INFUSRIA.
(Polygastria, Ehrenberg.) Numerous genera and multitudes of so-called species of free and locomotive microscopic organisms, which, because they do not present the distinctive characters of plants or animals, have been by turns referred to one or other kingdom, possess shells of flint, and consequently enter largely into the domain of fossil evidences of former life. The silicious shells of these infusory organisms present under the microscope the most definite as well as beautiful characters of form and sculpture, which are as recognisable and distinctive as those of the calcareous shells of Mollusca. The plates of the incom parable works and memoirs of Ehrenberg abound with exact figures of the delicate sheaths, shells, and shields of the lori eated Infusoria of past and present mras of life, the deposits of which, by reason of their pure, flinty, atomic constitution, were known in the arts long before science had detected their nature and vital origin. In 1836 portions of the stone called " tripoli" or "polierschiefer" (polishing-slate of lapidaries) were microscopically examined by Ehrenberg, who discovered it to be wholly composed of the silicious shells of Infusoria, and chiefly of an extinct species called Gaillonella distans. At Bilin, in Bohemia, there is a single stratum of polierschiefer, not less than fourteen feet thick, forming the upper layer of a bill, in every cubic inch of which there are forty-one thousand millions of the above-named organic unit. This mineral like wise contains shells of Havicula, Baeillaria, Actinocyclus, and other silicious organisms. The lower part of the stratum con sists of the shells compacted together without any visible cement ; in the upper masses the shells are cemented together, and filled by amorphous silicious matter formed out of dissolved shells. At Egea, in Bohemia, there is a stratum of two miles in length, and averaging twenty-eight feet in thickness, of which the uppermost ten feet are composed wholly of the silicious shells of Infusoria, including the beautiful Campylo discus ; the remaining eighteen feet consist of the shells mixed with a pulverulent substance. Corresponding deposits of the
silicious cases of Infusoria have since been discovered in many other parts of the world, some including fresh-water species, others marine species of Infusoria.
The conditions of such depositions will be readily under stood by examining the sedimentary deposits of bogs and of stagnant or slow-flowing sheets of water. In warm latitudes and seasons, such water swarms with infusorial life, and the indestructible cases of the loricated kinds are found in great quantities in the sedimentary deposits. Beneath peat bogs they have been found to form strata of many feet in thickness, and co-extensive with the turbary, forming a silicious marl of pure whiteness. A quantity of pulverulent matter is deposited upon the shores of the lake near Uranea, in Sweden, which, from its extreme fineness, resembles flour : this has long been known to the poorer inhabitants under the name of " berg mehl," or mountain-meal, and is used by them, mixed up with flour, as an article of food. It consists in great part of silicious shells of Infusoria, with a little organic matter. With regard to the source of fossil infusorial remains in sea-water, the fol lowing evidence is given in the United States Coast Survey, 1856 : Soundings of the gulf-stream near Key Siscayne, Florida, varying in depths from 147 fathoms to 205 fathoms, give a light greenish-grey mud composed chiefly of Foraminifers, Diatoms, Polycystins, and Geolites, in a profusion only sur passed by the fossil polycystinous strata of Barbadoes. The Foraminifers compose the largest part of these muds, including Textularia Americana, Marginula Bachei, and other forms, particularly many species of the Plicatilia of Ehrenberg, which had been supposed to live only in shallower haunts. The silicious shells of Diatoms abound in the residue, after the calcareous Foraminifers have been dissolved by acid. The inorganic portion of the soundings is chiefly quartz sand, and its proportion is quite small.