The Marble times and Crystal silver lance., 1901-06-07
Contact and Fissure Veins.

The following is an extract from
paper read by G. G. Longridge before
the British Institute of Mining En-
gineers:
What is contact lode? And why
should it be inferior to a fissure vein!
The contact vein is defined as one ly-
ing between two distinct kinds OF rock,
two dissimilar rocks, in which the dis
similarity has a genetic relation to
the deposit. The contact lode may
there be variously produced. 1t is
found between sedimentary rocks: be-
tween sedimeutary and igneous rocks;
between two igneous rocks or between
a dyke and the various sedimentary
or igneous rocks it traverses. 1t may
therefore at times be very difficult to
distinguish from an irregular deposit
a bedded or even tissure vein. For
it NAV exist in irregular masses filing
the spaces between the junction OF two
dissimilar rocks, or it may have been
deposited parallel to the stratification
between stratifed rocks, or it may be
scarcely distinguish able from fissure
vein, which by reason of g heavy throw
has two walls OF rock Of different kind
and age. 1t is probably due to these
difficulties of identification and to the
professional ignorance of experts that
the fssure vein once made and partly
still retains its reputation as the sole
deposit worth exploitation.
The facts of the case are very differ
ent. While subject to defects, common
also to the fissure vein, the contact
lode has certain favorable idiosyncras-
les. Like the former, it follows the
irregularities impaired TO the rock
faces in upheaval; it may also suffer
from sliding and faulting: but, on the
other hand, as it retains a constant
relation to the enclosing rock. it is
likely to be more uniform in structure
and filling-a very important point.
Another popularity of the contact lode
between sedimentary rocks or sedi-
mentary and igneous rocks is its GER
tainty to fatten out with depth. This
follows from the fact that no stratitied
bed can continue to descend indefinite
lv. This feature, more pronounced as
the inclination or dip of the contact
more closely approaches the horizon
tai, is of Importance in mining. 1t
obviates the necessity OF deep sinking,
with its attend ant costs and limitations
and it enables cheap and shallow
shafts to exploit large lode-areas.
Far, therefore, from being class OF
deposit from which the mining investor
should shrink. it is one that, demand
ing very careful study on the part OF
the mining engineer, may, II the re-
quired conditions are found to be pre-
sent, be exploited with as much, if not
more, confidence than the fissure vein.
As instances of valuable lodes of this
nature may be mentioned the deposits
of red oxide of copper between lime
stone, granite and slate in Arizona:
the copper pyrites between clay y-slate
and diorite at the Tanokuchi mines,
Tosa, Japan: the copper concentrated
at the junction of trap and sandstone
on the south shore of Lake Superior:
the deposits at Leadville, Colorado;
the auriferous deposits in Ouray coun-
ty, Colorado: the Comstock lode, Ne
vada; the copper-pyrites at Sulitelma,
Norway: the hematite deposits OF
Huelva, together with many other in
stances of like nature. 1t is, there
fore, in mining, as in most things,
obvious that every case should be
Judged on its own merits, not by its
accord or discord with some heeting
dictum OF the day.