VEGETABLE REMAINS. OG 
Besides this coal, many strata of the carboni- 
ferous order contain subordinate beds of a rich 
argillaceous iron ore, which the near position of 
the coal renders easy of reduction to a metallic 
state ; and this reduction is further facilitated 
by the proximity of limestone, which is requisite 
as a flux to separate the metal from the ore, and 
usually abounds in the lower regions of the car- 
boniferous strata. 
A formation that is at once the vehicle of two 
such valuable mineral productions as coal and 
iron, assumes a place of the first importance 
among the sources of benefit to mankind ; and 
deposited at the bottom of the sea. The fresh-water shells that 
occur occasionally in the upper regions of this great series show 
that these more recent portions of the coal formation were deposited 
in water that was either brackish or entirely fresh. It has lately 
been shown that fresh-water deposits occur also occasionally 
in the lower regions of the carboniferous series. (See Dr. Hib- 
bert's account of the limestone of Burdie House, near Edinburgh ; 
Transactions of the Royal Society of Edinburgh, vol. xiii.; 
and Professor Phillips's Notice of fresh-water shells of the genus 
Unio, in the lower part of the coal series of Yorkshire ; London 
Phil. Mag. Nov. 1832, 349.) The causes which collected these 
vegetables in beds thus piled above each other, and separated by 
strata of vast thickness, composed of drifted sand and clay, re- 
ceive illustration from the manner in which drifted timber from 
the existino; forests of America is now accumulated in the estua- 
lies of the great rivers of that continent, particularly in the estuary 
of the Mississippi, and on the river Mackenzie. See Lyell's 
Principles of Geology, 3rd edit. Vol. iii. Book iii. Ch. xv. and 
Prof. Phillips's Article Geology in Encyclopsedia Metropolitana, 
Pt. 37, page 596. 
GEOL. F 
