GEOLOGY. 239 



Prairie du Chien, the blue limestone rises at an average rate 

 of seventeen and a half feet per mile. The dip, however, is 

 subject to undulations ; for instance, at Dubuque, the blue 

 limestone does not show itself above low-water mark ; at 

 Eagle point, a mile and a half up the Missisippi, it rises ten 

 feet above low water ; at the mouth of the Little Makoqueta, 

 four miles farther up, its height above low-water mark is forty 

 feet ; at the mouth of Turkey river, twenty miles farther up, 

 it disappears again beneath the waters of the Missisippi ; a 

 few miles beyond this point, it emerges again to the surface ; 

 and, finally, at Prairie du Chien, twenty miles above Turkey 

 river, its upper surface has already attained an elevation of 

 more than four hundred feet above the level of the Missisippi. 

 The line of the greatest general dip is about south, ten to 

 twenty degrees west. 



" The importance of these observations on the dip of the 

 rocks, forming as they do the materials to calculate the thick- 

 ness of each stratum at any given spot, is very great. Indeed, 

 such observations are indispensable, before an accurate esti- 

 mate can be formed of the value and extent of a mineral 

 tract. They indicate, with much fidelity, the depth to which, 

 at different points, a productive vein of ore is likely to extend." 



" I have preferred and adopted the name of clifi" limestone 

 to designate this rock (though a popular rather than a scien- 

 tific term), because it aptly expresses its most striking external 

 characteristic, which imparts to the scenery of any country 

 in which the rock abounds a bold and romantic character. I 

 allude to its disposition to cleave vertically, and form per- 

 pendicular cliff's. 



" These mural escarpments, exhibiting every variety of 

 form, give to the otherwise monotonous character of the land- 

 scape in Iowa a varied and picturesque appearance. Some- 

 times they may be seen in the distance, rising from out the 



