58 Sioux City Academy of Science and Letters. 
Over a large area of country, on both sides of the 
Missouri River, through the State of Iowa and farther 
north, the sea bottom was upheaved and became dry 
land at the close of the Cretaceous Period. During the 
immensely long time from the close of the Cretaceous 
Period to the beginning of the Glacial Epoch, while all 
the Tertiary deposits were being laid down, this vast 
surface of land was exposed to erosion, and the plain 
was changed to valleys, hills and ravines; much the 
same in shape as we now see it. Then at the close of the 
Glacial Epoch, which had probably still farther changed 
the shape of the surface by ice action, all this region was 
covered by the Loess, left by the melting glaciers in their 
retreat to the north. Thus we now find in nearly all these 
river bluffs, which are on the surface loess, an old cre- 
taceous hill as a core. Most of the cretaceous deposits 
throughout this region have large quantities of Iron 
Pyrites and Sulphate of Lime or Gypsum all through 
them. The crystals of Gypsum, which are very plentiful, 
and of all the characteristic shapes of that mineral when 
crystallized, are probably what Lewis and Clark called 
cobalt. 
The larger part of the Iron Pyrites, which is so plen- 
tiful in the cretaceous deposits, is in a massive or un- 
crystallized form, and is in irregular lumps from the size 
of a pea up to those of many pounds in weight. When 
these masses are exposed to the air they soon disinte- 
grate and fall into dust. This is caused from the ab- 
sorption of oxygen from the air and the water held in 
suspension by the air. Such chemical action always pro- 
duces heat, and is nothing but slow combustion of the 
matter so acted upon. This heat volatilizes and drives 
out the sulphur contained in the pyrites, and thus disin- 
tegration is produced. By this chemical action sulphuric 
acid is also generated and by its action produces still 
more heat. I had opportunity of observing closely this 
action of disintegration of pyrites on quite a large speci- 
men which a friend of mine here in Sioux City had placed 
on a stone window sill outside of his home. It was ex- 
posed to the action of the air and the moisture contained 
in the air, but not to rain, as it was under a porch and so 
protected. When I first noticed it disintegration had 
