THE EARTH. bb 



of those Stony substances, with which this and almost every 

 cavern are incrusted -, it is enough to observe, in general, that 

 they are formed by an accumulation of that little gritty matter 

 which is carried thither by the waters, and which in time ac- 

 quires the hardness of marble. What in this place more imports 

 us to know, is, how these amazing hallows in the earth came to 

 be formed. And I think, in the three instances above mention- 

 ed, it is pretty evident, that their excavation has been owing to 

 water. 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 

 foimd without water, either within them, or near enough to point 

 out their formation. 



CHAP. vni. 



OF MINES, DAMPS, AND MINERAL VAPOURS. 



The caverns whioh we have been describing, generally carrj 

 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 to 

 venture down to their deepest recesses ; and some few 

 who have tried, have been able to bring back no tidings of the 

 place, for unfortunately they left their lives below. The ex 

 cavations of art have conducted us much farther into the 

 bowels of the globe. Some mines in Hungary are known to 

 be a thousand yards perpendicular downwards ; and I have been 

 informed, by good authority, of a coal mine in the north of 

 England, a hundred yards deeper still. 



It is beside our present purpose to inquire into the peculiar 

 contrivance and construction of these, which more properly be- 



