SLOPES OF TORRENTS AND RIVERS. 129 



enormous volume of water is then precipitated with extreme via 

 lence, and nothing can withstand the combined effects of its mass 

 and rapidity. All is overturned, and the most solid rocks, if they 

 project, in the least, across the direction of the current, are instantly 

 torn away, broken, and transported to great distances. The clear- 

 ing is so complete, at the origin of the current, and in the narrow 

 passages where the slope is rapid, that the exposed rock seems to 

 have been cut by the hand of man. 



13. Mud-torrents, from one cause or another, are also formed 

 which are not less terrible in their ravages. It sometimes happens, 

 as in Ireland, that turf-beds placed on a slight declivity, after being 

 swelled, more or less arched by retaining rain-water beneath them, 

 cannot resist the first heavy shower, and are set in motion. They 

 run then, in spite of th'e consistence of the mud, and the gentleness 

 of the descent, with prodigious rapidity, and sweep everything 

 they meet. Under other circumstances, the rain-waters soak in 

 loose, argilla'ceaus substances, accumulate in the midst of them, 

 and, at a certain moment, the dykes of the reservoir give way, and 

 a torrent of thick mud, filled with fragments of rock and even blocks, 

 suspended in the viscid mass, is formed, and rushes with fearful 

 rapidity, overturning everything, and cutting deep ravines. 



14. Slopes of torrents and rivers. The disastrous effects of 

 torrents are in proportion to the descent on which they move ; but 

 it does not necessarily follow that their bed must have a very con- 

 siderable inclination. The most rapid torrents* forming a continu- 

 ous bed and carrying rocks a half-yard in diameter, have a descent, 

 of only one or two degrees, and many rivers flow very swiftly on 

 a much less slope a descent of from three to four minutes 

 (sixty to a degree) is about the limit for navigable rivers. 



15. Rolled flints, or pebbles. In the ravages produced by 

 water-currents, the debris torn from mountains are transported to a 

 greater or less distance, accordingly as the inclination of the soil 

 permits the current to maintain its force for more or lesy couside- 

 rafyle distances ; but in proportion as the slopes dirmnisn, the swift* 

 ness decreases, and the larger blocks successively roxnuiu behind, 

 at the bottom of the valley, and then those of smaller size, and suc- 

 cessively the sand and mud, which are often carried enormous dis- 

 tances. In this rolling of different substances, the blocks and frag- 

 ments sinking during their transportation, rubbing against each 

 other and against the soil, gradually IOGU Ineir prominences ant) 

 angles, and in the end become compile!/ rounded, forming wha^ 

 are termed rolled flints, which im.y be more or less voluminous. 



13. How are mud-torrents formed? What are their effects? 



14. Upon what do the effects of torrents depend ? What is the rate of the 

 slope of neds of rivers that are nawgabie? 



15. How are rolled flints ai,d pebbles produced? What is gravel? 

 What is sand '> 



