14 AQUEOUS AGENCIES. 



This is well seen in the subjoined figure, representing an ideal cross- 

 section of the gorge below the falls. The dotted lines represent the 

 former bed and level of the river ; a a represent the banks of stratified 

 mud left on the margin of the gorge, as the river eroded its bed down 

 to its present level. 



Other Falls. — The evidence is completed by examination of other 

 great falls. In almost all perpendicular falls we find a similar arrange- 

 ment of strata followed by similar results. The Falls of St. Anthony, 

 in the Mississippi River, are a very beautiful illustration. Here we find 

 a configuration of surface very similar to that in the neighborhood of 

 Niagara. Above the falls the Mississippi River runs on a plateau which 

 terminates abruptly at the mouth of Minnesota River by an escarpment 

 about a hundred feet high. From this escarpment, backward through 

 the upper plateau, runs a gorge with perpendicular sides nearly a hun- 

 dred feet high for eight miles to the foot of the falls. The river above 

 the falls runs on a hard, silurian limestone rock, only a feAV feet in thick- 

 ness. Beneath this is a white sandstone, so soft that it can be easily 

 excavated with the fingers. This sandstone forms the walls of the gorge 

 as far as the escarpment. The recession of the falls by the undermining 

 and falling of the limestone is even more evident than at Niagara. 

 Tributaries running into the Mississippi just below the falls are, of 

 course, precipitated over the margin of the gorge. Here, therefore, the 

 same conditions are repeated, and hence are formed subordinate gorges, 

 headed by perpendicular falls. Such are the falls and gorge of Little 

 River (Minnehaha), which runs into the Mississippi about two miles 

 above the mouth of the Minnesota River. 



Another admirable illustration of the conditions under which per- 

 pendicular falls recede is found in the falls of the numerous tributaries 

 of Columbia River where the great river breaks through the Cascade 

 Range. The Columbia River gorge is 2,500 to 3,000 feet deep. The 

 walls consist of columnar basalt underlaid near the water-level by a 

 softer conglomerate. Every tributary at this point emerges from a 

 deep gorge, headed two or three miles back by a perjoendicular wall, 

 over which is precipitated the water of the tributary as a fall 200 to 

 300 feet high. The falling water erodes the softer conglomerate, un- 

 dermines the vertical-columned basalt, which tumbles into the stream 

 and is carried away ; and thus the fall has worked back in each case 

 about two or three miles to its present position.* All of this has taken 

 place during the present geological epoch, f 



* Gilbert has shown (American Journal, August, 1876) that comparative freedom 

 from detritus is another condition of the formation of perpendicular waterfalls. In 

 muddy rivers commencing inequalities are filled up by sediment, and waterfalls can not 

 be formed. 



f American Journal of Science and Art, 1874, vol. vii, pp. 167, 259. 



