252 PRINCIPLES OF STRATIGRAPHY 



is shown in the great delta of the Huang-ho. Avhich extends for 300 

 miles or more from the mountains which have furnished the sedi- 

 ment. The sands and round quartz pebble conglomerate at the 

 base of the Carbonic of North America (Pottsville) have been 

 spread over an area of more than 400 miles radius. With progres- 

 sive transport a constant decrease in the size of the fragments oc- 

 curs, owing to the grinding processes which they undergo. [See the 

 table given above for the reduction of the sediments of the Mur 

 River (p. 246).] 



Sorting Power of Rivers. When sands and gravel consist of a 

 variety of material, a gradual assorting and elimination of the more 

 destructible matter will result by river transportation, as well as 

 wave action. This sorting of sands by rivers and by the waves 

 along the shore is, however, always much slower and generally less 

 complete than when it is done by the wind. The gravels of the 

 River Saale at Jena, derived from the Thiiringer Wald, consist 

 chiefly of fragments of the Culm and Cambric formations. The 

 Siluric, Devonic, Zechstein and Pjuntsandstein formations of this 

 region are scarcely represented in the Saale gravels on account of 

 the preponderance of soft shales, sandstones, limestones and dolo- 

 mites, which in the course of transportation are destroyed. (Keil- 

 hack-40 : 75. ) 



Analyses (Mackie-46: 14Q) of the sands of the River Spey de- 

 rived from the fundamental gneiss of Scotland showed at Cromdale 

 18 per cent, of feldspar and i per cent, of mica, while further down 

 the river at Orton the percentage of feldspar was only 12. That 

 of the River Findhorn above Dulsie bridge showed 42 per cent, of 

 feldspar, while between Forres bridge and the sea the percentage 

 was reduced to twenty-one. This shows the gradual washing out 

 by rivers of the feldspar and mica. On the seashore, as already 

 noted, further assorting takes place. Thus the average per cent, 

 of feldspar in the sand furnished by the four principal rivers of 

 Eastern Moray (Spey, Lossie, Findhorn, Nairn) was 10, whereas 

 in the rivers themselves, near their mouths, the average per cent, 

 was 18. Prolonged sorting by either waves or river currents may 

 thus produce nearly pure quartz sand by the removal of the other 

 destructible minerals. The distance traveled by river gravels is often 

 very great. Pebbles from the hills bordering the Red Sea have 

 been found in the Nile Delta, 400 miles away. (See Chapter XIV.) 

 There are reasons for believing that the well-rounded quartz peb- 

 bles of the Sharon (Upper Pottsville) conglomerate of Ohio and 

 western Pennsylvania and the Olean of southern New York have 

 traveled over 400 miles from their original source. 



