56 ANKERITE VEINS OF LONDONDERRY—LOUIS. 
from the wall itself, are occasionally found embedded in the 
Ankerite. The general appearance too of the vein, its wedge- 
like shape, narrowing downwards, and the mode in which the 
minerals are arranged in it, all appear to me to suggest its for- 
mation by aqueous agency. 
As to the other minerals present, they are evidently of far 
later date than the vein itself, and have been formed by the 
action of water percolating through the fissures of the Ankerite. 
I may here mention as an instance of such action, that I have 
found near the Ankerite quarry conglomerates consisting of 
pebbles of Ankerite and other rocks united by a calcareous 
cement. In this connection too, it is interesting to note that 
whenever Aragonite and Calcite are present in the same fissure, 
the former invariably invests the latter, but never vice versa. 
The most generally accepted theory regarding these two minerals 
is, that Aragonite was deposited from hot water and Calcite from 
cold; this theory, if true, would lead us to infer that the Anker- 
ite vein was exposed to the action first of cold, and at a later 
period, of heated waters. 
When a mass of Ankerite or Sideroplesite is left exposed to 
the atmosphere for a number of years, a thin crust of brown or 
red hydrate of Iron forms upon its surface, the Calcic and Mag- 
nesic Carbonates being superficially carried off in solution. Here 
we have reproduced before our eyes the cheinuical phenomena, .to 
which we owe the Red ore. This ore has been produced by 
the joint action of air and moisture upon the Carbonates in the 
Ankerite vein. The air and Carbonic acid, dissolved in common 
water, would probably suffice for this reaction, which might 
perhaps be aided by a low degree of heat ; but at a red heat, as 
I have ascertained by experiment, anhydrous Ferric oxide, (and 
not Ferric hydrate,) is produced, even when steam is passed over 
the heated mineral. So gradual has the decomposition of the 
original minerals been, that their shape'and cleavage have been 
perfectly maintained during the course of the metamorphism. 
As to the Brown Heematites forming the bulk of the London- 
derry ores, their mode of formation is somewhat more obscure, but 
is probably as follows: Ata period subsequent to the formation of 
