544 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
is no evidence that the removed stratum was limestone,—the rock immediately 
beyond the void is a perfectly unaltered diorite, which therefore could not 
have supplied any lime ;—secondly, that, in Scotland at least, the limestones 
associated with serpentine are not dolomitic; and this because, according to the 
opposite view, the magnesia is retained in the resultant serpentine ;—and 
lastly, that there is no provision, according to such a view, for the removal of 
the alumina. 
For aid in the explanation, we must ascertain what are the circumstances in 
which silicate of alumina can either be decomposed or dissolved ; and what 
those in which silicate of magnesia can in any way be introduced :—we will then 
be in a position to say whether any of the said circumstances, in the case in 
question obtain. 
Silicate of alumina can be decomposed by chloride of magnesium or sul- 
phate of magnesium in solution,—-silicate of magnesia being formed. 
Silicate of magnesia, again,is also formed by the decomposition of bicarbonate 
of magnesia, by silicate of lime, or by silicates of the alkalies. 
Now the two first of these salts are present in river water, more largely 
in spring water, and most largely in sea water; while bicarbonate of magnesia 
is very frequently present in spring water. Though it is unnecessary to call 
in the operation of sea water, in presence of the fact that the waters which 
permeate all rocks are themselves charged with these potent agents of per- 
petual change, still, seeing that the altered rocks in question have been beneath 
the sea, and that their inmost pores would then be more urgently saturated 
by hydrostatic pressure, it may be very fittingly argued that diallagic rocks 
which have their felspar as well as their augite converted into serpentine, 
suffered the alteration during an epoch of marine submergence. 
BiscHor writes: “ It would be very illogical to suppose that the calcareous 
and magnesian salts dissolved in sea water do not take part in chemical 
alteration, when it is so manifest that alteration is effected by these salts dis- 
solved in much smaller quantity in the water percolating through rocks. It 
would be inconsistent with the relation of mutual compensation perceivable in 
all natural phenomena to suppose that saline substances were continually 
carried into the sea, without being consumed in the formation of new sub- 
stances.” To apply this to the present case, there would seem to be no readier 
way of accounting for the abstraction of the vast quantities of magnesian salt 
present in the sea, and constantly being added to it, than to use it up, so to 
speak, in the formation of serpentine. How else abstract so soluble a salt as 
chloride of magnesium? Its extreme solubility and deliquescence would 
negative its being abstracted through the direct formation of minerals ; but, m 
virtue of the above interchange, lime salts replace it in the ocean, to be con- 
tinuously removed in turn by crustaceans, mollusks, and coral insects. 

