34 GEOLOGY OF ILLINOIS. 



Terrapin Ridge, about two miles south of Elizabeth, is the northern 

 projection of another high table land of exactly similar character, ex- 

 tending south and a little west, between Apple river and Rush creek, 

 nearly or quite to the Carroll county line ; but this table land does not 

 approach close to either of these streams. 



About two and one-half townships in the south-east corner of the 

 county, are underlaid by this rock. This field extends from the cast 

 and south county lines to the Rush creek valley, on tbe west, and nearly 

 to Morseville on the north. Plum river valley and Dutch Hollow, in 

 this field, cut down to the galena, in places, and show the gentle talus- 

 covered slopes and outcrops of the Cincinnati group at many places 

 along their sides. 



The probable extent of the county covered by this formation, is a 

 little less than one-third. There are many places throughout this ex- 

 tent where the eroding streams have cut down through the Niagara, into 

 the Cincinnati shales, and even reached the Galena limestone below 

 both. 



Such is the superficial area covered by this rock, stated approximately. 

 Its lithological character has been so often written that it seems su- 

 purfluous to speak of it here. The rock is generally massive, irregularly 

 bedded; tough; of a yellowish color on fresh fracture, but weathering 

 to a reddish-brown. It is full of chert bands ; and some of the Niagara 

 hills are macadamized with a thick floor of finely broken, dendrite- 

 speckled flints, which remain from the decay of the strata formerly en- 

 closing them. These flint hills, or flint covered hills, are characteristic 

 of the Niagara limestone formation. The maximum thickness of the 

 Niagara limestone in this county, cannot be accurately stated. The 

 denudation which has taken place on its top, and the difficulty of 

 ascertaining the bottom, make it almost impossible to measure its thick- 

 ness correctly. Its heaviest outcrop is probably along Small-pox creek, 

 where it reaches a thickness of over two hundred feet. As developed 

 in this county, it is exceedingly homogenous in character the varieties 

 observed at Racine, Le Clare and Cordova, being wanting. In chemical 

 analysis, lithological character, and general appearance, it is very simi- 

 lar to the Galena limestone. If a difference can be detected, it is less 

 sandy and crystaline, and tougher than the latter formation. Its type 

 or characteristic fossils are also different. 



These are chiefly Fentamenis olrtonyux ; Favoxites favosa ; Halynitw 

 catenularia ; Astrocerium venmtion ; and one or two species of 8tro)n<tto- 

 pora formed corals. The Pentamerw are the traditional " petrified 

 hickory nuts," so often spoken of by the miners and well diggers. Huge 

 blocks of the stone, in places, are sticking full of them. On the silex 

 sown hills, bushels of rough weather-stained specimens of the Favoaites 



