16 Indiana Natural Qas Field. — Lever ett. 
rock out-crops and deep borings to the north and south indi- 
cate that such an axis exists. At Monon the Niagara limestone 
out-crops at an altitude 675 feet A. T. If we assume that the 
Niagara has here its known maximum thickness, 4D0 feet, and 
that the lower shales have a thickness of 350 feet, — which is a 
liberal allowance since they are but 318 feet in one of the bor- 
ings at Delphi, and since borings in Illinois, cited on a subse- 
quent page, as well as those given in the tables, show that the 
shales decrease in thickness toward the west, — the Trenton 
should be struck at Monon at about 75 feet below tide. 
Three miles east of Kentland is an out-crop of limestone 
which is thought by S. S. Gorby, Esq., of the Indiana Geologi- 
cal Survey, ' to be Niagara, but which may possibly be a Lower 
Silurian limestone. If we consider it Niagara and give it the 
known maximum thickness of this formation, 400 feet, and as- 
sume that the lower shales are 300 feet in thickness, the Tren- 
ton would be about 25 feet above tide at this point. 
From Kentland the axis probably bears northwest. We find 
evidence of an anticlinal north of the Kankakee river in the 
borings made at Chicago, Joliet and Kankakee. In the well at 
the Union stock-yards in Chicago the Trenton is struck at 93 
feet A. T. At the state penitentiary near Joliet the bottom of 
the lower shales is reached at 175 feet A. T. Here a rock de- 
scribed as a "sharp sand rock" is struck. We think it probable 
that this is Trenton limestone, for we have frequently tested 
rock drillings pronounced by the well drillers sandstone and 
found them to be granular limestone. A boring near Kankakee 
reaches the bottom of the lower shales and enters what is prob- 
ably Trenton at 17 feet below tide. There are indications that the 
crest of the anticlinal is south of Joliet. One and a half miles 
west of Manteno a prominent ridge of Niagara is exposed at 
an altitude of about 750 feet, A. T. The Niagara at Joliet is 
355 feet, and at the Kankakee boring is 388 feet in thickness. 
The lower shales at Joliet are 110 feet, and at the Kankakee 
boring 213 feet in thickness. Manteno is about 20 miles south 
of the Joliet and 15 miles north of the Kankakee boring. If 
we assume a uniform increase in the thickness of the limestone 
and shales toward the south from Joliet the former would be 
375 feet and the latter about 170 feet, and the combined thick- 
ness 545 feet. Deducting this from the altitude of the rock 
' See Geological Report of Indiana. 1885-6. pp. 236-7. 
