No. 161.] 
201 
oxides may hereafter be found, as they greatly resemble carbonate 
of lime, and are often confounded with it. These latter ores are 
alone used in the arts, the sulphuret requiring to be roasted be- 
fore it can be of value. 
In concluding this very brief, or mere announce of the metallic 
minerals of the 4th district, I am induced to make a few observa- 
tions upon the theory of veins* as a number of our geologists are 
disposed to attach more importance to their igneous origin than the 
facts in relation to it merit. The subject, moreover, is of great 
practical importance, the origin from igneous injection from below, 
being calculated to lead to error, from its consequence, that the 
deeper we descend, the nearer we approach to the source or ori- 
gin of the deposite. 
Were the opinion correct, that veins have been filled from be- 
low by matter injected, one would suppose that the volcanic rocks 
would be the richest in metals, whereas like all modern rocks they 
are the poorest. In the three classes or orders of extinct volca- 
noes of France, no other metallic mineral than a little oligiste iron 
is to be found, occurring in rifts, cracks or hollows of the lava, 
having no connection with one another, the iron appearing to have 
separated from the mass, at the time of its cooling. 
The two classes of facts, that of metallic minerals existing in the 
rock, as well as in the veins, and secondly that the predominant 
. crystallizing mineral or minerals of the rock, forms the material of 
the vein, seem to me conclusively to prove, that the only satisfac- 
tory way of accounting for their origin, is by segregation and la- 
teral infiltration, the metallic and other minerals having been fur- 
nished by the rock which encloses the veins. 
That metallic minerals form a part of rock is certain from the cir- 
cumstance, that in a large class of metallic veins, sometimes called 
beds, from having their sides parallel with the layers of the rock, 
their stony materials are no wise diflferent from the crystallized ma- 
terial of the rock. 
2d. From the existence of another class of contemporaneous me- 
tallic deposites, called by the French geologists amas, or blocks, 
* Meaning those veins which usually cut the rocks or layers at a right angle, which 
from being formed after the consolidation of the rocks in which they exist, are called non 
contemporaneous, and are generally the depositories of metallic minerals. 
[Assem. No. 161.] 26 
