1 877.] 
459 
in its Geological Relations . 
due to carbonic acid entrapped by rain-water and carried 
down into the ocean. On the one hand, the carbonate of 
lime previously conveyed by river waters is held in solution, 
and kept in a fit state for assimilation by marine organisms. 
On the other, the dead shells while sinking through great 
depths are attacked, forming, as Sir Wyville Thomson tells 
us, if the depth is not sufficient to give time for complete 
decomposition, a calcareous ooze ; at greater depths the 
deep sea muds.* Thus a very great amount of the carbonate 
of lime in the ocean owes its existence entirely to atmo- 
spheric carbonic acid, either from the diredt adtion on 
calcareous rocks, whether old limestones or silicates, — or 
indiredtly through a series of changes whereby carbonate of 
soda would be produced, and this being brought into contadt 
with the chloride of lime so abundant in the ocean, carbonate 
of lime would result. There can be no question but that 
such effedts are going on extensively day by day. 
^Influence of Vegetation. 
If we follow the series of rock-metamorphisms due to the 
simple absorption of carbonic acid by a plant the result will 
be seen to be more than interesting. The carbon is assimi- 
lated by the plant, an equivalent of oxygen being exhaled. 
The plant dies, and may become either a part of a coal-bed 
or may be separately imbedded amongst layers of sediment 
of some kind. Slow decomposition will now set in, sooner 
or later, and, if there be a reducible compound near it, 
chemical changes result. Say the strata contains sulphate 
of iron : this is reduced to sulphide, commonly known as 
iron pyrites, a very common mineral in coal-seams — as 
colliery owners know too well — or in other strata where 
plants abound. The redudtion is effedted by the carbon of 
the plant abstradting the oxygen from the sulphate, and the 
resulting carbonic acid either is taken up by percolating 
water, and penetrates farther into the heart of the rock, 
effecting new changes, and producing carbonates, or it finds 
its way to the surface through some crevice or by the aid of 
a mineral spring, and once more mingles with the atmo- 
sphere, to be perhaps again absorbed by vegetation, and 
pass through a round of similar changes afresh. Carbonic 
* It now appears, however, that a considerable portion of these muds is 
derived from the gradual disintegration of pumice and other volcanic debris 
very widely spread over the sea-bottom. See Mr. John Murray’s paper on the 
“Distribution of Volcanic Debris” (Proc. Roy. Soc. Edinb.). The result 
still due, however, to the adtion of carbonic acid dissolved in the ocean. 
