MINERAL VEINS. 
87 
Some interesting specimens in the following Case (30) exhibit 
certain peculiarities in the succession of the deposits. In many 
of the examples crystals of a later-formed mineral have been 
deposited on particular sides only of the pre-existing mineral, 
the cause determining this selective action being in many cases 
far from obvious. 
Vein-deposits after their formation have often been subjected 
to the action of certain dislocating forces, which have again 
established fractures, and in these new fissures other minerals 
have been deposited. Several examples of “ comby ” lodes 
are introduced, in which the succession of plates shows that the 
opening must have been several times repeated at distinct 
intervals. 
The mineral deposited in a vein occasionally serves to cement 
together angular fragments of the neighbouring rocks, and 
even of other veins, In Case 31 is a large collection of such 
brecciated lodes, in which a non-metallic mineral has acted as 
the cement ; fragments of sandstone, for instance being cemented 
by heavy-spar; whilst Case 32 is devoted to the exhibition of 
those in which a metalliferous mineral has been introduced as 
cement, as when sandstone fragments are bound together by 
galena. In the upper part of Case 33 are some interesting 
breccias containing fragments of pre-existing lodes ; pieces of 
copper pyrites, for instance, being embedded with fragments of 
quartz in a “ flucan ” or clay. 
The fine pendent forms of the stalactites, introduced in 
Case 33, will naturally attract attention, and it will be observed 
that many of the Cornish specimens exhibit a regular sequence 
of deposits, the stalactite having been coated by a succession of 
newer- deposited minerals. 
The polished and striated surfaces of the “ slickensideS,” in 
Case 34, evidently point to some sliding or grinding motion in 
the mass constituting the lode, affording evidence of movement 
since the formation of the metalliferous matter, such movement 
as would be sufficient to account for the fissures and breccias to 
which allusion has been made. 
From these evidences of mechanical disturbance in mineral 
lodes, we may pass to those changes of a chemical character 
which vein-deposits frequently suffer, as especially attested by 
the phenomena of pseudomorphism. In this case is arranged a 
large collection of the so-called “ displacement ” pseudomorphs, or 
those in which one mineral is deposited either upon or in the 
place of another. Of such changes the curious “ boxes ” from 
Virtuous Lady Mine, near Tavistock, in Devon, are interesting 
examples : a coating of carbonate of iron has been deposited 
upon cubic crystals, probably of fluor-spar, and by the subse- 
quent removal of these crystals the encrusting carbonate has 
been left in the form of large hollow cubes, in which quartz 
and copper pyrites have finally crystallised. It may thus be 
readily understood how a cellular character may be imparted to 
