GEOGNOSY AND GEOLOGY. 199 
n, m. Rock salt, or saliferous formation, strata 2'7, 28 
i-g. Jura formation, © 9489-393 
f—a. Cretaceous, “« ~ 34-38 
6-3. Tertiary masses, « 39-61 
2, 1. Upper series, + a-e 
InrLuENcE or Water upon Rocks. 
Water exerts a very great influence upon the masses composing the 
earth’s crust. Water descending from the clouds in the form of rain, 
naturally contains such substances as are floating in the atmosphere ; 
among these are carbonic acid, traces of ammonia, and, under certain 
circumstances, very slight traces of sulphuric acid. The inorganic particles 
originally contained by water are deposited or separated from it in its 
evaporation. When the water again descends, it not only retains its 
original inherent property of dissolving and disorganizing portions of rock, 
but will be found to have derived additional power in this respect from the 
carbonic acid. Water by itself, or chemically pure, is incapable of dissolving 
carbonate of lime, but after obtaining carbonic acid from the atmosphere, 
and still more from the humous particles of the soil, it can effect this solution 
in considerable quantity. The portions of lime taken up are generally 
deposited in fissures, veins, caves, druses, &c., in the form of calcareous spar, 
stalactites, &c. The chemical effect of the carbonic acid is to form a soluble 
bicarbonate of lime with the original carbonate of the limestone. Similar 
influences may be exerted upon other masses besides limestone, so that a 
gradual destructign of all rocks is taking place with greater or less rapidity. 
The greatest mountains will, then, in time, be completely dissolved, like 
sugar in water. Rain water, while thus decomposing rocks chemically, and 
disintegrating them mechanically, acts upon them afresh in transporting 
them towards the ocean or still lakes, where they are again deposited in the 
form of strata. The natural tendency of things, then, is to elevate the 
valleys and low regions, and depress the elevated, and so to reduce all to 
the same level, or to the regular spheroidal solid. Another mode in which 
the destruction of rocks is effected is by the force of waves and currents. 
The breakers of the sea, dashing with irresistible force upon the rock-bound 
shore, shatter the rocks, and breaking them into blocks of various size, 
spread them upon the bottom. Wherever, then, the coast is lined with rocks, 
these generally are fissured, cleft, or otherwise affected so as to be exhibited 
in every variety of form. Innumerable instances might be adduced. We 
shall only refer to the curious serpentine rocks on the coast of Cornwall, in 
the bay of Mullian, not far from Lizard Point ( pl. 53, fig. '7), and the rock 
groups on the Faroe Islands ( pl. 49, fig. 3). 
The force of waves not rarely results in the production of caves, some of 
them of considerable dimensions. Among these may be mentioned Fingal’s 
cave on the Isle of Staffa, inclosed by the most beautifully symmetrical 
columns of basalt ( pl. 52, fig. 6); the fresh-water cave (pl. 51, fig. 4), 
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