46 
IOWA ACADEMY OF SCIENCES. 
of from 110 to 140 feet. Nowhere in this immediate neighbor- 
hood is the drift known to have a greater depth than this. At 
Ddwson a coal shaft shows the superficial deposits to be eighty 
feet thick at that point. At Angus, a few miles northeast of 
Dawson, there is a thickness of fifty to 100 feet and in southern 
Greene county borings show between sixty and seventy feet of 
drift. There seems to be considerable evidence, therefore, that 
the gravel is at the base of glacial deposits and that it rests 
directly on the coal measure shales. In this case it would be 
possible that the gas, originating in these black carbonaceous 
shales, may have passed up into and accumulated in the gravel 
and sand beds above. 
But it seems much more probable that the gas at Herndon 
and Dawson has its source in the vegetable accumulations of 
the drift, as is undoubtedly true for the gas at Letts. 
It is not necessary to suppose that it has been formed directly 
in the place where it is now found. It may have originated 
from the decomposition of vegetable material some considerable 
distance off and later have diffused itself laterally through the 
gravel beds until reaching a place favorable for its accumulation. 
There is another interesting fact concerning the distribution 
of these gas wells. They are found not far from the border of 
the upper drift sheet of the region. Thus, for example, at 
Dawson and Herndon the wells are only a few miles back from 
the edge of the Wisconsin lobe and at Letts the Illinois ice 
seems to have extended but a short distance in the west. Orton 
mentions the same fact concerning the distribution of wells in 
Ohio, where as already stated, they are found along the border 
of the glacial deposits or back twenty to forty miles. 
The most favorable conditions for the preservation of forest 
beds and like accumulations of vegetable material would seem 
to be near the edge of the ice, where it was the thinnest, and 
where, during its advance, there would have been less disturb- 
ance of the materials beneath. During its advance only a com- 
paratively few miles of the ice sheet would pass over the drift 
near its border, while back 50 or 75 miles the ice would doubt- 
less be considerably thicker and a vastly greater amount of ice 
would pass over the surface, and as a result the underlying 
deposits would be more disturbed. The forest bed, if present, 
might be carried away or mingled with the clay of the drift. 
Concerning the origin of natural gas little need be said. It 
is now generally admitted by all geologists and most chemists 
