Origin of the Toira Lead and Zinc Deposits. — Leonard. 291 
place. By such a process the present zinc deposits would seem 
to have accumulated and to represent the originally diffused 
ores in many formations, possibly as high in the geologic 
scale as those of Missouri or the Lower C'arboniferous. This 
is, however, improbable, owing to the dense and impervious 
nature of the intervening Hudson River (Maquoketa) shales." 
It would seem that this impervious character of the shales 
constitutes a serious objection to Winslow's theory as applied 
to the Iowa deposits. The latter are commonly overlain by 
these shales and occur mostly near the top of the Galena lime- 
stone. Grranting that the overlying Niagara and Maquoketa 
formations were impregnated with lead and zinc, it would 
hardly have been j)ossible for the mineral-bearing solutions to 
make their way through the impervious shales. In other words, 
there could not have been in this area a very extensive down- 
ward flow and re-composition of the ores. The process has 
doubtless been going on within the Galena formation itself 
and may have caused some local concentration, but the lime- 
stone has not undergone very extensive decomposition in situ, 
and the ore is found largely in the upper beds. For these rea- 
sons, while ('hamberlin's theory of ocean currents may appear 
somewhat too hypothetical, it furnishes on the whole the most 
plausible explanation yet olfered for the localization of the 
Upper Mississippi deposits. 
Formation of creviees. Cavities and crevices in rocks are 
formed in several different ways. They may result from con- 
traction due to solidification, drying or cooling. A familiar 
example of this process is seen in the cracks found in basalt. 
It is probable that some of the joints of sedimentary rocks 
have had the same origin. But the most important cause of 
fracture is found in the movements of the earth's crust pro- 
ducing a folding and crum[)ling of the strata. When such 
anticlinals and synclinals are formed the rocks are fissured by 
the strain to which they are subjected. Should the walls of 
the fissure slip over each other, one side being raised or low- 
ered, a fault would result. The fractures when once formed 
become the channels for subterranean drainage, and these are 
enlarged and modified by the dissolving power of water. 
The crevices of the r])per Mississippi region are apparently 
due to the second en use. Extending east and west through 
