44 NATURAL HISTORY. 



ed by the sinking of rocks, the base of which sometimes 

 gives way more on one side than the other, by the 

 action of air and frost, which splits and divides them ; 

 and by the impetuous fall of torrents, which opens 

 passages, and carries along with them whatever op- 

 poses their violence. But these abysses, that is, these 

 vast and enormous precipices found at the summit of 

 mountains, and to the bottom of which it is not pos- 

 sible sometimes to descend, although they arc above 

 a mile or half a mile round, have been formed by the 

 operation of fire. They were formerly the funnels of 

 volcanoes, and all the matter which is there deficient 

 has been ejected by the action and explosion of these 

 fires, which are since extinguished for want of com- 

 bustible matter. The abyss of mount Ararat, of which 

 M. Tournefort gives a description in his voyage to the 

 Levant, is surrounded with black and burnt rocks, as 

 the abysses of /Etna, Vesuvius, and other volcanoes 

 will be when they have consumed all the combustible 

 ^natters they include. 



Plot, in his natural history of Staffordshire, mentions 

 a kind of gulph, which has been sounded to the depth 

 of 2600 perpendicular feet, without finding either 

 water or bottom. 



Great cavities and deep mines are generally m 

 mountains, they never descend to a level with the 

 plains ; so we learn from them the internal structure 

 of the mountain only, and not that of the globe. 



It was long thought that the cliains of the highest' 

 mountains run from west to east, till the contrary di- 

 rection was discovered in the new work! ; but no per- 

 son before Mr Bourguet discovered the surprising re- 

 gularity of the structure of t!,o-e ijjreat masses. After 

 having crossed the Alps thiiiy tunes in fourteen dif- 



