Origin of the Iowa Lead (hkI Zinc De/j(j.sifs. — LeonnnL 
•298 
since it does not necessitate the presence of profound tissures 
or faults, nor the rising of the heated waters through these 
from great depths. 
A third view as to the origin of ore deposits is mentioned 
by Professor J. F. Kemp* It is held by a number of careful 
observers and was brought into prominence by Emmonsf in 
his report on the Leadville region. According to the replace- 
ment theory, as it is called, no large cavity is supposed to have 
previously existed. There is a circulation of ore-bearing so- 
lutions which interchange their metallic contents, molecule 
by molecule, for the substance of the rock. The ore body in 
this case has no well defined limits but shades off gradually 
into the barren country rock. 
We are now ready to enquire which one of the above theo- 
ries explains in the most satisfactory wa}^ the source of the 
Iowa deposits. There seems to be little doubt that to the 
process of lateral secretion is due the deposition of the ores 
in the crevices and that they have thus been derived from the 
limestone whence they have been leached by surface waters. 
The view that the metal-bearing solutions came from below 
is strongly advocated by Professor Jenney;J; who holds that 
the Mississippi valley ores have been deposited by waters ris- 
ing through fissures. 
But there are numerous objections to this theory as applied 
to the region under consideration, and among them may be 
mentioned the following: 1. No true fissures extending to 
great depths have been discovered. 2. P'aults are of rare oc- 
currence, and when they are occasionally found have no aj)- 
parent connection with the tleposits. 8. The ores exist only 
in comparatively small aniounts in the underlying Saint Peter 
sandstone and Oneota liiiK'stoiie. and are almost altogether 
absent from the Saint Croix or Potsdam formation. 
On the other hand there are many facts connected with the 
mode of occurrence of the ores which go to prove that the 
waters came from above. Masses of Galena are frequently 
found suspended from the roof of the openings. These could 
only have been formed by waters that reached the crevices 
*Ore Deposits of the United States, New York, 1895. 
tGeology and Mining Industry of Leadville, with atlas. Monograph 
XII, U. S. Geol. Surv., Washington, 1886. 
ILead and Zinc Deposits of the Mississippi Vallev: Trans. Am. Inst 
Ming. Eng.. vol. xxii, p. 171, 1894. 
