Burton: The Cleveland Ironstone. 163 
ally, which illustrate this diversity, and it should be noted that 
the diagrams deal only with the Ironstone series of the Spinatus- 
zone of the middle lias. 
If we examine any one of this series of either shale or iron- 
stone, or indeed almost any of the ironstone beds, we find, in 
more or less abundance, fossils of many forms of marine life ; 
and as most of the shell remains consist of carbonate of lime, 
often in an altered condition, we are forced to the conclusion 
that the deposit of iron-bearing strata cannot have been 
formed in the condition in which it now exists, and indeed that 
no iron solution could have contemporaneously permeated 
the beds as they were laid down. The very abundance of 
shells throughout the whole zone negatives any idea that there 
was a ferruginous sea existing when and where the ironstone 
beds were deposited. 
Many writers have carefully considered the question of the 
nature of the original deposit and in almost all cases they have 
arrived at the conclusion that the beds were of carbonate of 
lime. The evidence is not absolutely beyond question, as 
there are difficulties not easily explained, but by a process of 
eliminating every other imaginable idea of the original con- 
dition of the beds, it seems fairly well established that they 
were at first an impure limestone. 
This is known as the replacement theory, and if it is correct, 
we have only advanced one step, as the origin of the interstra- 
tified limestones and shales has to be sought, and then the 
question as to how the lime has been substituted by iron, and 
whence the iron was derived must be answered. 
Limestone has many sources. First, the Chemical. Car- 
‘bonic acid from the atmosphere combines with rain water, 
and as it permeates the surface of the earth, the solution is 
strengthened by decaying vegetable matter. This weak 
solution of Carbonic acid dissolves certain mineral constituents 
of the soil, and, in particular, lime. The lime-charged water 
issues as springs, loses part of its Carbonic acid by evaporation 
of the solvent medium, and deposits the lime as a carbonate. 
A very similar action takes place between Carbonic acid and 
iron. The chemically formed limestone, however, is com- 
paratively small in extent. 
The second source is the accumulation of abandoned 
‘house-boats’ of organisms living in the sea. These house- 
boats are composed of carbonate of lime which the owners 
built by abstracting the material from the waters carried 
down to the sea by inland streams. 
The third source is the destruction by denuding, but not 
necessarily dissolving forces, and the carrying away in suspen- 
sion by streams of the denuded material, which has again been 
deposited. 
1913 April 1. 
