Such manifestations of life, with its mineral results, have been detected from the earliest sedimentary deposits to the present time ; but as regards the Infusoria, they are given on the grandest scale in formations of the tertiary age. The town of Richmond, in Virginia, United States, is built on barren silicious strata of marine origin and tertiary age. The strata are twenty feet in thickness, composed chiefly of infu sorial flint-shells, including the well-known and beautiful microscopic objects, Actinocyclus and Coscinodiscus.
Most of the infusorial formations, as the polishing-slates at Cassel, Planitz, and Bilin, are astounding monuments of the operation of microscopic organisms at former periods of the history of this planet. The minute size, elementary struc ture, tenacity of life, and marvellous reproductive power of the Infusoria have enabled them to survive as species those destroying causes which have exterminated contemporaneous higher forms of organism. Species of Bacillaria still exist which were in being at the period of the deposition of the chalk. Existing species of Diatomaceoc have been detected as low down as the oolite. The discovery by Ehrenberg of more than twenty species of silicious-shelled Infusoria, fossil, in the chalk and chalk-marls, which are identical in species with some now living in the bed of the Baltic, is an instructive addition to the obscure history of the introduction of species of living things in this planet, and must add greatly to the interest of the infusorial class in the eyes of the geo logist and philosopher. "For these organisms," writes Ehren
berg, " constitute a chain which, though in the individual link it be microscopic, yet in the mass is a mighty one, connecting the life-phenomena of distant ages of the earth, and proving that the dawn of the organic nature co-existent with us reaches further back in the history of the earth than had hitherto been suspected." " The microscopic organisms are very in ferior in individual energy to lions and elephants, but in their united influences they are far more important than all these animals." If it be ever permitted to man to penetrate the mystery which enshrouds the origin of organic force in the wide-spread mud-beds of fresh and salt waters, it will be, most probably, by experiment and observation on the atoms which manifest the simplest conditions of life.