114 GEOLOGY OF ILLINOIS. 





 nearly across the county, preserving the general course of the stream. 



The broad blue band represents the part of the county along the stream 

 underlaid by the Blue limestone. All the small streams falling into 

 Eock river from both sides, so far as I examined them, present the fol- 

 lowing succession of the rocks. At their mouths, especially from three 

 miles above Oregon to Grand De Tour, the St. Peter's sandstone comes 

 to the surface ; a short distance up stream the Buff limestone outcrops 

 along the banks and on the sides of the ravines ; farther up, the lime- 

 stone under consideration is met and continues to outcrop for two, three 

 01 four miles ; then the Galena rises like a rocky wall along the waters' 

 edge, and continues the surface rock until the headwaters of the streams 

 are reached. Some of the hill sides show all three of these resting con- 

 formably upon each other, as in the ravines about Oregon, and along 

 the lower part of Pine Creek. Kite river and the next stream below it 

 south of Rock river, Leaf river, Pine creek, and almost any of the small 

 brooks, present the same succession of the rocks. 



On Pine creek, from a mile below r the crossing of the highway leading 

 directly east of Polo, to about Sharp's mill, the upper thin-bedded layers 

 of the limestone Tinder consideration outcrops in rocky faced abrupt 

 bluffs, reaching a thickness of forty or fifty feet. The heavier blue 

 layers of the Polo beds w r ere not here observed. They resemble the 

 outcrops of the same rocks above Dixon, except that fossils are rare, 

 and the rocks have a dry, baked appearance. At Sharp's mill, the St. 

 Peter's sandstone and the Buff limestone begin to outcrop along the 

 base of the hills. Above Byron the river hills are capped with the 

 Blue, changing into the Buff toward their bases. 



The Blue limestone at Dixon and many other places is full of fossils. 

 Slabs of thin stone are there found covered so thickly with fragments 

 of small trilobites, corals, stems of encrinites, and mollusca of various 

 genera and species, that one cannot help wondering at the great abund- 

 ance of the lower forms of animal life, which sw r armed in the ocean of 

 the lower Silurian era. These thin fossiliferous strata are compact, and 

 solid, and when dressed and polished look like a beautiful variegated 

 marble. Dr. EYEKETT, of Dixon, has in his cabinet specimens of this 

 polished marble, which will compare in beauty with any marble we ever 

 saw. In Ogle county, however, we could nowhere find in the Blue lime- 

 stone the same abundance of fossils. At Polo, a large chambered shell 

 known there as an Ammonite, but probably the Lituites undatus of Hall, 

 is occasionally found; also an Orthoceras, which sometimes reached the 

 great size of nine inches in diameter and eight or ten feet in length. 

 But the thin fossiliferous layers, such as are found at Dixon, were not 

 found. A heavier working of the outcrops along Pine creek, might dis- 

 close them. A barrenness of good fossils seems to characterize all the 

 formations in Ogle county. 



