88 ILLINOIS ACADEMY OF SCIENCE. 
in the anticlines and synclines. The "terrace", strictly speaking, 
is an interruption in the uniform dip of the sides of a basin, giv- 
ing rise to an approximately horizontal plane. Such benches are 
to be found upon the sides of the great structural basin in south- 
ern and central Illinois. The water in the basin enables the oil 
to rise to the "terrace", where it is trapped by friction. Any oil, 
originally in the sloping sand above the bench, may migrate farth- 
er up the general incline until it is caught by some other bar- 
rier. 
In studying the theories and the actual facts in the oil fields it 
is found that the oil and gas in the main fields occur along the 
crest of the LaSalle anticline. That in Marion County occurs on 
a "terrace" or irregular dome. Enough information is at hand 
to show that wet sands predominate in Illinois and that the oil 
and gas are captive in the highest portions of raised structure, 
according to the theory stated. It has been further shown that 
practically all localities in Illinois, yielding oil and gas to date, lie 
along anticlines, domes, or terraces. Therefore, the general con- 
clusion follows that any area within the State, or out of it, for 
that matter, underlain by suitable formations and these structural 
features, will bear investigation for the presence of oil and gas. 
INVESTIGATIONS OF THE GEOLOGICAL SURVEY. 
The Survey began an investigation of the main fields in 1908, 
with a view of determining certain peculiar conditions existing in 
the oil horizons. The elevations above sea level and logs of 
about 5,000 wells were taken in the southern half of the produc- 
tive area. These records were plotted and contour maps were 
made upon each sand. The maps, about 15 in number, are being 
studied at the present time and an effort is being made to find 
the relation of the quantities of oil, salt-water, porosity of the 
sand, etc., to the structural features of the sand. It is believed 
that these maps will be of large service in the further develop- 
ment of this important field and also that they will yield valuable 
data for the study of the laws governing the genesis and accumu- 
lation of oil and gas. The report will include also the commer- 
cial features peculiar to the Illinois fields. 
The work upon the report of the main fields has been inter- 
rupted to prepare a paper in response to many inquiries received 
by the Survey, relative to the conditions and localities favorable 
for the accumulation of oil and gas outside of the main fields. 
They have been, chiefly, questions regarding structural features, 
