222 GEOLOGY OF ILLINOIS. 



middle division of the Cincinnati group, in the south part of the county, ap- 

 pears well fitted for potter's use, but I can not learn that any attempts have been 

 made to utilize it. The results of experiments made by the " Mound Compa- 

 ny," with the various beds of the neighborhood, are well summarized in the 

 following letter from a son of one of the proprietors, for which I am indebted 

 to our mutual friend, Mr. H. M. Bannister, of the Survey : 



PORTLAND, Oct. 6th, 1868. 

 H. M. BANNISTER, Assist Geologist of Illinois. 



DEAR SIR: As regards the Joliet Mound, situated one and a-half miles southwest of the city 

 of Joliet : It is about one-fourth of a mile in length, and two to three hundred feet in width. 

 At its northeast extremity is solid limestone rock, overlaid with a thin stratum of blue clay, 

 above which is about twenty feet of fine gravel, containing a large per centage of cement, and 

 many boulders of various sizes and species. The rock dips toward the southwest, and when it 

 reaches the gravel pit, at or near its extreme end, the gravel bed is forty or fifty feet in thick- 

 ness in the center, while beneath it is a bed of fine, blue, earthern clay, six feet in thickness, 

 and remarkably free from stones and other impurities, though strongly impregnated with salts 

 and lime, and so solid as to require a sharp pick to excavate it. The top of the bed is strati- 

 fied and colored with oxide of iron, producing a fine slip or glaze for pottery ware. The lower 

 portion of the bed is solid, and rather an impure clay. The bed dips with the rock, and in- 

 creases in thickness in the same proportion as the gravel. 



Many Indian remains have been exhumed while excavating the gravel, and an old flint-lock 

 pistol was found ten feet in the gravel, while excavating the clay. I have seen toads jump out 

 of the solid bank and hop off. 



Under this bed of clay are boulders, gravel and clay, and under that a stronger brown clay, 

 beneath which are strong evidences of the same formation as that above it, and then rock. 



One-half mile further to the southwest, is Mount Flat-head, one mile in length, one-quarter 

 in width, and about sixty feet in hight, composed of boulders and gravel, with very little ce- 

 ment and no clay under it. The rock in this mound dips in directly the opposite direction from 

 that in Mount Joliet, 



The clay used in the manufacture of tile is from a ridge one-quarter of a mile northwest of 

 the mound, and forming one of the boundaries of the DesPlaines valley. It is a red, earthern 

 clay, formed in cubes, strongly impregnated with iron, and a little lime and some fine gravel 

 mixed with it. (I found the same bed at White Lake, Michigan.) The bed is ten or fifteen 

 feet thick, and is a good, strong, earthern clay, and can be used as a high-fired slip clay, or a 

 lower fire if a flux be mixed with it. Its formation is very irregular, as is all that region. 

 Under it are fine, yellow and blue loams, and under them gravel and boulders and then the rock. 

 Not one hundred feet from this bed is one of brown clay, of great depth and filled with lime 

 pebbles. The internal arrangement of the whole ridge is similar to rolling prairie, and of 

 every species of drift. Two miles below the mound, in a railroad cut, you will find a bed of 

 hard, stratified or shaly clay, brown, red and green, with which we experimented largely, but 

 it was so full of lime and lime-dogs as to be of very little value, although it stands a heavy 

 fire to a certain point, and then suddenly gives way, and in burning checks badly by fire and 

 air. 



All down the DesPlaines valley, on either side, are extensive beds of the same material. At 

 Channahon, on Mr. Althower's place, in his low land, is a bed of fine, greasy, blue clay, which 

 is very good for a glazing clay, and not far from it is a bed of white marl. On the Rock Island 

 railroad, near Mokena, is a bed of green clay, and you will find pockets of it in the rock at 

 Lockport. 



I know very little of the Goose lake clay, save that they have had great trouble with it. At 



