4 68 
The Atmosphere Considered 
[October, 
to deliberate as to the possible duration of “ Our Carbonic 
Acid Supplies.” But should a necessity ever arise, it is 
comforting to reflect that it is not likely to occur until some 
ages after the travelled New Zealander has been gathered 
to his fathers, and even the very sites of Auckland and 
Otago perhaps long a subject of curious speculation amongst 
Central African savants. I say it is comforting to take this 
to heart in these days of sensational cosmogony, when one 
day we are threatened with destruction from the sweep of 
a comet’s tail, and the next an unfavourable eruption of 
sun-spots may entail unheard-of miseries upon us. All the 
information we are in possession of goes to show that the 
trifling changes that are now observed in the condition of 
the atmosphere would perhaps require a continuance 
throughout many millions of years before making them- 
selves disagreeably apparent. 
Geological Influence of Oxygen. 
This comes next in importance as a geological agent.* I 
have dwelt first upon the results wrought by the carbonic 
acid, because the work done by it is immensely greater in 
proportion to its amount. But oxygen also has its mission. 
Percolating the rocks, dissolved in rain-water, which is able 
to absorb a very large quantity of it, it quickly reaCts on all 
oxidisable substances. Carbonates and proto-salts are con- 
verted to peroxides ; sulphides are changed into sulphates, 
and sometimes this is accompanied by the production of 
double salts, such as alums. A familiar instance may be 
referred to as occurring in the spoil banks of coal-pits, where 
quantities of aluminous shales, with refuse coal containing 
iron pyrites, are heaped up together and exposed to the in- 
fluence of the weather. The oxidation of the iron pyrites 
results in sulphate of iron, and the sulphuric acid so formed 
— reading on the alumina, potash, &c., of the shales — forms 
a more or less complex alum, which may be observed in 
small stellate crystals between the laminse of the shales. 
Alum slates and earths are very common, and all owe their 
origin to the oxidation of iron pyrites, or some other sul- 
phide, under circumstances akin to the above. 
Ores and Metalliferous Deposits. 
The peroxides of iron and manganese are of considerable 
importance, both commercially and from a scientific point 
of view. In many cases their formation may be traced 
* The amount of oxygen in the atmosphere is about two trillions of pounds 
(Bischof, op. cit., i., 204), equal to about 892,857,000,000,000 tons. 
