94 THE MEDALS OF CREATION. Cuar. VI» 
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 diatomacez. 
Lien. 4. Srr1cEous FrustuLEs OF DIATOMACEZX, AND SPICULES OF SPONGILLZE; 
from a deposit on the banks of the river Bann, Ireland. 
(Seen by transmitted light, and highly magnified.) 
Fossth DIATOMACEH FROM IRELAND, Lign. 4.—Dr. Drum- 
mond 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 diatomaceze of the usual fresh-water species, without any 
admixture of inorganic matter.* 
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 
* Mag. Nat. Hist. New Series, vol. iii. p. 353, July 1839. 
