AQUEOUS AGENCIES. 21 



200 years ago,, the falls have steadily worked their way 

 back toward Lake Erie. The rate of recession has been 

 estimated at one to three feet per annum. The cause 

 is easily perceived. The strata at the falls consist of 



E.P. 



FIG. 5. Section of Niagara Falls and gorge. O.P., Ontario plateau ; E.P., Erie 

 plateau ; L.O., Lake Ontario ; /, fall ; mb, stratified mud-banks. 



solid limestone, represented in the figure by the jointed 

 structure, underlaid by softer shale. The force of the 

 dashing water cuts away the soft shale and undermines 

 the limestone, causing it to project as overhanging rocks, 

 which fall from time to time into the abyss below. Thus 

 the falls work backward, but remain perpendicular. 



Gorge formed by Recession. There can be no doubt 

 that the whole gorge has been formed in this way ; that 

 the river once fell over the cliff which runs across its 

 course near Lake Ontario, and then worked its way back 

 to its present position ; and the work is still going on. 

 The general configuration of the country suggests this 

 origin even to the casual observer, and close examination 

 entirely confirms it. It is a familiar fact that stratified 

 mud-banks are found in spots along the margins of all 

 rivers, evidently formed by deposits from the river. These 

 stratified muds often contain the shells of the mussels 

 which inhabit the river. Now, in several spots (mV) 

 along the top of the gorge-cliff, from the falls to Lake 

 Ontario, are found such stratified mud-deposits contain- 

 ing shells. The deposits were evidently made when the 

 river ran at that level. 



