IOWA ACADEMY OF SCIENCES. 
45 
Much, more frequently marsh gas is struck in the vegetable 
deposits (of Ohio) and sometimes escapes in large volume and 
with great force when first released. It not infrequently gives 
rise to a small but persistent supply. Gas wells are of com- 
mon occurrence in all the border areas above mentioned. 
In Illinois natural gas in the drift has been found in com- 
mercially valuable quantities at Bloomington, Kankakee, Men- 
dota, and other points. 
The reports of the Irdiana survey also contain accounts of 
the discovery of gas in the superficial deposits of that state. 
From what has been said above it will be seen that it is by 
no means an uncommon thing to find gas in the Pleistocene 
deposits. It has been discovered at a number of different 
points in at least four states and doubtless there are unrecorded 
occurrences in other parts of the country. 
We are now prepared to consider the question as to the 
source of this natural gas and later its origin. 
There are two possible sources of the gas found in the drift. 
(1) It may have been derived from the underlying rock and the 
drift then serve simply as a reservoir for its accumulation and 
storage, or (2) it may have been derived from the vegetable 
accumulations of the drift and thus have its source in the 
Pleistocene deposits where it is now found. The latter source 
is doubtless much the more common and in most instances 
there is little doubt that the gas has been derived from the 
decomposition of the vegetable remains in the drift. But 
examples of the drift serving as a reservoir only, are occasion- 
ally found. Thus, Orton mentions several such instances in 
Ohio and it is possible, though hardly probable, that at Hern- 
don and Dawson the gas has been derived from the underlying 
coal measures shales. 
That it may have such a source the gas-bearing rocks must 
be overlaid by porous beds of drift. Then during the long 
periods since they have had this relation the porous beds have 
become charged with gas when there were suitable conditions 
of level. As we have already seen the arrangement of the beds 
at Herndon and Dawson are such that it is possible that the gas 
might be derived from the rocks underlying the drift sheet. At 
both of these localities the gas occurs in a stratum of sand and 
gravel at the base of the drift and apparently directly overlying 
the coal measures. As already stated the wells have a depth 
