FRESH-WATER STREAMS. 655 



Mr. Babbage observes that, taking four kinds of detritus, of such a 

 size, shape, and density that they would sink — the Jirst kind 10 feet 

 an hour, the second 8, the third 6, the fourth 4, then, if a stream con- 

 taining this detritus were 100 feet deep at mouth, and entered a lake 

 having a uniform depth of 1,000 feet, and a rate of motion of two 

 miles an hour, the first kind would be carried 180 miles, before the 

 first portion would reach bottom, and would be distributed along for 

 20 miles ; the corresponding numbers for the others would be for (2) 

 225 and 25 ; (3) 360 and 40; (4) 450 and 50. Thus, four kinds of 

 deposits would be formed from the same stream and be distributed for 

 different distances along the bottom. 



Thus, as a stream decreases in velocity, it assorts according to size of 

 particles deposited; so that, (1) the rate of flow determines the order 

 of deposition and the arrangement of material ; and conversely, (2) 

 the coarseness or fineness of a bed is evidence, in general, as to the rate 

 of flow of the depositing waters. 



An important exception to this relation between size of particles 

 and hydraulic value, noticed and made the subject of special investiga- 

 tions by Professor Hilgard, arises from the tendency of the finer kinds 

 of sediment, if the water is not absolutely quiet, to agglomerate their 

 particles, if not over 1 mm. in diameter, into larger particles, or to 

 flocculate, as he terms the process, and so take the hydraulic value of 

 coarser sediments. He shows that fine river deposits consist largely 

 of such flocculated particles, and that the fitness of soils for tillage de- 

 pends largely on the porous condition thus derived. 



Assorting according to hardness is another effect of erosion and 

 transportation. The softer minerals are ground to the finest powder 

 before the harder ; and hence the former are most likely to constitute 

 the finest deposits. Of rock materials, the two chief kinds are feldspar 

 and quartz. The feldspar is most easily ground up ; so that the two 

 become widely parted by prolonged water movements, the quartz be- 

 ing left in coarser particles behind, while the feldspar is often carried 

 on to make fine beds by itself. Quartzytes and felsytes are hence nat- 

 ural results of erosion, in regions of metamorphic rocks, when the 

 feldspar has not become decomposed. 



3. Amount of Material transported and deposited by Rivers. 

 The amount of transportation going on over a continent, especially 

 in seasons of floods, is beyond calculation. Streams are everywhere 

 at work, rivers with their large tributaries and their thousands of little 

 ones spreading among all the hills and to the summit of every moun- 

 tain. And thus the whole surface of a continent is on the move to- 

 ward the oceans. The amount transported is a measure of the amount 



