The miners' express. (Dubuque, Iowa) 1849-1854, 1851-11-12
Pref. CYaix,s Lectures.

|
Prof. Craig on Thursday evening gave the
4.h lecture of his course. The subjects en-
braced were the character, and formation of the
various rocks which enter into the composition
Of the crust Of the earth, and the relative an.
tiquity Of the formations in which they occur
This part ofthe lecture was given extempora.
niouslv-,he latter part relating as it does 10
subject Of great importance 10 this community,
namely Mineral veins, we have been kindly
permitted to copy, particularly that part which
our own mineral fields.
awe feel justified from the facts stated in con- /
rocks which are more or less IN contact with
rocks of gneous, or volcanic formation. The |
lead bearing strata OF the valley Of the Mssis
sippi, would seem to form a remarkable excep-
ton to this general law. rs the beds appear to
preserve an almost universal hariz"n.ay in
their mode Of qualification thing almost
| unknown in other regions where stratihed rocks
occur in contact, in the vicinity Of rocks Of
an igneous origin. Ii rocks of this character
occur in connection with the qualification of
this country, they must be very deeply seated
and the disturbing forces have been remarkably
quescent since the period Of the formation of
| the super inc mbent masses. Such universal
llnrizontality, in the beds of the seluran rocks
of Iowa, appears to justify at first sight the
opinion that their metallic deposits originated
not Immediately from ignlous agency, but from
metallic matter held in solution in water which,
pcrcolated through the strata. But even under
this supposton the uestion recurs whence
came the metallic matter originally? For GER
{ tainly if we analyze the limestone or the sub
soil clay beds we find that there is nothing in
their composition TO warrant The idea tina,
they were the source from whence the metallic
matter issued. From this it seems to me iha,
we must infer that the original matter was de-
rived from a subterrancan source.
1t is we'll known that certain lakes IN Russi
constantly deposit iron ore, in the state of a car
Donate or oxide of iron. The iron so deposited
is obtained from springs holding iron in solution
similar springs may have existed in ancien
, times, and have held in solution not carbonate
, Of iron, but sulphurate Of lead. The waite
; containing this solution while percolating thro
he strata of the earth, may have deposited thu
metallic matter in the assures land formed crystal
line masses such as we now find; but whence
would such springs obtain their metallic con
tents, The general character Of the prenome
ma connected with metallic VEINS in other dis
tricts, as we have already seen, would seem t
justify the opinion that the metals are origi
! u r.
frame work Of the globe. Now, as I formerly
mentioned, we have direct evidence that tn
central mass of the earth is in state of intens
heat, from the fact, that in descending all dee]
mines the thermometer rises one degree 10
every so feet Of descent from the surface,
heat must exist at no great depth, quite suffi
cent to melt lead or any other ore whatever
and even reduce it, particularly lead, to case
Gus form
Sulpher is one of the most common substan
n ces emitted in volcanic exhalations. All tn
II metals have a strong affinity for sulphur, henc
most of the ores sulphurtes. Such gasse.
, , arising through the rents in The crust of tn
e earth, might either become solid by crystalizin
f on the sides Of the assures, or they might i'm
n pregnate the waters on the surface into whcl
:, they issued, which, percolating through tn
I rocks, might snvey the metallic matter to sub
It terrancan regions, where, acted on by the thermo
2 electrical currants, passing through the earth
: and acting more intensely on one class of rock
J. than another, would effect, not only consolida
e ton, but crystalzation.
II To o< b,,fh hoe
;. able to explain the phenomena manifested, ant
which have given origin to many conf1ictin!
e opinions-but the idea never having been sug
$ gested to me till after my examination of Mr.
5 Levens' mine | offer the theory it involve
; with hesitation
q The immens caves in which the lead is sc
often deposited, seems another popularity o
the lead mines of this country. These cavitie
are certainly the result OF the wearing down BY
under currents Of water, derived like the wa
tiers of Springs, from the surface, and still ex
hibited in many places by the sudden dsap
r pearance of streams of water and the rapid gush
ngs out of others.
= At the bottom of MIR. Levcns mine ihere i
deposit of lead containing masses of lead ore
. and fragments of limestone intermixed with cal
. careous SPAR, over which there are in some pla
$ ces nearly two feet of finely laminated clay
Il Over this hangs the heavy crystalline veins o
Galena. In examining this deposit, at first con
!, cluded that the lead in the bed at the bottom o
the cave had been derived from the vein, ant
the appearances of the masses extracted from it
. seemed to justify the conclusion and also tina
the crystalline carbonate of lime, intermixe s
with it, had been formed after its beinr imbed
= ded. This bed Of sand, lead, and calcareou
spar, contains pebbles Of rocks which have bee,
= washed into the cave from the surface-and i
y was mentioned to me, the bones of huge AM
NAL had been found in it, or in similar bed :
We must conclude, therefore, it to be of compare
lively recent origin to the rocks in the caves o
which it occurs But in examining this bed ano
the fine clay everything it, we find It at every
little distance crossed by the veins of iron and
lead ORE the latter occurring in sheets, and the
former bearing strict resemblance to bog iron
ore. These veins cross the principal lead vein
they are therefore, newer than the limestone
rocks, the south and ives VEINS and the bed con.
tanning the lead ore, and ANIMATED clay in short
they have been formed since the excavation oil
the cave.
The rents so filed, are also posteror to the
formation of the south and west veins and
these rents could only have been produced from
some disturbing cause from ben~ath-that cause
was too slight to disturb the general horizontal
ity Of the strata, but it has produced faults OIL
slips at least foot in ex'ent. 1t seems to me
therefore, reasonable to conclude that the ore
contained in the cross courses, which are a,
right angles to the principal vein, are due to
subterranean causes, that is to matter coming
up in state of gass, and reconsolidating in the
assure. The occurance of the mineral manga
nese in the dendritic form In the fints, which
traverse the limestone, to justify the same
conclusion, for have found the same form of
that mineral, always to be associated with rocks