FIVE TYPES OF EARTH-SURFACE IN THE UNITED STATES. 



311 



raged all day; the ground is strewn with the dead; nothing but the rear guard of the beaten 

 array is to be seen ; the victors arc about to bivouac on the field ; now the king's jester 

 rushes forward, waving his sword, and shouting, " Follow me, to victory or death !" We 

 have only to turn our eyes from the dentritic erosion of the Alleghany coal-area, as rep- 

 resented imperfectly upon our map, to contemplate the outspread of Cretaceous and Ter- 

 tiary country between the Blue Ridge and the sea, to place a proper estimate on the avail- 

 ability of the supposed ice-cake of the glacial era, for sculpturing such continental areas 

 as that of the Coal, reaching from the 43° or 45° down to the 33° or 32 i° of latitude, espe- 

 cially remembering how the sculpturing was nearly finished before the glacier was born. 



The erosion of the wonderful Green River ravines, in the Colorado country, to a depth 

 of five thousand feet, is not more evidently the product of ordinary meteorological causes 

 than is the erosion of any given segment of the Appalachian or Alleghany districts 

 exhibited upon the map. But the Coal Measure areas show the truth most plainly. The 

 ravines which debouche upon the Blue Grass area south of the Ohio, along a line extend- 

 ing from Portsmouth in Ohio, to Somerset near the Tennessee line, all have vertical walls 

 of Conglomerate (Millstone grit), supporting a sadly wasted Coal Measure country above, 

 and overhanging steep slopes of Subcarboniferous shales and limestone below. This lime- 

 stone, horizontally outcropping on each side of every ravine, and with a delicate dip towards 

 the southeast, is full of caverns ; while sink-holes in the Coal Measure country above 

 show that their ramifications extend beneath the; whole surface. Fine springs issue from 

 them; robbers have always haunted them; and guerilla bands, during the late Rebellion, 

 made many of them famous for deeds of blood. 



Towards their outlets these ravines are gradually cut down through the Devonian sands 

 and shales into the Silurian limestones next below. An opportunity is thus afforded for 

 the escape of all the waters concealed within a second system of brandling caverns 

 belonging to this formation, far larger than the one above. These lower limestone 

 waters are highly magnesian. The underground erosion, the concealed dilapidation of 

 the Silurian limestones by their disintegrating reactionary chemistry, is extensive beyond 

 conjecture and growing daily. A world of natural catacombs must underlie the many 

 thousands of square miles of surface represented by the northwest quarter of the map. 



I believe that this chemical element of erosive energy has been slighted even in discus- 

 sions the most recent I ascribe to it nine- tenths of the wastage of the Blue Grass area. 

 I believe that the erosion went on, chiefly underground, along those narrow belts where 

 the lime-rock formations approached the surface at whatever level the surface happened at 

 the time to stand. The caverns grew ; their roofs fell in ; streams washed the debris con- 

 tinually away. As the surface thus kept filling piecemeal into the cavernous traps every- 

 where laid for it below, the general level of the area, was slowly and insensibly let down 



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