kes, probably attracted by the
temperature and the quantity of food in which they abound; but they
usually confine themselves to the banks, as the carbonic acid disengaged
from the surface would be fatal to them if they ventured to swim upon it
when tranquil. In May, 18--, I fixed a stick on a mass of travertine
covered by the water, and I examined it in the beginning of the April
following for the purpose of determining the nature of the depositions.
The water was lower at this time, yet I had some difficulty, by means of
a sharp-pointed hammer, in breaking the mass which adhered to the bottom
of the stick; it was several inches in thickness. The upper part was a
mixture of light tufa and the leaves of confervae; below this was a
darker and more solid travertine, containing black and decomposed masses
of confervae; in the inferior part the travertine was more solid and of a
grey colour, but with cavities which I have no doubt were produced by the
decomposition of vegetable matter. I have passed many hours, I may say
many days, in studying the phenomena of this wonderful lake; it has
brought many trains of thought into my mind connected with the early
changes of our globe, and I have sometimes reasoned from the forms of
plants and animals preserved in marble in this warm source to the grander
depositions in the secondary rocks, where the zoophytes or coral insects
have worked upon a grand scale, and where palms, and vegetables now
unknown are preserved with the remains of crocodiles, turtles, and
gigantic extinct animals of the _sauri genus_, and which appear to have
belonged to a period when the whole globe possessed a much higher
temperature. I have, likewise, often been led, from the remarkable
phenomena surrounding me in that spot, to compare the works of man with
those of Nature. The baths, erected there nearly twenty centuries ago,
present only heaps of ruins, and even the bricks of which they were
built, though hardened by fire, are crumbled into dust, whilst the masses
of travertine around it, though formed by a variable source from the most
perishable materials, have hardened by time, and the most perfect remains
of the greatest ruins in the eternal city, such as the triumphal arches
and the Colosaeum, owe their duration to this source. Then, from all we
know, this lake, except in some change in its dimensions, continues
nearly in the same state in which it was described 1,700 years ago by
Pliny, and I have