THE CRUST OF THE EARTH. 



The waters of the ocean evaporated by heat rise into the- 

 superior parts of the atmosphere, and are carried by atmospheric 

 currents in their course towards the most elevated parts of the- 

 land, where they are condensed, and upon which they are pre- 

 cipitated in the liquid state ; thence they descend along the 

 declivities, forming rivers and lakes, and sometimes penetrating 

 the crust so as to form springs, until at length, sweeping over the 

 land, they return to the deep, carrying with them, however, a 

 large quantity of detritus of the solid crust, over which they have 

 passed, and which they deposit in the bottom of the sea to form 

 new systems of strata. 



Independently of water, air itself by its mechanical action upon 

 solid matter detaches, fractures and abrades more or less of it. 

 The water suspended in the form of vapour in the atmosphere 

 penetrates the pores and interstices of rocks to a greater or lesser 

 extent, according to their density and structure. In times of 

 drought it is again expelled by evaporation, and being thus alter- 

 nately and incessantly absorbed and dismissed, it at length dis- 

 integrates the superficial strata of the rocks, to whatever depth it 

 may penetrate. 



Such effects are observable in all cases where extensive sections 

 of the solid crust are made, whether by natural or artificial! 

 causes. They are thus seen upon the face of the cliffs which 

 overhang the sea, in the escarpments of ravines which pass 

 through mountain-chains, and in the sides of the vast cuttings 

 artificially produced in quarrying, and still more in the con- 

 struction of roads, railways, and canals. These effects are, of 

 course, the more prompt and sensible, as the matter composing the 

 rocks is more susceptible of imbibing humidity and of being 

 deprived of it by evaporation. All mountains exhibit traces of 

 such effects in some forms, determined by the various degrees in 

 which their strata are susceptible of them. Thus, while some, like 

 volcanic cones, assume uniform slopes in a conical iprm, fig. 75 A; 

 others, those composed of gneiss, for example, assume the forms of 

 pointed and dentated peaks, fig. 75 r,. Numerous examples of 



these are seen in the chains of the Alps, where they take the 

 names of needles, teeth, and horns, (aiguilles, dents, and cornes,} 

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