236 APPENDIX. 



" Of these groups, the mountain limestone particularly 

 claims our attention, as almost all the rocks of Iowa and 

 Wisconsin are referrible to that subdivision. 



" In the western States, above-mentioned, these subdivi- 

 sions generally vary in thickness from one hundred to one 

 thousand feet, with the exception of the cliff limestone, 

 which, in some districts, is hardly distinguishable, and, in 

 general, does not exceed one hundred feet in thickness. 



" Now, this cliff limestone, so sparmgly developed else- 

 where, swells, in the Wisconsin lead region, into the most 

 remarkable, most important, and most bulky member of the 

 group. It becomes, as it were, the Aaron's rod, swallowing 

 up all the rest. It attains to a thickness of upward of five 

 hundred and fifty feet, while the underlying blue limestone, 

 which, in Ohio, has usually from eight hundred to one thou- 

 sand feet of thickness, shrinks, in many places, to less than 

 one hundred feet, and, in others, seems wholly wanting ; 

 while, at the same time, the black slate, commonly found 

 above the cliff limestone, seems also deficient ; and it is 

 doubtful whether the fine-grained limestone, or the oolitic 

 limestone, or the conglomerate, can be detected at all through- 

 out the entire tract of country which has been subjected to 

 exploration. 



" In a word, in the region now under consideration, the 

 cliff limestone, with a variable and usually thin substratum 

 of blue limestone, seems to engross the entire mountain lime- 

 stone group ; and the coal-measures, where found (namely, 

 in the extreme southern boundary of the tract), occur in 

 immediate contact with it, instead of being separated, as usual 

 m Ohio and the neighboring States, by three distinct mem- 

 bers, occupying about one thousand feet in thickness. 



" This enormous development of one of the members of the 

 mountain limestone group, and the almost complete oblitera 



