290 The American Geologist. November, ih<»5 
tration of the deposits to the currents of the old Silurian sea. 
The oceanic waters impregnated with metallic salts derived 
from the leaching of the adjacent lands were borne by cur- 
rents to areas where there was an abundance of oi'ganic life, 
in the presence of which the metals would be extracted and 
thrown down along with the sediments. 
Mr. Arthur Winslow* has recently advanced a somewhat 
different hypothesis concerning the origin of the Missouri ore 
bodies. He holds that the concentration is due to the surface 
decomposition of the rocks. "According to our theory the 
concentration is entirely secondary. It is primarily a result 
of great and long-continued surface decay of the rocks ; and 
secondarily, the result of the presence of local favorable, 
physical and chemical conditions." The hypothesis starts 
with the proposition that the minerals existed in the Archean 
rocks, and with the decay of these became diffused through 
the later formed sediments. It will be noticed that this the- 
ory agrees with that of Chamberlin in recognizing the pres- 
ence of minerals in the country rocks and the derivation of 
the deposits from them; but it differs in maintaining a con- 
dition of general diffusion, rather than one of concentration, 
over certain favored areas. 
The evidence is abundant that very extensive sub-aerial 
decay has befallen the rocks in the Missouri region, and dur- 
ing successive geological periods many hundreds of feet have 
been removed. Mr. Winslow believes that in the Wisconsin- 
Iowa area the same processes were long operating to concen- 
trate the ores. It has already been stated that the district is 
unglaciated, and thus has been long exposed to atmospheric 
agencies by which the rocks were extensively decomposed. 
Mr. W^ P. Blake,f who is familiar with the Wisconsin de- 
posits, seems to hold something of the same view as Winslow, 
if we may judge from the following words: "The evidence is 
strongly in favor of the view of the long-continued decompo- 
sition, downward flow and re-composition of not only the ores 
of zinc but of lead and of the pyrite from the upper forma- 
tions to the lower, as the general water-level of the region 
subsided and as the upper formations by long continued ex- 
posure through geologic ages were gradually decomposed in 
*Mis8ouri Geol. Surv., vol. vii, p. 477, 1894. 
tTrans. Am. Inst. Ming. Eng., vol. xxii, p. 621, 1894. 
