TH’S DECOMPOSITION OF DOLOMITE. 
BY NICHOLAS KNIGHT. 
The dolomites of Iowa and other sections of the country, belonging to the 
Niagara period of the Upper Silurian, in the course of centuries apparently under- 
go decomposition. The rock loses its massive structure and becomes crumbly 
and mealy in appearance. In the vicinity of Mount Vernon, Iowa, the top layer 
to a depth of 3 or 4 inches seems to be quite thoroughly disintegrated and 
powdery. The same phenomenon is observed in lower strata where a layer pro- 
jects, by which it is especially exposed to the elements. A similar disintegration 
is observed in many other localities. 
It seemed reasonable to suppose that some constituent or constituents of the 
rock are more soluble than the others, or more easily acted upon by atmo- 
spheric agencies. The analysis of a more or less disintegrated air-dried specimen 
resulted as follows: 
Si02 
PeoOs and AI2O3 
CaCOs 
MgCOa 
0.56 per cent 
0.74 per cent 
48.43 per cent 
50.56 per cent 
Total 
100.09 per cent 
The analysis of a typical massive specimen gave: 
Si02 0.83 per cent 
Pe203 0.59 per cent 
CaC03 53.62 per cent 
MgC03 44.96 per cent 
Total 
100.00 per cent 
It is thus seen that about 10 per cent of the calcium carbonate has been re- 
moved, and the magnesium carbonate is relatively stable. Other portions of the 
rock would doubtless show greater disintegration. 
Immediately above the disintegrated layer referred to are found portions of 
what appears like a ferruginous clay. In all likelihood it formerly existed as a 
distinct layer, but w^as largely removed by the ice sheet. It has accumulated in 
various cavities and depressions in the rock. This formation is quite widely ex- 
tended. In Northeast Iowa it is several feet in thickness and in Southwestern 
Wisconsin, on broad uplands, where it has been but little disturbed by the ice- 
sheet, it has a thickness of 13 feet. ' 
An analysis of the ferruginous clay resulted as follows: 
Si02 16.68 per cent 
Pe203 60.87 per cent 
AI2O3 6.18 per cent 
CaO 0.22 per cent 
MgO 0.03 per cent 
H2O 17.14 per cent 
Total 101.12 per cent 
(107) 
