although Lippi took advantage of this removal, and met the
argument of his antagonists by requiring them to prove the fact. There
is decisive evidence that no stream of lava has ever reached Pompeii
since it was first built, although the foundations of the town stand
upon the old leucitic lava of Somma; several streams of which, with tuff
interposed, had been cut through in excavations.

_Infusorial beds covering Pompeii._--A most singular and unexpected
discovery has been recently made (1844-5) by Professor Ehrenberg,
respecting the remote origin of many of the layers of ashes and pumice
enveloping Pompeii. They are, he says, in great part, of organic and
freshwater origin, consisting of the siliceous cases of microscopic
infusoria. What is still more surprising, this fact proves to be by no
means an isolated or solitary example of an intimate relation between
organic life and the results of volcanic activity. On the Rhine, several
beds of tuff and pumiceous conglomerate, resembling the mass incumbent
upon Pompeii and closely connected with extinct volcanoes, are now
ascertained to be made up to a great extent of the siliceous cases of
infusoria (or Diatomaceae), invisible to the naked eye, and often half
fused.[552] No less than 94 distinct species have already been detected
in one mass of this kind, more than 150 feet thick, at Hochsimmer, on
the left bank of the Rhine, near the Laacher-see. Some of these Rhenish
infusorial accumulations appear to have fallen in showers, others to
have been poured out of lake-craters in the form of mud, as in the Brohl
valley.

In Mexico, Peru, the Isle of France, and several other volcanic regions,
analogous phenomena have been observed, and everywhere the species of
infusoria belong to freshwater and terrestrial genera, except in the
case of the Patagonian pumiceous tuffs, specimens of which, brought
home by Mr. Darwin, are found to contain the remains of marine
animalcules. In various kinds of pumice ejected by volcanoes, the
microscope has revealed to Professor Ehrenberg the siliceous cases of
infusoria often half obliterated by the action of heat, and the fine
dust thrown out into the air during eruptions, is sometimes referable to
these most minute organic substances, brought up from considerable
depths, and sometimes mingled with small particles of vegetable matter.

In what manner did the solid coverings of these most minute plants and
animalcules, which can only originate