16 
into green carbonate of copper. Crystals of Galena (sulphide of lead) are con- 
verted into sulphate of lead by the addition of oxygen. Anhydrite by the addition 
of water is converted into gypsum. 
5th. Pseudomorphs by exchange of ingredients are by no means uncommon. 
Galena (sulphide of lead) becomes Pyromorphite (or phosphate of lead) by the 
exchange of sulphur for phosphoric acid, and vice versa. Carbonate of lead 
(cerusite) becomes sulphate of lead (anglesite) by exchange of carbonic acid for 
sulphuric acid. Malachite, which is a carbonate of copper, becomes sulphide of 
copper by the exchange of carbonic acid for sulphur. These resulting minerals 
appear in a crystallized form not belonging to them, but belonging to the original 
mineral, and they are thus Pseudomorphs. We can explain many of the changes 
that have taken place underground. The decomposition of sulphide minerals, 
such as iron pyrites, results in sulphuric acid being formed, and the action of this 
acid greatly changes a the character of the minerals. All the carbonates in that 
lode or vein would be dissolved and afterwards crystallized as different substances 
and in different forms. Thus, calcite would be connected with gypsum. Oxides 
and carbonates would become sulphates, and so on. It is, however, impossible to 
say what cause induced the first decompositions. 
I have now given a brief description of Pseudomorphism, and come to the 
subject to which I alluded in the commencement of my paper, viz., how to account 
for the occurrence of an incrustating Pseudomorph of Calcite after Arragonite. 
I had perhaps better repeat what I have previously said respecting these minerals. 
Both are exactly the same chemical substance, namely, carbonate of lime ; both 
are chemically expressed by the same formula, Ca O, C0 2 But they differ minera- 
logically in these respects— Calcite crystallizes in the hexagonal system, and has a 
hardness of 3 inthe mineralogical scale,whilst Arragonite crystallizes in the fourth or 
rhombic system, and possesses a hardness of 3' 5 to 4, and a slightly greater specific 
gravity than Calcite. Both are soluble in acids, but Calcite perhaps more readily 
so than Arragonite. 
"When at Cleator Moor, near Whitehaven, some years ago, I got a goodly 
number of specimens of Arragonite, which occur there nicely crystallized in fair 
abundance, associated with Hematite. I also obtained some Pseudomorphs by 
incrustation of Calcite after Arragonite. The Arragonite had entirely decomposed, 
and had left the Calcite as a hollow form. 
Now, the question is, by what agency was this effected ? We have seen that 
both minerals are soluble in acids, how is it, then, that the investing mineral 
Calcite remains, whilst the original substance, which is somewhat more difficultly 
soluble, has decomposed ? "Would it not seem to us probable that the exposed 
Calcite would have been more liable to decomposition than the protected Arrago- 
nite, and should we not be inclined to say that the agent which affected one sub- 
stance would have affected the other ? 
Quartz pseudomorphs by incrustation, after soluble minerals such as Calcite, 
Dolomite, Chalybite, Fluor spar, &c, are common, but these we can understand 
because the investing substance, Quartz (i.e. silica), is insoluble in acids, and 
further it is probable, if not certain, that the decomposition of these minerals is 
caused by the deposition of the Quartz ; for a hot solution of silicic acid decom- 
poses Carbonates. 
