[ S8a ] 
phureous vapours. But the examples of this kind are 
fo frequent, that there are few inflances to be pro- 
duced of fingle volcanos, without evident marks, 
either that there have been others formerly in their 
neighbourhood, or that there are, at prefent, fub- 
terraneous fires near them. 
37. This frequency of fubterraneous fires, in the 
neighbourhood of volcanos, will appear fiill more 
probable, if we confider the internal ftrudture of the 
earth ; and, as it will be neceffary alfo, in order to 
underhand what follows, to know a little more of 
this matter, than what falls under common obferva- 
tion, I fhall endeavour to give the reader fome ac- 
count of it. 
38. The earth then (as far as one can judge from 
the appearances), is not compofed of heaps of matter 
cafually thrown together, but of regular and uniform 
flrata. Thefe ftrata, though they frequently do not 
exceed a few feet, or perhaps a few inches, in thick- 
nefs, yet often extend in length and breadth for 
many miles, and this without varying their thick- 
nefs confiderably. The fame firatum alfo preferves 
a uniform character throughout, though the flrata 
immediately next to each other are very often totally 
different. Thus, for inflance, we fhall have, per- 
haps, a firatum of potters clay; above that, a ftra- 
tum of coal ; then another firatum of fome other kind 
of clay ; next, a fharp grit fand flone ; then clay 
again ; next, perhaps, fand flone again ; and coal 
again above that; and it frequently happens, that 
none of thefe exceed a few yards in thicknefs. There 
are, however, many inflances, in which the fame 
kind of matter is extended to the depth of fome 
hundreds 
