VADOSE ORE DEPOSITION 
391 
pod shells and corals are now composed entirely of zinc carbonate; 
and all the delicate surface sculpturings are preserved as sharply 
as when the animals were alive. For a period of 30 years lead- 
ores were extracted extensively from these mines until they were 
though to be exhausted. That they were also the largest zinc- 
carbonate deposits developed on the continent was beyond fancy. 
The zinc-carbonate had replaced the country-rock so completely 
that the latter was always regarded by the miners as the original 
limestone. One day an assayer more inquisitive than his fellows 
undertook to find out just how much metal the country-rock 
might contain. To his great astonishment he found 40 per cent 
zinc; and the camp renewed its life as a zinc-producer. 
In moist countries where metasomatic phenomena !are not 
commonly recognized in the vadose region, there are beds in 
which beautiful septate corals are perfectly preserved in sphal¬ 
erite^®; and Van Horn notes gasterpod shells replaced by 
galena. 
Metasomatic replacement is to be regarded as a wide-spread 
phenomenon in the vadose zone. Two distinctions may be ad¬ 
vantageously made concerning the replacements by metallic min¬ 
erals ; stratal and mural. 
Although it frequently occurs that ore deposits follow definite 
stratigraphic planes and that certain beds are especially liable 
to become cavernous, it is not so common to find a given stratum 
or set of beds entirely replaced by ore-materials. This, however, 
appears to be the case with certain of the zinc carbonates of the 
Magdalena district already mentioned. Beneath a well-known 
local guide-bed called the Silver-Pipe Lime, the zinc ores com¬ 
pletely take the place of a particular limestone layer. 
The Leadville silver-lead deposits so fully described by Em¬ 
mons seem to display similar features; as do many other zinc- 
carbonate deposits recently examined in southwestern United 
States and northern Mexico. In the present connection more 
than mere mention need not be made. 
The alteration of the wall-rocks by vadose metasomatism is in 
many respects different from that of individual strata. A number 
ofl new factors enter into account. It is noteworthy that quite 
40 Proc. Iowa Acad. Sci., Vol. X, p. 103, 1903. 
41 Missouri Geol. Surv., 2iTd series, Vol. Ill, p. 97, 1905. 
42 Monograph XII, U. S. G. S., 1886. 
