GENERAL VIEW. 151 



know the mass and the mean density, and we are able to 

 compare the latter with the density of the materials con- 

 stituting the upper terrestrial strata, which alone are 

 accessible to us. Where the knowledge of the chemical 

 and mineralogical properties of substances in the interior of 

 the Earth fails us, we find ourselves again limited to the field 

 of mere conjecture, as in the case of the remotest planetary 

 bodies. We can determine nothing with certainty respect- 

 ing the depth at which the materials of which our rocks 

 are composed exist either in a softened though still 

 tenacious state, or in complete fusion, respecting cavities 

 filled with elastic vapours, the condition of fluids heated 

 under enormous pressure, or the law of the increase of 

 density from the surface to the center of the earth. 



The notice of the increase of heat with increasing depth 

 in the interior of our planet, and of the reaction of the in- 

 terior on the surface, will lead us to the consideration of the 

 long series of volcanic phsenoinena : these manifest them- 

 selves to us as earthquakes, emissions of gas, thermal 

 springs, mud volcanoes, and streams of lava flowing from 

 craters of eruption. The reaction of the internal elastic 

 forces shews itself also in alterations of the configuration 

 and of the level of the surface of the globe. Vast plains 

 and deeply indented continents are elevated or depressed, 

 and thus the reciprocal limits of land and sea, of solid and 

 liquid surface, are frequently and variously modified. Plains 

 have undergone an oscillatory motion, being alternately ele- 

 vated and depressed. Subsequently to the elevation of con- 

 tinents above the sea, mountain chains have risen from long 

 clefts, and these are mostly parallel, in which case the 

 elevations were probably cotemporaneous. Salt lakes and 



