INFUSORIA OF TRIPOLI. 
51 
the recent Teredo, The wood in this fossil specimen is now 
converted into a stony mass, a mixture of clay and lime; 
but it must once have been buoyant and floating in the sea, 
when the Teredince lived upon, and perforated it. Again, 
before the infant colony settled upon the drift wood, part of 
a tree must have been floated down to the sea bv a river, 
uprooted, perhaps, by a flood, or torn olf and cast into the 
waves by the wind: and thus our thoughts are carried 
back to a prior period, when the tree grew for years on dry 
land, enjoying a fit soil and climate. 
Strata of Organic Origin. —It has been already remarked 
that there are rocks in the interior of continents, at various 
depths in the earth, and at great heights above the sea, al¬ 
most entirely made up of the remains of zoophytes and tes- 
tacea. Such masses may be compared to modern oyster- 
beds and coral-reefs; and, like them, the rate of increase 
must have been extremely gradual. But there are a varie¬ 
ty of stone deposits in the earth’s crust, now proved to have 
been derived from plants and animals of which the organic 
origin was not suspected until of late years, even by natural¬ 
ists. Great surprise was therefore created some years since 
by the discovery of Professor Ehrenberg, of Berlin, that a 
certain kind of siliceous stone, called tripoli, was entirely 
composed of millions of the remains of organic beings, which 
were formerly referred to microscopic Infusoria, but which 
are now admitted to be plants. They abound in rivulets, 
lakes, and ponds in England and other countries, and are 
termed Diatomaceae by those naturalists who believe in 
their vegetable origin. The subject alluded to has long 
been well known in the arts, under the name of infusorial 
earth or mountain meal, and is used in the form of powder 
for polishing stones and metals. It has been procured, 
among other places, from the mud of a lake at Dolgelly, in 
North Wales, and from Bilin, in Bohemia, in which latter 
place a single stratum, extending over a wide area, is no 
less than fourteen feet thick. This stone, when examined 
with a powerful microscope, is found to consist of the silic¬ 
eous plates or frustules of the above-figured Diatomaceae, 
united together without any visible cement. It is difficult 
to convey an idea of their extreme minuteness; but Ehren¬ 
berg estimates that in the Bilin tripoli there are 41,000 mil¬ 
lions of individuals of the Gaillonella distans (see Fig. 16) in 
every cubic inch (which weighs about 220 grains), or about 
187 millions in a single grain. At every stroke, therefore, that 
we make with this polishing powder, several millions, perhaps 
tens of millions, of perfect fossils are crushed to atoms. 
