MINERAL VEINS. 
9\ 
reasons for concluding that great and repeated movements of the rock 
masses have taken place. 
Another kind of structure is presented by the brecciated parts of 
lodes, where, as in the well-known breccia marble, angular fragments 
are cemented by a paste of another mineral. Most frequently such 
fragments are nothing else than pieces of the ruptured adjoining rock 
or “ country,” and thus fragments of clay-slate, black, blue, or grey, 
occur in the breccias of copper lodes in Cornwall, and of the lead lodes 
of Cardiganshire and the Tamar; granite, greenstone, and porphyry in 
those of Wicklow, Hungary, and parts cf Cornwall. The material 
which has enveloped the fragments is sometimes one of the non-me- 
talliferous kind, as barytes, quartz, or fluor, at others a metallic ore, as 
copper pyrites, oxide of tin, iron pyrites, galena, &c. Of these the 
museum possesses a very instructive series, thanks to the liberality of 
several of the Western mine owners and the attention bestowed on the 
subject many years ago by the late Sir H. De la Beche, C.B. 
The fragments in a breccia, when they are evidently derived from 
an earlier consolidated lode, bear evidence corroborative of what has 
above been said on the repeated opening of the same lines of fracture 
in the earth’s crust. The first stage, as it were, of this action is seen 
when a lode mass is broken irregularly or fissured across and the cracks 
are filled in and the broken parts held together by some newer mineral, 
often iron pyrites or zinc blende. In rarer instances the fragments 
have fallen farther away from their original places, and help to make up 
a lode in which the principal constituent may be the newer introduced 
matter. 
Another special character is introduced into certain lodes by the 
tendency of particular minerals to form stalactites and stalagmitic 
incrustations. Calamine, asphalt, psilomelane and hydrous oxide of 
iron are among the most notable in this respect, and we may invite 
particular attention to the specimens from the large and interesting lode 
of Restorrnel, near Lostwithiel, which was visited and admired, under¬ 
ground, by Her Majesty and the late Prince Consort. 
If farther evidence were required of the violent grinding action 
which has accompanied the formation of the lode fissures, it may be 
found in the slickensides or polished surfaces, which in certain veins 
run wfith the walls, especially in the hanging or upper side, but in 
others intersect variously the substance of the lode. Scored and grooved 
as they are in the direction in w’hich sliding motion has taken place, 
they remind the collier of the slip-faces so often found in and about the 
faults or throws, where we have the most palpable proof of the descent 
of one side of a fissure relatively to the other. 
It would be out of place in a brief introduction to do more than 
glance cursorily at the chemical changes which have taken place in the 
minerals constituting lodes, and of which the evidence consists princi¬ 
pally in pseudomorphs, or mineral substances exhibiting a form foreign 
to themselves but characteristic of some other mineral whose place they 
have assumed. The false form may be a mere coating of one mineral by 
another, as of fluor spar by quartz, the latter thus appearing in cubes 
or octohedrons, figures in which it cannot crystallize : more frequently 
the internal mineral has been naturally removed, leaving the other 
as a thin crust. Into the hollows thus formed other substances have 
