Tlie Blue Mound Quartzyte. — Hubbard. 167 
the lime was removed until the rock became almost entireh' 
silica. The small amount of iron present as carbonate, or 
more likely as sulphide, was oxidized to the h}'droxide and 
remained in the rock. If kaolin were present it would re- 
main. It therefore follows that the kaolin did not occur in 
the Niagara limestone, here originally, but that aside from 
silica the rock was nearly a pure calcium and magnesium car- 
bonate. But the long continued action of the water has re- 
duced the lime to a minimum and has left almost nothing 
but silica with iron stains. This was accomplished long be- 
fore erosion had so nearly completed its work as at present. 
This process is no new thing. It is recognized in man)' 
places. One of much consequence is the region containing 
the extensive development of the Clinton iron ore. This 
deposit is derived from a ferruginous limestone by the gradu- 
al removal of the lime and the accumulation of the iron to a 
point where the iron oxide constitutes ninety-five percent, of 
the rock. Another very common one is accumulation of re- 
sidual clays b}^ the removal in solution in lime and mag- 
nesia from argillaceous limestones, so marked a feature in the 
Jura-trias regions of New Jersey, the Silurian areas in Alabama 
andGeorgia, and theTertiary at Richmond, Va. Another ver}- 
similar alteration is that occurring in the Devonian lime- 
stone at the falls of the Ohio at Louisville, Ky. Here im- 
mense numbers of corals occur but all are siliceous now and 
the limestone is very largely silica also. A result of sub- 
stitution largely in this case. 
To sum up and apply the illustrations and explanations 
given, to the case in hand, we will say that the quartzyte on 
Blue mound, Wisconsin, is a stratum of the Niagara lime- 
stone and is in no way the result of volcanic or eruptive 
agencies; that it was laid down in the Silurian sea at a time 
and place where silica was comparatively abundant, giving 
rise to a siliceous limestone. The silica probably occurred 
in the form of tests of various plant and animal organisms, 
and was incorporated as such into the limestone. Subse- 
quent to the elevation of the limestone to land altitudes the 
percolating earth water, with the proper well known solv- 
ents took the silica into solution and then re-deposited it as 
chert and flint while the lime was bcin<^ removed. This 
