Sioux City Academy of Science and Letters. 59 
proceeded so far that most of the mass had fallen to 
powder, only a small piece in the center of the original 
specimen being still solid. On clearing away the dust, 
I found that the sulphuric acid had eaten into the stone 
window sill more than an eighth of an inch over the 
whole surface where the pyrites had lain. I have also 
often seen this process of disintegration of pyrites going 
on in specimens in my own cabinet, where they were 
entirely protected from exposure to outside air. 
A few years since, while collecting geological speci- 
mens near Sioux City, I was at work on the side of a 
bluff which had been cut down to make room for a road 
between it and the Sioux River. The exposure faced the 
west and the day was warm and sunshiny. While rest- 
ing under the shade of a tree on the top of the bluff at 
noontime I noticed a light mist or smoke rising from the 
ground or river which ran far below me. Thinking it 
came from the water I paid no further attention to it at 
the time. But when I again went down to the side of 
the hill below me I could not see the mist or smoke, but 
noticed quite a strong odor of burning sulphur. I at 
once thought of the smoking hills I had heard and read 
of before, and believed that the same phenomena was 
being manifested here before me. The ground was very 
warm as far down as I could dig with the tools I had. 
During the same summer I was collecting near Jack- 
son, Neb., and was down in a deep ravine of erosion 
where a small stream had cut through the Loess and far 
down into the cretaceous. While searching for speci- 
mens I found a place near the top of the cretaceous that 
had the appearance of a chimney or vent from some 
source of heat somewhere down in the earth, at and 
near the top the clay had been burnt to a bright brick- 
red color, while farther down, five or six feet below the 
surface, where it was as strongly altered by heat, it was 
a deep black. I took a number of pieces from this place 
and they were all very porous and light, strongly resem- 
bling pumice. 
In the region about Sioux City the cretaceous de- 
posits generally consist. of dense blue-black clay in dis- 
tinet layers, in the lower beds. As one ascends toward 
