116 ON EROSIONS OF THE EARTH'S SURFACE. 



Further north, on the same continuous ridge, is at least one other cavern in 

 limestone, which is said to have been penetrated 150 feet in depth, without reach- 

 ing its bottom. But I have not visited it, and know neither its height above the 

 valley, nor whether it was an ancient river bed : though every such cavern, which 

 I have visited in the limestone of New England, has seemed to have been thus 

 produced. 



If this valley in Dorset was formed by aqueous erosion, it is highly probable 

 that the many other deep and narrow valleys in the same metamorphic rocks in 

 Vermont and Massachusetts, especially in Berkshire county, were formed in a 

 similar manner. On no other theory could I explain their existence, even had 

 we not this striking fact of the eroded cavern on the top of Dorset mountain. 



I might extend this inference to a large part of the deep valleys of our country. 

 At least, such facts afford a presumption that many of them were probably the 

 beds of rivers on former continents. Here, then, it appears to me, is an interesting 

 field of geological inquiry, rarely entered, yet capable of exploration. I mean 

 the determination of the period and manner in which our ancient valleys have 

 been formed. 



20. Gorge on Neio River, a tributary of the Kenaioha, in western Virginia. — Dr. 

 Hildreth describes this gorge as having nearly perpendicular walls of 800 feet in 

 height, and its whole length is 50 or 60 miles. Indeed the entire valley of the 

 Kenawha river, so far as I have ascended it, appears manifestly to have been worn 

 out in the nearly horizontal sandstone, shale, and fire-clay, of the coal formation. 

 The hills along its lower part, however, rarely rise higher than 400 or 500 feet. — 

 American Journal of Science, vol. XXIX. p. 91. 



21. The Valley of the Mississippi, for two hundred miles above the mouth of 

 the Missouri. — I select this part of this valley, because the proof of its erosion 

 seems quite obvious, by looking at Professor Owen's geological map, appended 

 to his Report upon Wisconsin, Iowa, and Minnesota; or upon a similar map in 

 Taylor's Statistics of Coal. On the east side of the river and at some distance, is 

 exhibited the Illinois coal field ; and on the west side, that of Missouri and Iowa. 

 Approaching the river from either side, we find the coal measures swept away, and 

 the carboniferous limestone, the next rock beneath, brought to light. Still nearer 

 the river, we find rocks of a still older date, because the valley is deeper. How 

 obvious that these coal fields were once united, and that the coal measures have 

 been swept away by the action of water ! What portion is gone I am unable to 

 state : but the fact that powerful erosion has taken place seems too evident to be 

 doubted. Most other river beds present similar facts : but they do not usually 

 stand out so distinctly. 



22. Big Canon on the Rio Colorado of the West. — This occurs in W. long. 115° 

 and N. lat. 36° ; but I have not been able to find any detailed account of its 

 extent. Where Capt. Sitgreaves struck a canon on the Zuiii, or Little Colorado, 

 which he was assured extended to the Rio Colorado, its depth was 120 feet, less 

 probably than that of the Big canon. — Sitgreaves Report to Government, p. 8. 



23. Dalles of the Wisconsin River, in Wisconsin. — The length of this gorge in 

 sandstone, is five or six miles, and the height of the wall from 40 to 120 feet. — 

 Owen's Report on a Survey of Wisconsin, &c, p. 517. 



