3-10 EXPEDITION TO THE 



to the three superior strata, viz. the slaty limestone with or- 

 ganic remains, the blue limestone destitute of these, and the 

 sandstone with a loose texture. The falls are occasioned by 

 the fissures which occur in the superior limestone, and which 

 allow the water to penetrate through this bed to the sand- 

 stone, which, being of a loose texture, is soon washed 

 away: in this manner thick plates of limestone are left 

 unsupported, and soon fall by their own gravity. This 

 process is constantly causing the fall to recede towards its 

 source. What time has been required, what lapse of cen- 

 turies has been consumed, in bringing the falls to their 

 present situation, it is not in the power of man to decide ; 

 but we may well see that it must have been immense. The 

 difference of level between the head of the fall, and the 

 level of the river at the fort, being, as we have stated, es- 

 timated at about one hundred feet, and the strata running 

 in a horizontal position, we can readily account for the 

 additional strata observed under the sandstone at the fort, 

 and which are concealed at the falls. 



It would remain for us, in order to complete this view 

 of the geology of the falls, to inquire whether the lime- 

 stone, observed at the falls, corresponds with that superior 

 to the sandstone south of the Wisconsan, and if that, found 

 near the level of the river at the fort, be analogous to that 

 observed under the sandstone, between the Wisconsan and 

 Wassemon. We shall not affect a degree of certainty 

 which we do not possess, but we may be permitted to ad- 

 vance an opinion that the sandstone is probably of analo- 

 gous formation, and that, therefore, the strata of limestone, 

 which we found at the falls, correspond with that stratum 

 of whose existence at a former period, between the Was- 

 semon and the Wisconsan, we think we have evident 

 proofs. Wc have in our possession, specimens taken i» 



