[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.
On the banks of the river Bann, in the same county, there is an extensive stratum of a similar earth, and which, from being in much request for polishing plate, is locally known as Lord Roden's plate powder. This earth is wholly made up of the siliceous frustules of many kinds of this tribe of Algæ, and a few grains under the microscope yield a great variety of exquisite forms: figures of several are given in _Lign. 4_, from specimens of this earth, with which I was favoured by the Countess of Caledon. They comprise two or three species of Navicula, Galionella, Coscinodiscus, Gomphonema, Bacillaria, Stauroneis, &c., and spicules or spines of fresh-water sponges.[67]
[67] The names of the usual kinds of Diatomaceous frustules may be learnt by reference to Mr. Andrew Pritchard's abstract (with coloured figures) of Ehrenberg's Infusoria. The splendid work of Mr. Ralfs, on the British Desmidieæ, 1 vol. 4to, with coloured plates, is the best guide for those who wish to study the recent plants.
Beds of siliceous marl--that is, of argillaceous earth combined with a large amount of minute particles of silex, all of which prove to be organisms when examined by a high magnifying power,--have been found in numerous places not only in England, but all over the world, since M. Ehrenberg first directed attention to their nature and origin.
Near Bryansford (Newcastle), Binstwick in Holderness, and in the Fens of Lincolnshire and Cambridgeshire, extensive fresh-water microphytal deposits have been discovered and examined.