of the JJland of Ponza. 379 
communication, as well as in this, have been capable of con- 
verting tuffa’s, lava’s, and pumice ftone, into the pureft 
clay. 
I have remarked, that young obfervers in this branch of 
natural hiftory are but too apt to fall into the dangerous error 
of limiting the order of nature to their confined ideas : for 
example, (hould they fufpeCt a mountain to have been a vol- 
cano, they immediately climb to its fummit to feek for the 
crater, and if they neither find one, or any figns of lava or 
pumice ftone, direCtly conclude fuch a mountain not to be 
volcanic : whereas, only fuppofe Mount Etna to have ceafed 
erupting for many ages, and that half of its conical part 
(hould have mouldered away by time (which would naturally 
be the confequence) and the harder parts remain in points, 
forming an immenfe circuit of mountains (Etna extending at 
its bafis more than one hundred and fifty miles) ; fuch an ob- 
ferver as I have juft mentioned would certainly not find a crater 
on the top of any of thefe mountains, and his ideas would be 
too limited to conceive, that this whole range of mountains 
were only part of what once conftituted a complete cone and 
crater of a volcano. It cannot be too ftrdngly recommended 
to obfervers in this, as well as in every other branch of natural 
hiftory, not to be over-hafty in their decifions, nor to attribute 
every production they meet with to a (ingle operation of na- 
ture, when perhaps it has undergone various, of which I have 
given examples in the ifland which has been the principal fub- 
je£t of this letter. That which was one day in a calcareous 
ftate, and formed by an infeCt in the fea, becomes vitrified in 
another, by the aCtion of the volcanic fire, and the addition 
of fotne natural ingredients, fuch as fea falts and weeds, and 
is again transformed to a pure clay by another curious procefs 
Vol. LXXVIo Ddd of* 
