540 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
formed by the alteration of an aluminous mineral must be a more or less 
aluminous serpentine. And inasmuch as it is in hornblende and not in augite 
that alumina replaces silica, we may, if we find on analysis that a serpentine is 
notably aluminous, be able to say that it was formed by the transmutation of 
diorite or other hornblendic rock. 
Again, inasmuch as carbonic acid does not combine with the peroxide of 
iron, if that substance is either present, or once formed in a mineral, it cannot 
be removed by such a process; and so it is that we have the iron in these 
serpentinous products unremoved if it be thrown into the state of peroxide. 
Its retention as such may even aid us in determining the depth at which 
the transmutation took place. 
If the change was effected near the surface, we know that the transmuting 
water was aerial,—“ meteoric water,” as it has been called. Each gallon of 
such water holds in solution 2 cubic inches of oxygen and 1 of carbonic acid. 
This water, holding so small a charge of carbonic acid, could effect the trans- 
formation with extreme slowness. We have seen that the lime was removed 
in the first place: if the quantity of acid did not suffice to remove both lime 
and iron, during the time that that acid was engaged in taking up the lime, 
the oxygen in solution would simultaneously be engaged in peroxidising the iron. 
So we would be entitled to hold that serpentines with red or brown 
colours, and such as retained iron as peroxide, had been formed near the surface. 
So can we explain, also, the ferruginous crust which is so characteristic of 
most serpentines at their outcrop. 
Spring waters, again, much more highly charged with carbonic acid, but 
not carrying so large a supply of oxygen, would effect the change more 
rapidly, and sweep away, more or less, perhaps, all of the iron as proto- 
carbonate, leaving only residual traces of protosilicate, which impart the green 
coloration. 
We have at Portsoy the most direct evidence conceivable of the conversion 
of a diallagic rock into serpentine, in the fact that one end of the stratum still 
remains as gabbro; and in immediate contact with it we have limestone, here 
very siliceous. Now, the frequent association of thin beds of limestones with 
serpentine supplies very direct evidence of the conversion of hornblendic and 
augitic rocks into serpentine. In that fact we have a ready answer to the 
question, ‘‘ What becomes of the carbonate of lime necessarily formed during 
such an alterative process as the above ?” 
I will not say that limestone is always to be found in such association ; 
we do not always find limestones even where we have indubitable evidence 
that they once existed; for here the very thing that makes can unmake, or 
sweep away. The carbonate of lime thus fashioned out of the rock forms a 
belt beneath the residual serpentine, thicker or thinner in accordance with the 

