EFFECTS OF WATER. FALLS OF NIAGARA. 127 



9. Erosion. Something analogous happens when waters, which 

 cashing the foot of a mountain, meet there with substances that 

 they can easily soften or disaggregate. These substances being 

 destroyed, the upper parts of the soil are soon undermined, and 

 more or less considerable falls occur. This takes place on sea- 

 coasts, on the shores of Jakes or rivers where more or less elevated 

 escarpments are formed, and more and more degraded. The 

 same thing happens sometimes at the foot of cascades which fall 

 over rocky peaks (fig. 210), forming alternately calcareous and 

 argil la'ceous deposits ; the latter are disaggregated, and borne away 

 little by little by the waters which exude on the parietes or jet 

 forth after the fall, and other layers being undermined must fall 

 sooner or later from their own weight. In this case the cascade 

 cuts deep into the soil, and the same being successively repeated, 

 necessarily forms a gorge or bed the whole length of the rivulet, 

 which deepens more and more. It is in this way that the falls of 

 Niagara, by which the waters of lake Erie are precipitated into 

 those of lake Ontario, have sensibly receded since the discovery 

 by Europeans, and probably have excavated the deep bed through 

 which they afterwards escape. 



" The waters, after cutting through strata of limestone, about fifty feet 

 thick in the rapids, descend perpendicularly at the falls (of Niagara) over 

 another mass of limestone about ninety feet thick, beneath which lie soft 

 shales of equal thickness, continually undermined by the action of the spray, 

 driven violently by gusts of wind against the base of the precipice. In 

 consequence of this disintegration, portions of the incumbent rock are left 

 unsupported, and tumble down from time to time, so that the cataract ia 

 made to recede southwards. The sudden descent of huge rocky fragments 

 of the undermined limestone at the Horse-Shoe Fall, in 1828, and another 

 at the American Fall, in 1818, are said to have shaken the adjacent country 

 like an earthquake. According to the statement of our guide in 1841, 

 Samuel Hooker, an indentation of about forty feet has been produced in the 

 middle ledge of limestone at the lesser fall, since the year 1815, so that it 

 has begun to assume the shape of a crescent, while within the same period 

 the Horse-shoe Fall has been altered so as less to deserve its name. Goat- 

 Island has lost several acres in area in the last four years (prior to 1841) ; 

 and I have no doubt that this waste neither is, nor has been, a mere temporary 

 accident, since I found that the same recession was in progress in various 

 other waterfalls which I visited with Mr. Hall, in the state of New York. 

 Som^ of these intersect the same rocks as the Niagara for example the 

 Genesee at Rochester ; others are cutting their way through newer forma- 

 tions Allan's creek, below Le Roy, or the Genesee at its upper falls at 

 Portage. Mr. Bakewell calculated that, in the forty years preceding 1830, 

 the Niagara had been going back at the rate of about a yard annually ; but 

 I conceive that one foot per year would be a much more probable conjecture, 

 in which case 35,000 years would have been required for the retreat of the 

 falls from the escarpment of Quecnston to their present site, if we could 

 assume that the retrograde movement had been uniform throughout. This, 

 however, could not have been the case, as at every step in the process of 

 excavation, the height of the precipice, the hardness of the materials at its 



9. What is meant by erosion ? What are the effectg of erosion ? 

 32 



