GEOLOGY. 239 



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



of seventeen and a lialf feet per mile. The dip, liowever, 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 Missisip|)i ; 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. '^fjfjl 



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, [ndeed, 

 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 whicli, 

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



*' I have preferred and adopted the name of cliff 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 cliffs. 



" These mural escarpments, exliibiting every variety of 

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

 scape in Iowa a varied and ])icturcs(jue appearance. Some- 

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



