382 Observations on an 
stances to the present day; for almost every 
cavern is the place of union to a number of 
secret brooks, which enter it in different di- 
rections, some of them being perennial, but 
others depend on the weather. |The impetu- 
osity of these currents is very apparent in 
some caverns, which are filled with water in 
wet seasons; for the bottoms of them are 
covered with large masses of stone; the edges 
and, angles of which are worn away, like 
those of a pebble, that has been rolled im the 
channel of a rapid river. 
I have already remarked that the caves of | 
the North of England are commonly found 
in calcareous strata. This circumstance may 
be traced to natural causes; for the rain water 
descends with great ease through the vertical 
fissures of these rocks; which generally rest 
upon a base of gray schist, and in some places 
on a soft argillaceous substance of a lamf- 
nated texture. This base is not uniformly 
flat: for it swells occasionally into lumps’ or 
hillocks ; some of which appear above the 
surrounding limestone. Such of these hillocks 
as were originally situated under one, or a 
number of subterranean brooks formed in the 
calcareous strata, have been washed away 
long ago; and the caverns, which remain ‘at 
present, shew the extent and form of these 
