88 
THE PRINCIPAL FLOOR. 
a lode by the hollows formed on the removal of the original 
minerals. 
In addition to the displacement and substitution-psendomorphs 
just noticed, there is yet a third group in which the change of 
mineral is readily explicable by chemical action, and of these 
“ alteration ’’-pseudomorphs a series is exhibited in the succeed- 
ing Case ( 35 ). These may result either from the addition or 
from the removal of a constituent, or from a partial exchange of 
ingredients. Thus the crystal of calamine in the form of a large 
scalenohedron of calcite has been formed by the exchange of 
lime for oxide of zinc. So obvious in many cases is the forma- 
tion of these “ epigenic ” pseudomorphs that it may be, to a 
certain extent, successfully imitated, and an interesting series 
of such artificial pseudomorphs, prepared by Dr. Sorby, F.R.S,, 
is placed in this Case. In nature, such chemical action occurs 
in many cases on a very extended scale, affecting mineral masses 
often of enormous extent. To illustrate such changes there is 
introduced into this Case a large series of minerals occurring 
mostly in the upper part of lodes, where the original deposits 
have been subjected to atmospheric influences. Thus the 
crystals of anglesite often found in the shallow workings of a 
lead vein are true products of alteration, having resulted from 
the absorption of oxygen by the galena forming the body of the 
lode, and the consequent oxidation of the sulphide of lead to a 
sulphate. A common alteration in veins is that in which iron 
pyrites is superficially converted into brown iron ore, by elimina- 
tion of sulphur and addition of oxygen and water. On the 
“ back ” or outcrop of a lode such a change is exceedingly common, 
the superficial deposit of loose brown iron ore, which thus acts as a 
cap to the ore beneath, passing under the name of “ gossan.” 
The greater part of the following Case ( 36 ) is devoted to the 
display of varied samples of this gossan, which, although not 
perhaps very attractive to the eye, are nevertheless of the highest 
importance to the miner, who is frequently enabled to judge 
from them of the probability of cutting ore in the deeper 
workings. (For pseudomorphs, see also p. 55.) 
The remaining portion of this Case ( 36 ) is occupied by a 
collection of specimens illustrating certain chemical changes in 
metallic minerals, resulting in the production of native metals : 
thus there will be found, among the specimens, crystals having 
the characteristic octahedral form of red oxide of copper, but 
consisting entirely of the native metal ; the copper having been 
reduced from its combination with oxygen ; and it is indeed 
probable that many of the metals which now occur in a native 
state have been reduced by successive stages from various 
combinations, often of considerable complexity. 
In connexion with the veinstones, the visitor may study a 
series of specimens recently placed in the Model Room A, 
(Case IV.) intended to illustrate the various modes in which 
metalliferous minerals occur in nature. 
