152 COSMOS. 



planets of our solar system. We only know the mass of the 

 whole Earth and its mean density by comparing it with the 

 open strata, which alone are accessible to us. In the interior 

 of the Earth, where all knowledge of its chemical and minera- 

 logical character fails, we are again limited to as pure conjec- 

 ture, as in the remotest bodies that revolve round the Sun. We 

 can determine nothing with certainty regarding the depth at 

 which the geological strata must be supposed to be in state of 

 softening or of liquid fusion, of the cavities occupied by elastic 

 vapour, of the condition of fluids when heated under an 

 enormous pressure, or of the law of the increase of density 

 from the upper surface to the centre of the Earth. 



The consideration of the increase of heat with the increase 

 of depth towards 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 pro- 

 ducing alterations in the level of the sea.* Large plains and 

 variously indented continents, are raised or sunk, lands are 

 separated from seas, and the ocean itself, which is permeated 

 by hot and cold currents, coagulates at both poles, convert- 

 ing 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 

 frequently 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 con- 

 temporaneous ; 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 connexion. 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 eruptions on its surface, and by the pressure of elastic 

 vapours, give rise to burning streams of lava that flow from 

 open fissures. 



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

 so-called mud volcanoes, and the reasons advanced in favour of adopting 

 the term "salses" to designate these phenomena.] 2V. 



