228 On Mineral Veins. 
Art. XVII.—On the Formation of Mineral Veins and the | 
Deposit of Metathe Ores and Metals im them. By 
Mr. H. A. THompson. 
[Read 12th August, 1867.] 
For the purpose of this paper mineral veins may be 
divided into two classes—those more or less vertical, found 
in all metalliferous strata; and the flat or horizontal veins, 
most distinct and regular in the carboniferous limestone, 
but also occasionally found in the diorite dykes traversing 
the schistose rocks, or in these rocks themselves. The 
bounding walls of mineral veins are usually well defined, 
but not universally so, there being some irregularity in this 
respect ; and the veins vary from a mere thread up to 50, 
60, or even 100 feet in width. In some cases the space 
between the walls of the vein is filled up with solid mate- 
rials—quartz in the silurian rocks, and carbonate of lime and 
sulphate of baryta in the carboniferous limestones; in 
others a portion of the space is occupied by slate, clay, or 
earthy matter. The ores of the different metals are found 
mixed with these substances, sometimes in masses without 
any perceptible order, at others in layers parallel to the 
sides of the vein, and also in the cleavage planes and 
fissures or joints of the adjoining rock. Veins are in some 
cases continuous for many miles in length, and have been 
worked to over two thousand feet in depth without giving 
any sign of closing; in others they are limited both in 
length and depth. Some veins retain a regular width over 
large areas, others open out into bunches, and then contract 
into mere joints at irregular intervals. 
With such a diversity of conditions it is not surprising to 
find that there has been an equal diversity of opinion as to 
the mode in which these veins have been formed ; but the 
theory now generally supported is, that they have been 
faults caused by some disruptive force—the fissures thus 
opened having been afterwards filled up with the minerals 
now constituting the vein. Acting on this theory, the veins 
of different districts have been classed in series in accordance 
with their presumed relative age, the latter being fixed by 
means of the dislocations which occur at their intersections 
with each other. 
