14 Indiana Natural Gas Field. — Leverett. 
vonian limestones set in, than it is near the east line 
of the state. How much of this difference is due to original 
inequality and how much to erosion has not been determined. 
Variation in thickness of 'Hower shales-''^ — The "lower 
shales" become gradually thicker from west to east both in In- 
diana and Ohio. By reference to the accompanying tables it 
will be seen that along the western portion of the district 
which they cover, the thickness in Cass and Carroll counties 
falls below 400 feet, and in Clinton, Boone, Hendricks and 
Morgan counties their usual thickness lies between 400 and 
500 feet. Passing eastward from these counties we find a 
gradual increase, the thickness in the eastern range of coun- 
ties, (south of the Wabash river,) being from 700 to 900 feet. 
Continuing eastward into Ohio, a similar increase is noted, the 
thickness in Madison and Union counties being over 1100 feet, 
as is shown by Prof. Orton's report. (Geology of Ohio, Vol. 
vi). ' A belt taken in the same manner across the north- 
ern portions of Indiana and Ohio shows nearly as great 
an increase in the thickness of these shales, but it lies 
mainly in Ohio, there being at Goshen, Indiana, 560 feet 
and at Bryan, Ohio, but 650 feet. In Ottawa, Seneca, 
Sandusky, Marion and Wyandotte counties, however, the 
thickness averages more than 1,000 feet. This thickening of 
the shales toward the east was not known previous to the gas 
explorations and consequently a mistaken idea has prevailed 
as to the trend of the main axis of the Cincinnati anticlinal, 
north from the Ohio river. Its main axis was supposed to pass 
east of north past the western end of lake Erie into Canada, 
through this belt of thick shale ; but it is now known that the 
main axis passes northwest into Indiana, as we shall presently 
show. There is, however, a minor axis in western Ohio along 
the line formerly considered to be that of the main axis, the 
value of which is shown in the table of altitudes of the Tren- 
ton limestone. 
Cincinnati anticlinal. — The column of the table devoted to 
the altitude of the Trenton brings out the main features and 
also some of the minor phases of the topography of this inter- 
esting formation. 
' The table which pertains to the Ohio district was compiled from 
data given in the volume referred to, and is inserted for purposes of 
comparison. 
