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sition of rain in different parts of the country have shown the very 
general prevalence of chlorides, and particularly of chloride of 
sodium, in the air. As might be expected, the proportion of 
chlorides is greatest nearest to the sea, though abnormally large 
quantities are found in the air of manufacturing towns as the 
result of the combustion of coal, &c. On the west side of Britain 
the proportion of hydrochloric acid in rain was found sometimes to 
amount to nearly four grains per gallon, or 56 parts in a million. 
On the east side of Scotland the proportion sinks to *9 grain per 
gallon, or 12 parts in the million. There can be little doubt that 
most of this is chloride of sodium. Dr Smith has pointed out the 
curious fact that this salt cannot be conveyed into the atmosphere 
merely in spray driven from the surface of the sea by high winds, 
for if that were the case the composition of the rain should be 
approximately like that of sea-water. But the saline ingredients 
do not occur at all in similar proportions. It seems reasonable to 
suppose that the superficial parts of rocks liable to be saturated with 
rain-water must thereby receive an appreciable amount of chloride 
of sodium. In this case it is evident that much care should be 
exercised in procuring for analysis portions of rock which lie 
beyond the reach of this surface saturation. Possibly in some of 
the instances cited by Kennedy, in the paper already referred to, 
the common salt may have been introduced by the action of rain. 
2. The Deposit of Salts on the Floor of Old Lakes and Inland Seas. 
— This mode of origin is doubtless by much the most important 
source of the chloride of sodium in rocks. When we consider the 
large proportion of marine strata in the stratified part of the earth’s 
crust it is surprising, as De la Beche remarked long ago,* that 
saline waters are not more abundant than they really are. Dr 
Sterry Hunt has pointed to the mineral waters of Canada and the 
North-Eastern States as probably deriving their salts from the 
original sea-water of palaeozoic times still imprisoned within the 
pores of the rocks. f I need not refer to the abundant deposits of 
rock-salt and gypsum, as well as of saliferous and gypsiferous 
clays, which occur in so many districts of the world. 
3. Volcanic Sublimations. — The occurrence of incrustations and 
* Researches in Theoretical Geology, 
t “Essays in Chemical Geology, 1875. : 
