Ore Fonnatioii by Surface Decomposition. — Keycs. 350 
of the vein matter is lixiviated in a given time than would re- 
sult from slower wasting of the land. Such enrichments are 
favored hy high altitudes. Moreover, the mountainous re- 
gions are those in which secondary fractures are most apt to 
be found." 
Blake's observations''' on the formation of the lead and zinc 
deposits of the Wisconsin district have an import similar to 
that of Winslow's for Missouri. 
Of like suggestion is Winchell's viewt regarding the Wis- 
consin lead deposits and the Minnesota iron ores. This au- 
thor regards the Cretaceous strata as having extended over the 
whole of the upper Mississippi valley region and in their re- 
moval to have allowed the ore materials to lodge in the porous 
and gashed Silurian rocks beneath. 
These special examples have been noted in this connection 
for the reason that in all there is the same effort to seek an 
adequate and direct .source for the ore bodies. All start with 
the assumption that the present ore bodies were derived from 
the ore materials- that had settled downward as the great rock- 
masses above were removed through erosion. In other words, 
the existing ore bodies of the region mentioned are claimed to 
be deposits the formation of which has depended wholly (ir 
largely upon the general metallic content of the country rock 
once existing above the present ground surface of the respec- 
tive districts. 
In none of these cases is it believed that the conclusions 
reached regarding the local source of the ore materials are 
either warranted ])y the arguments adduced, the facts as thev 
are presented in the field, or the general geological conditions 
known to be prevalent in ore-producing regions. 
In the case of the Ozark region, we have unusually com- 
plete data regarding the geological times of ra])id erosion and 
the periods of peneplanation. There is abundant evidence 
showing that immediately after the deposition of the Lower 
Carboniferous rocks — the country rock of the southwest Mis- 
souri zinc district — the Ozark region was not the highland it 
now is. Between the time when the last Lower Carboniferous 
limestones were deposited and that when the first Coal T^Teas- 
ures of Missouri were laid down, the strata were tilted and so 
* Trans. American Inst. Min. Bng., vol. xxii, p. 628, ISg*. 
t Minnesota Geol. Sitr.. Bull. 6, p. 153, 1891. 
