85 



Just why some V-sliaped valleys occur in older, wide, flat ones is not 

 clear to the writer. Whether the inner gorge of the Grand Canyon is the 

 result of rejuvenation or not, there has been little lateral erosion accom- 

 panying the great vertical cutting. Whether lateral cutting takes place or 

 not, may depend ui)on the acceleration of the stream's force, which in 

 turn would depend upon the rate and amount of elevation ; or, it may de- 

 pend upon the character and structure of the rocks. Unmetamorphosed, 

 horizontal rocks of alternating hard and soft beds would favor lateral 

 erosion, while metamoi'phosed crumpled beds, such as occur in the inner 

 gorge of the Grand Canyon, probably would retard it. 



Limestone ISi)il:-]toles. Tlie common notion, and the teaching of most 

 ttxt-books, that limestone sink-holes are formed by the collapsing of the 

 roofs of caves, is erroneous. That some sink-holes have had such origin 

 doubtless is true, but they are the rare exception. Most of them are the 

 result of solution by descending groundwater. As this has been discussed 

 somewhat at length elsewhere, it will be only mentioned here. 



Anticlinal Vallci/s. Tlie common explanation of anticlinal valleys is 

 that streams have gradually shifted from synclines to anticlines, the shift- 

 ing having been invited by the excessive fracturing of the latter over the 

 former. The writer believes that most anticlinal valleys have had a differ- 

 ent history. It will be conceded that most folds had their inception while 

 yet submerged. This granted, the first part of the folds to appear at sea 

 level were the crests of the anticlines. Except at considerable depths, all 

 the sedimentary material but that of calcareous nature was in the incoher- 

 ent state at the time of elevation, and consequently was easily eroded. As 

 soon as the anticlinal crests came within the effective force of the waves, 

 they were thereby truncated. The rate of rise was greater than we are 

 accustomed to admit, if the truncation did not for a long time equal the 

 elevation. As the truncated material was shifted to the synclinal troughs, 

 the whole process was a leveling one. It is not unreasonable to suppose 

 that many folded areas emerged as practically level plains, and that streams 

 were at least as free to flow along anticlines as synclines. 



In those cases where the rise of any anticlines was rapid enough to 

 overtake the erosive action of the waves, that action was still effective on 

 the sides of the resulting islands. Added to this, was the work of the 

 subaerial agencies. On the whole, the direction of the resulting small 



1 A. H. Purdue, Science, Vol. XXVI, p. 120. 



