. 



The consideration of the increase of heat with the increase 

 of depth toward the interior of our planet, and of the reaction 

 of the interior on the external crust, leads us to the long series 

 of volcanic phenomena. These elastic forces are manifested 

 in earthquakes, eruptions of gas, hot wells, mud volcanoes and 

 lava currents from craters of eruptions, and even in producing 

 \alterations in the level of the sea.* Large plains and vari- 

 ously indented continents are raised or sunk, lands are sep 

 arated from seas, and the ocean itself, which is permeated by 

 hot and cold currents, coagulates at both poles, converting 

 water into dense masses of rock, which are either stratified and 

 fixed, or broken up into floating banks. The boundaries of 

 sea and land, of fluids and solids, are thus variously and fre- 

 quently changed. Plains have undergone oscillatory move- 

 ments, being alternately elevated and depressed. After the 

 elevation of continents, mountain chains were raised upon long 

 fissures, mostly parallel, and, in that case, probably cotem- 

 poraneous ; and salt lakes and inland seas, long inhabited by 

 the same creatures, were forcibly separated, the fossil remains 

 of shells and zoophytes still giving evidence of their original 

 connection. Thus, in following phenomena in their mutual 

 dependence, we are led from the consideration of the forces 

 acting in the interior of the Earth to those which cause erup- 

 tions on its surface, and by the pressure of elastic vapors give 

 rise to burning streams of lava that flow from open fissures. 



The same powers that raised the chains of the Andes and 

 the Himalaya to the regions of perpetual snow, have occa- 

 sioned new compositions and new textures in the rocky masses, 

 and have altered the strata which had been previously de- 

 posited from fluids impregnated with organic substances. We 

 here trace the series of formations, divided and superposed ac- 

 cording to their age, and depending upon the changes of con- 

 figuration of the surface, the dynamic relations of upheaving 

 forces, and the chemical action of vapors issuing from tha 

 fissures. 



The form and distribution of continents, that is to say, of 

 that solid portion of the Earth's surface which is suited to the 

 luxurious development of vegetable life, are associated by in- 

 timate connection and reciprocal action with the encircling 



* [See Daubeney On Volcanoes, 2d edit., 1848, p. 539, &c., on the so- 

 called mud volcanoes, and the reasons advanced in favor of adopting the 

 term " salses to designate these phenomena.] Tr. 



