i877-j 
469 
in its Geological Relations. 
direcftly to the adtion of atmospheric oxygen. In other in- 
stances this action is but veiled by a series of complications. 
Many valuable deposits of iron and manganese are formed 
in cavities of rocks through the means of water containing 
carbonic acid and oxygen. The first dissolves the minerals 
as bicarbonates ; then, the excess of carbonic acid escaping 
as opportunity permits in open fissures, they are oxidised, 
and deposited at once in an insoluble form, while such other 
carbonates as happen to be in solution, and which — like 
lime, magnesia, and the alkalies — have a stronger affinity 
for carbonic acid than for oxygen, are carried away. 
By such a process as this immense beds of limonite have 
been deposited, and the liberated carbonic acid restored to 
the atmosphere. Bog iron-ores and the well-known lake 
iron-ore deposits of Sweden are cases in point. Some of 
these deposits are assisted by organic agency, some of the 
Diatomacese — Gallionella in particular — being very adtive in 
this way ; but they are only accessory aids, the real work being 
due to chemical reactions between carbonic acid, oxygen, 
and soils or rocks. The extensive beds of hematite associ- 
ated with the Antrim basalts are unquestionably lake- 
deposits, as Prof. Hull has suggested,* and must be due also 
to the reciprocal chemical adtion of the carbonic acid and 
oxygen from the atmosphere. These beds are now interca- 
lated between the sheets of basalt, and sometimes reach a 
considerable thickness, consisting of beds of rich ore, poorer 
ore, and “ lithomarge,” which is a highly ferruginous clay. 
Prof. Hull considers that all these were deposited in a large 
lake or series of lakes. Assuming this, the modus operandi 
was probably this : — The highly ferruginous basalt forming 
the shores of these lakes being subjedt to the adtion of atmo- 
spheric water, the iron existing as proto-silicate in the 
augitic rock, was dissolved out as carbonate and carried into 
the lake. The excess of carbonic acid then escaping, 
oxidation ensued, as in the case already referred to, and the 
iron was precipitated as a hydrated peroxide. At the same 
time fine sedimentary aluminous matter was also carried 
down and deposited, and, according as the amount of this 
was greater or less, a bed of lithomarge or workable ore was 
laid down. A fresh volcanic outburst eventually taking 
place, the lakes were covered in, and the ore bed preserved 
from denudation. 
The ore must have been precipitated in the hydrated state, 
* Brit. Assoc. Report, Belfast, 1874; also Ex. Mem., Sheets 21,28,29 
Geol. Surv. Ireland, 
