EVIDENCES OF HIGHER SEALEVEL. 149 



iiioiintains is the most considerable. The evidence we liave in hand 

 points to the conclusion that ordinary mountain-folding, if not limited to 

 the seashore, prevailingly begins when and where a tract of country has 

 been subjected to the erosion and transference of materials, such as 

 occurs in a coastal belt. I have already noted-'' how considerable is the 

 evidence from the distribution of mountains going to show that they do 

 not originate on the deep-sea floors. It is worth while also to note the 

 fact that wliere mountain folds involve a number of formations the evi- 

 dence goes to show that interrui)tions of deposition have occurred such 

 as are explicable only on the supposition that the region in which they 

 developed was at or near a coastline. It is easy to see that the develop- 

 ment of mountains next the shore necessarily tends to destroy the marks 

 of ancient sea-margins. This result is accomplished in two ways : In the 

 first place, the irregular uplifting of the surface will necessarily throw 

 the planes of the old shore out of their original horizontal attitude, while 

 the intensified erosion due to the institution of the new steeps hastens the 

 destruction of the old marine benches. 



Criteria indicatixNg higher Sealevel. 



marixe cliffs and benches. 



The criteria by which we may determine the former presence of the 

 sea at a higher level than it now has may best be judged by the study of 

 existing shorelines where the coast is unprotected by detritus, so dis})osed 

 as to form a l)arrier to wave-action. There are two elements usual in the 

 topograi)hy of such a coastline, each of great but diversely enduring value. 

 These are the marine scarf or cliff and the corresponding greater or less 

 bench, where the materials disrupted by the waves which are not dis- 

 solved in the water are accumulated. When the coastline is elevated the 

 atmospheric processes rapidly operate to destroy the cliffs. With most 

 rocks a few thousand years will serve so to break down the steeps and to 

 convert them into taluses of a relatively low angle that even the eye most 

 practiced in the interpretation of such phenomena may fail to detect in 

 them satisfactory evidence of marine erosion. So fjir as my observation 

 goes, the most enduring evidences of marine action are the detached 

 elevations (" monadnocks," as they have been termed by Professor Davis), 

 whicli were islands in the ancient seas at the time when the waters beat 

 at or near their leases. 



MARINE CA VES. 



Next in lasting value, though much less enduring, are marine caves, 

 wliich in favorable positions sometimes penetrate a considerable distance 



•See this Bulletin, vol. v, p. 203. 

 XXI — Hii.r., Gkol. Sor. Am.. Vor.. »;, l«<j4. 



