EQUILIBRIUM MAINTAINED. 47 



up the interior structure of our planet. Their function is at 

 once to wear down the old and to reconstruct the new ; to 

 scatter abroad in one region and to accumulate in another. 

 And the new rocks reconstructed must necessarily bear the 

 closest relationship to the old from which they were derived. 

 Their hardness and compactness and crystalline texture is 

 merely a matter of time. Given time and the fitting con- 

 ditions, and the loosest sand will be converted into the most 

 compact sandstone, the softest mud into the hardest slate, 

 and the earthiest chalk into the most crystalline marble. 

 Though here separated for the purposes of elucidation, these 

 agents are ever working hand in hand the atmospheric 

 with the aqueous, the aqueous with the chemical, and the 

 chemical with the organic and igneous. And it is this com- 

 plicated working that renders the composition of the solid 

 crust so varied, the aspect of its rocks so different, and the 

 task of unravelling their history at once the trial and the 

 triumph of geology. 



Such is a brief sketch a mere indication as it were of 

 the great forces by which the earth's crust is incessantly 

 modified, its rock -matter wasted and reconstructed, and 

 the equilibrium of its terraqueous distribution sustained. 

 Sketchy as the outline has been, the careful reader will have 

 perceived not only the nature of the modifying agents, but 

 the manner in which they operate and must feel convinced 

 that, small as may be their results over a given area or 

 during a given time, yet comprising the whole globe and 

 allowing for ages, they are sufficient to accomplish any 

 amount of change to destroy, in fact, the whole of the 

 existing continents and to reconstruct new ones from the 

 bed of the ocean, and this by degrees, and over and over 

 again, according to the course of Time, which is illimitable 

 and beyond all computation. He will also have perceived 



