THE LIAS MARLSTONE OF LEICESTERSHIRE, 
153 
contained in the Maidstone could have been deposited in the 
open sea in the state in which we now find it in that rock, 
because the carbonate, if introduced into such an area, would, 
on account of its strong affinity for oxygen, inevitably become 
converted into the peroxide. The presence, in abundance too, 
of mollusca in the Marlstone Bock precludes the idea of its 
contemporaneous deposition, for these animals could never 
have lived in such a concentrated solution of this iron com¬ 
pound as a percentage of thirty parts of the metal implies. 
The iron must, therefore, have been introduced subsequently 
to the formation of the rock. Speaking of the Cleveland 
ironstone, which, for all practical purposes, may be considered 
as identical with the Leicestershire stone, Dr. Sorby says, the 
calcic carbonate of many of the fossil shells it contains is 
often found to be partially or wholly replaced by the carbonate 
of iron, and the exteriors of the oolitic grains are also 
similarly replaced, indicating that they, too, have been altered 
after deposition. He concludes that the Cleveland ironstone 
was once an oolitic limestone in ter stratified with clays con¬ 
taining a large amount of oxide of iron and organic matter, 
which, by their mutual reaction, gave rise to a solution of 
bicarbonate of iron—that this solution percolated through 
the limestone, and, removing a large part of the carbonate of 
lime by solution, left in its place carbonate of iron (see Report 
Geol. and Polytech. Soc. of Yorkshire, Yol. II., 1856). With 
slight modification we may, I think, accept this theory as the 
most feasible one to account for the origin of the Leicestershire 
ironstone. For the source of the iron, however, we shall, I 
think, have to look beyond the beds themselves. If carbonate 
of iron were by any process developed in the impervious clays 
enclosing the Marlstone Bock, as suggested by Dr. Sorby, it 
would, in all probability, be deposited within those clays in 
the same way, for example, that it was in the clay-ironstone 
nodules of the coal measure shales, and, as a matter of fact, 
such nodules occur in the underlying Middle Lias shales. 
We have yet a great deal to learn as to the origin and distri¬ 
bution of metallic compounds in the sedimentary rocks. 
What the precise source of the carbonate of iron contained in 
the Marlstone of Leicestershire was, and when and how 
introduced, it is at the present time, and perhaps always will 
be, impossible to say. 
