But the Tertiary formations contain strata of this nature, which far surpass in the abundance and variety of their organic contents, any of the modern deposits we have noticed. The _Polierschiefer_, or _polishing-slate_ of Bilin, is stated, by M. Ehrenberg, to form a series of strata fourteen feet in thickness, entirely made up of the siliceous shells of _Galionellæ_, of such extreme minuteness, that a cubic inch of the stone contains forty-one thousand millions. The _Berghmehl_ (_mountain-meal_, or _fossil farina_), of San Flora, in Tuscany, is one mass of these organisms. In Lapland a similar earth is met with, which, in times of scarcity, is mixed by the inhabitants with the ground bark of trees, for food; some of this earth was found to contain twenty different species of algæ. In the district of Soos, near Egra, in Bohemia, a fine white infusorial earth occurs, about three feet beneath the surface; this substance, when dried, appears to the naked eve like pure magnesia, but under the microscope is seen to be mainly constituted of elegant disciform cases of a species of _Campilodiscus_, of which figures are given, _Lign. 111, figs. 1, 2_. Some beds of porcelain-earth M. Ehrenberg found to be in a great measure made up of concentric articulated rings, entire and in fragments (see _Lign. 6_), which he believes to be bacillariæ. [Illustration: Lign. 6 Organic Bodies in Porcelain Earth; _highly magnified_.] [Sidenote: FOSSIL DIATOMACEÆ OF VIRGINIA.] Fossil Diatomaceæ of the Richmond-earth; Virginia.--The town of Richmond, in Virginia, is built on strata of siliceous marl of great extent, which earth; highly magnified. have a total thickness, beneath and around the town, of more than twenty feet. These marls, whose organic composition was first detected by Professor W. B. Rogers, are referred by that eminent American geologist, to the older tertiary (_eocene_, or _miocene_) formations. They occupy considerable districts, spreading out into sterile tracts along the flanks of the hills, their siliceous character rendering them unfavourable to vegetation. The investigations of Dr. Bailey have shown that the frustules so abundant in this earth, consist of several species of Navicula (_Lign. 1, fig. 1, 1a_.), Galionella (_Lign. 1. fig. 3, 3a_.), _Actinocyclus_ (_Lign. 1, figs. 4, 5_), &c.