246 PRINCIPLES OF STRATIGRAPHY 



the current very slight. Then a sudden change in the slope of the 

 river bed, causing a descent of 55 feet in less than half a mile, trans- 

 forms the river into a rushing torrent — the rapids above the falls- 

 culminating in a drop of 160 feet at the cataracts, over which 22,- 

 400,000 cubic feet of water fall per minute. Below the falls strong 

 currents and eddies continue for a while, due to the disturbance of 

 the water at the falls. Then, however, the river becomes relatively 

 calm again with moderate current, easily navigable for about two 

 miles. The channel is from 1,200 to 1,300 feet wide at the top, and 

 the water from 160 to 190 feet deep. At Suspension Bridge the 

 channel suddenly contracts to 700 or 750 feet at the top, while the 

 water is not over 35 feet deep. In this narrow and shallow gorge 

 the whirlpool rapids are situated, the great volume of water rushing 

 through it with indescribable force, far exceeding that of the upper 

 rapids. The descent at the same time is slightly more than 50 feet 

 in a distance of about a mile, or about half the descent of the 

 upper rapids. The curious eddy of the whirlpool is entirely due to 

 the conformation of the channel, wuich here bends at right angles. 

 A second narrowing at Fosters Flats again produces rapids, but be- 

 low this the river becomes relatively quiet and placid again, and 

 navigable for seven miles of its lower course. 



Erosive Pozver of Rivers. The erosive power of a river, i. e., 

 the ability to overcome cohesion, varies as the square of its velocity. 

 Pure water does little or no actual erosion, this being accomplished 

 by the transported material. The rock fragments carried by the 

 stream corrode its bed, while at the same time they abrade each 

 other. In the part of a stream bed unsupplied by new material 

 from tributaries it is noticeable that there is a progressive diminu- 

 tion in size of the fragments downstream, the reduction being pro- 

 portional to the weight of the rock in water and the distance trav- 

 eled. The following measurements made on the river Mur show 

 the progressive reduction in average size of the fragments in ac- 

 cordance with distance (Hochenburger-33 :5j, quoted by Penck- 



At Graz . 224 cb. cm. 



At Gossendorf ( 10 km. below Graz) 184 cb. cm. 



At Wildon (26 km. below Graz) 132 cb. cm. 



At Landscha (43 km. below Graz) . .' 1 17 cb. cm. 



At Unterschwarza (56 km. below Graz) 81 cb. cm. 



At Dippersdorf (71 km. below Graz) 60 cb. cm. 



At Leitersdorf (83 km. below Graz) 50 cb. cm. 



At Mauth-Eichdorf . . . . (loi km. below Graz) 33 cb. cm. 



At Wernsee (112 km. below Graz) 37 cb. cm. 



At Unter-Mauthdorf . . . (120 km. below Graz) 21 cb. cm. 



