150 TERRESTRIAL PHENOMENA. 



crystalline masses erupted from active volcanoes, and for the 

 most part resembling the rocks at the surface of the earth, 

 come from absolute depths which, though they cannot be accu-. 

 rately determined, are assuredly sixty times greater than 

 any which have been reached by our artificial works. In 

 situations where strata of coal dip beneath the surface, 

 and rise again at distances determined by careful measure- 

 ment, we are enabled to assign numerically the depth of 

 the basin formed by them ; and we thus learn that such 

 coal measures, together with the ancient organic remains 

 which they contain, often reach (as in Belgium, for ex- 

 ample) depths exceeding five and six thousand feet ( 125 ) 

 below the present level of the sea ; and that the mountain 

 limestone, and the strata of the Devonian basin, attain a 

 depth fully twice as great. If we now combine these depths 

 beneath the surface with those mountain summits which 

 have hitherto been regarded as the highest portions of the 

 crust of the Earth, we obtain nearly 40000 English feet, 

 or a measure equalling about -5-^- of the Earth's radius. 

 This, therefore, would be the whole range in a vertical 

 direction of our geological researches, or of our knowledge 

 of superimposed rocks, even if the general elevation of the 

 surface of the Earth equalled the height of the Dhawalagiri 

 in the Himalaya, or of the Sorata in Bolivia. All that is 

 situated at a greater depth beneath the level of the sea than 

 the deepest wells or mines, or the basins I have referred 

 to, or than the bed of the sea where it has been reached 

 by sounding (James Ross sounded with 4600 fathoms, or 

 27600 feet of line, without finding bottom), is as unknown 

 to us as the interior of the other planets of our solar system. 

 In the case of the Earth, as in that of the other planets, we 



