i6 POPULAR SCIENCE MONTHLY. 



to the ridges or breaking through them, are the result of the denuda- 

 tion of the rock surfaces of the country, and accordingly, when 

 similar forms occur beneath the sea, they suggest the same origin. 



On the MxiGNiTUDE OF Land Valleys. — Concerning the mag- 

 nitude of land valleys, they vary from the smallest ravines to valleys 

 such as the Mississippi, whose flood plains are from thirty to eighty 

 miles wide for the lower five hundred miles of its length. Several 

 comparatively short rivers, crossing the coastal plains of the south- 

 eastern Atlantic States, have valleys from two to four miles wide, 

 even at a hundred miles from the sea, and upon nearing the coast 



^ •^■-~,^ ---..Little Sand Mt. 



FiQ. 3. — Cross-seeticm vi n complex valley iu the southern Appalachians, showin<; it to be in- 

 dependent of the geological structure, which is represented by the shading. Dotted lines 

 mark what was the upper limit of strata which Lave been denuded away. The straight 

 lines {F) illustrate some of the faults affecting the region. 



they may be from five to ten miles wide, bounded by only broken 

 hills. All the rivers crossing the coastal plains are flowing over 

 deeply buried channels. That of the Savannah River is buried be- 

 neath two hundred and fifty feet or more of superficial deposits, and 

 the old Mississippi Yalley is now known to reach one thousand feet 

 below sea level at New Orleans. These buried channels prove that in 

 recent times the continent has sunken to a great extent. The valley 

 of the St. Lawrence River differs from that of the Mississippi in be- 

 ing drowned but not subsequently filled with the mud brought down 

 by the streams. In its lower reaches it is seventy miles wide. These 

 examples of continental valleys are greater than any of the drowned 

 ones to be considered in this paper. The examj)les cited are those of 

 valleys crossing extensive plains at no great elevation above the sea. 

 with their lower reaches depressed even below high tide. 



The Colorado River of the West flows from table-lands eight 

 thousand to ten thousand feet above the sea. Its canon section is 

 about two hundred and twenty miles long. This is not a simple 

 gorge, but a broad valley from five to twelve miles wide, bounded by 

 walls rising two thousand feet above its floor. This floor was an old 

 base level of erosion formed at no considerable altitude, so that the 

 streams meandered sluggishly over it, and the rains and rills widened 

 it to broad proportions; but, owing to subsequent elevation of the 

 region, the river has cut down its channel to a still lower base level, 

 and in doing so it has been deepened thirty-five hundred or four 



