"This fresh-water plant is seen in immense quantities in the mountain brooks around West Point, the bottoms of which are literally covered in the first warm days of spring with a ferruginous-coloured mucous matter, about a quarter of an inch thick, that, on examination by the microscope, proves to be filled with millions and millions of these exquisitely beautiful siliceous organisms. Every submerged stone, twig, and spear of grass, is enveloped by them; and the waving plume-like appearance of a filamentous body covered in this manner, is often extremely elegant. Alcohol completely dissolves the endochrome (_soft colouring matter_) of this species, and the frustules are left as colourless as glass, and resist the action of fire."[64] [64] Trans. Amer. Assoc. Geolog. 1843, p. 152. The yellow or ochreous scum observable in ponds, ditches, and stagnant pools, is an aggregation of diatomaceæ, whose frustules are feriniginous, and of such extreme minuteness, that a billion of their cases would not be more than a cubic inch in bulk.[65] [65] Ehrenberg. Fossil Diatomaceæ.--From this notice of a few recent types, we proceed to the investigation of the fossil remains of this tribe of Algæ. In peat-bogs and swamps, both of modern and ancient date, masses of a white marly or siliceous paste (_hydrate of silica_), are often observed, and these are found upon microscopical observation to be wholly made up of the frustules of _Naviculæ_, _Bacillariæ_, _Galionellæ_, &c., with an intermixture of the needle-like spicules of fresh-water sponges. Many of the peat-bogs of Ireland contain layers of a white earthy substance, which, when dry, is of the appearance and consistence of friable chalk, and entirely consists of the siliceous cases of various kinds of diatomaceæ. [Illustration: Lign. 4. Siliceous Frustules of Diatomaceæ, and Spicules of Spongillæ; from a deposit on the banks of the river Bann, Ireland. (_Seen by transmitted light, and highly magnified._)] [Sidenote: FOSSIL DIATOMACEÆ OF IRELAND.] Fossil Diatomaceæ from Ireland, _Lign. 4._--Dr. Drummond describes a bed of this kind near the base of the Mourne Mountains, in the County of Down, Ireland. It consists of a very light white substance, resembling in appearance carbonate of magnesia: it has a coarse and somewhat fibrous fracture, and is easily reduced to powder. It is almost entirely siliceous, and is composed of the cases of diatomaceæ of the usual fresh-water species, without any admixture of inorganic matter.[66] [66] Mag. Nat. Hist. New Series, vol. iii. p. 353, July 1839.