THE EARTH. 65 



It is enough to observe, in general, that they ate 

 formed by an accumulation of that little gritty 

 matter which is carried thither by the waters, 

 and which in time acquires the hardness of mar- 

 ble. What in this place more imports us to 

 know is, how these amazing hollows in the earth 

 came to be formed- And I think, in the three 

 instances above mentioned, it is pretty evident 

 that their excavation has been owing to waters. 

 These, finding subterraneous passages under the 

 earth, and by long degrees hollowing the beds in 

 which they flowed, the ground above them has 

 slipt down closer to their surface, leaving the 

 upper layers of the earth or stone still suspended ; 

 the ground that sinks upon the face of the waters 

 forming the floor of the cavern; the ground or 

 rock that keeps suspended forming the roof: and, 

 indeed, there are but few of these caverns found 

 without water, either within them, or near enough 

 to point out their formation. 



CHAPTER VIII. 



OF MINES, DAMPS, AND MINERAL- VAPOURS. 



THE caverns which we have been describing, 

 generally carry us but a very little way below the 

 surface of the earth. Two hundred feet at the 

 utmost, is as much as the lowest of them is found 

 to sink. The perpendicular fissures run much 

 deeper ; but few persons have been bold enough 

 VOL . i. E 



