536 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
attacked are carbonates and silicates of the alkalies, the waters thus becoming 
charged with most potent graving tools. 
Next we have silicates which contain lime, protoxides of iron, and of man- 
ganese. 
Lastly, we know that silicates of alumina and magnesia are the most stable 
of all; for carbonated water has no action upon silicate of alumina, and but a 
slight one on silicate of magnesia. 
In virtue of the above,—from compound silicates carbonated waters will 
abstract the silicates of lime, iron, and manganese ; leaving the. silicates of 
magnesia and alumina as residues. 
In virtue of the above,—the rock masses which we find in nature to be least 
prone to decomposition, are either immediately silicates of alumina and magnesia, 
or they are such as have originated from the alteration of the less stable silicates. 
Such are—steatite, talc, silicate of alumina, clay, kaolin, and sand itself— 
among simple silicates; and mica, chlorite, serpentine, asbestus, and mountain 
leather—among compound ones. 
These, however subject they may be to more complex changes induced by 
saline or alkaline waters, are no longer liable to further alteration through the 
operation of atmospheric agents—such as oxygen, carbonic acid, and water. 
Thus it is, then, that serpentine asserts itse// wherever occurring; protruding 
as lines of rugged eminences,—fitter types of the attribute assigned to the “ever- 
lasting hills” than the lordlier granitic masses around it; thus it is that the mica 
crystal, which, torn from that granite, and mechanically comminuted but intrin- 
sically unchanged, had served its purpose of giving continuity and sparkle to 
sandstone of newer and still newer epoch, glitters yet untarnished mid the sands 
of the sea-shore ; and thus it is that these sands themselves, buffetted by the 
waves of Cambrian, and Old Red, and Coal Measure, and Permian, and it may 
be still more recent epochs, amid many surrounding changes have known none, 
but, atomies though they be, seem quite large enough and hard enough again 
to complete a like extensive cycle. 
Thus it is that the clay which, as impure kaolin, the rain drop has gouged 
out of the felspar of that granite,—which, soft as mud, gives way to everything, 
but can be changed by nothing,—is seized upon by man to be fashioned into a 
structure, harder, less compressible, more durable than stone itself. 
Thus, then, the mere passage of a current of carbonated water over minerals 
containing, or rock containing lime, iron, and silica, is sufficient to sweep these 
substances in solution out of the rock, and to do so, moreover, with great 
rapidity. 
Some years ago I had an opportunity of noting this. In an investigation 
into the relative excellence and the durability of different paving stones, I first 
ascertained the quantity of water which perfectly fresh unaltered pieces thereof 

