NATURAL HISTORY. 27 



Perpendicular fissures vary greatly as to the extent 

 of their openings. It is clear, ho>wever, that the fis- 

 sures, whose openings are small, have been occasioned 

 solely by drying. But those which extend several feet 

 are partly owing to the sinking of the foundation upon 

 one siue while that of the other remains firm. When 

 rocks are founded on clay or sand, they sometimes slip 

 a little to a side ; and the fissures are of consequence 

 increased by this motion. I have not taken notice of 

 those prodigious cuts which are found in rocks and 

 mountains, arid which could be produced by nothing 

 but ths sinking of immense subterraneous caverns that 

 were unable to support the load any longer. But these 

 cuts in mountains are not of the same nature with per- 

 pendicular fissures : They appear to have been ports 

 opened by the hand of mature for the communication 

 of nations. This appears to be the intention of all 

 openings in chains of mountains, and of those 

 t?l raits by which different parts of the ocean arc con- 

 nected ; as the straits of ThermOpyle, of Gibraltar, &c. 

 :>s or ports in Mount Caucasus, i he Cordeliers, c. 



These great sinkings, though owing to accidental 

 and secondary cantos, are leading facts in ihe history 

 of the earth, and have contributed much to change 

 the appearance of its surface. Most of them have 

 been produced by subterraneous fires, whose explosions 

 ;;ive birth to earthquakes and volcarioes. But though 

 the f;>rce of inflamed matter pent up in the bowels of 

 the earth be great, and though its- effects appear to 

 hi 1 protfigious, we v car,not suppose that these subterra- 

 neous fires are only branches of an immense abyss of 

 'flame in the centr: of the t-a-th ; nor do we believe 

 that those fires have their seat at a great depth helovr 

 dite surface, asmsLter carsnct begin to burn,, cr at least 



