BUEEAU COUNTY. 181 



lower thin vein at Tiskilwa probably is the same as the lower La Salle 

 vein, being Xo. 2 of the section; and this is the only place, so far as I 

 know, where a third vein has been discovered in the county. The seam 

 worked at Sheffield ; the lower seam at or near Buda 5 the middle seam 

 at Tiskilwa, and the lower or worked seam at Biermau's shaft, are all 

 identical with each other, I think, and are identical with the upper 

 Peru and La Salle coals. This seam, according to the new section for 

 Northern Illinois, would belong to coal Xo. 6. The upper seam at Tis- 

 kilwa, at Bierman's shaft, and at the shaft of Eobinson, Dinks & Co., 

 is probably the equivalent of coal Xo. 7 of the new section of the Illi- 

 nois coals. These seams are assigned to the horizons of the La Salle 

 coal seams, not on paloeontological evidences; characteristic fossils 

 seem to be scarce at all the localities examined ; the coal seams them- 

 selves, however, and their associate rocks and shales, seem to justify 

 such classification. The position of the Sheffield coal, near the surface 

 of the ground, and no seam being found above it, would seem to iden- 

 tify it with the upper instead of the middle seam ; but its place without 

 doubt, I believe, is with the coals of Xo. 6 in the section referred to. 



The general level of these seams varies greatly. The railroad track 

 at Sheffield is eighty-eight feet above, and at Tiskilwa it is sixty-six 

 feet below the level of the surface of Lake Michigan. Estimating from 

 the position of the coal seams, as compared with the railroad track at 

 these places, there must be a diflerence in the level of the coal of from 

 seventy-five to one hundred feet, in a lineal distance of some twenty 

 miles, showing a dip of about five feet to the mile to the south-east- 

 ward. 



SILURIAN FORMATIONS. There are no exposures or outcrops of the 

 Silurian rocks in this county ; but the northern one-third of it is under- 

 laid by these rocks in about the following order: 



The Trenton or Blue Limestone. These rocks outcrop rather heavily 

 at Homer, about two miles east of the Bureau county line, in La Salle 

 county. They doubtless continue the underlying rock west, or a little 

 south of west, along the north line of the coal field, until they sink be- 

 neath the Coal Measures opposite to Princeton, and extend northward 

 nearly to the C. B. & Q. B. E. 



The Galena Limestone. This limestone outcrops at Lee Center and 

 near Sublette, in Lee county, and is probably the underlaying rock in 

 that part of Bureau county about and on both sides of Bureau creek, 

 and between that creek and Green river, and north of the Coal Mea- 

 sures, with the exception of some elevated ground about " Dad Joe's 

 Grove." 



The Cincinnati Group. These shales would doubtless be found under- 

 laying the grove just named, and may also underlay small patches in 

 the north-west corner of the county, west of the Green river swamps. 



