and the Geology of the Surrounding Country. 129 



outline, whereas formerly it was nearly in a straight line. 

 The successive descent of large masses of limestone, and the 

 still continued overhanging of the table rock, prove very con- 

 clusively the unremitting action of water and air upon the 

 shale below. 



In the absence of established landmarks, we are compelled 

 to leave the rate of recession unsettled for the present ; and 

 the only mode by which this can be determined accurately is, 

 by a systematic survey and triangulation of the form of the 

 cascade, and the establishment of permanent marks of refer- 

 ence for future observers. 



Leaving out of view the time or rate of recession, we have 

 sufficient data to establish Avith certainty the future changes 

 which will supervene, allowing the recession to go on as it is 

 now doine. The lower half of the rock at the cascade, or 

 about eighty feet, is of soft shale, the limestone above being 

 of equal thickness ; higher still is about forty feet of thin, 

 bedded limestone, forming the rapids. These different rocks 

 are represented in the section as 7, 8 and 8', respectively. 

 Now these beds dip to the south at the rate of about twenty- 

 five feet in the mile, and the declivity of the bed of the river 

 is about fifteen feet in the mile, from the falls to Lewiston. 

 It follows, therefore, that as the falls recede, there will be a 

 less amount of shale above water, owing to the dip, and to 

 this must be added the amount of declivity in the river bed, 

 both together making forty feet. So that when the fall has 

 receded one mile, the surface of the water will stand at k.^ 

 or a point in the shale half way between the present surface 

 of the water and the bottom of the limestone. Going on at 

 this rate for another mile would take away from the fall forty 

 feet more of the shale, so that the surface of the river would 

 stand at p. or the base of the limestone. 



The cataract would then have a solid wall of limestone to 

 wear down, the river beneath protecting, in a great measure, 

 the undermining action upon the shale. During this time, 

 and at the end of the first mile, the falls would have arrived 

 at the present site of the commencement of the rapids, and 

 thus about forty feet more of limestone would be added to 



17 



