150 N. S. SHALER — EVIDENCES AS TO CHANGE OP SEALEVEL. 



from the surface, and beiDg well placed to avoid erosion, they endure, as 

 fresh-water caverns may, until the downwearing surface penetrates to 

 their level. 



DETAILED DISCUSSION OF CRITERIA. 



Danger from, imperfect Discrimination of Phenomena. — Both these evi- 

 dences of ancient marine steeps are likely to be confounded with other 

 phenomena. Isolated mountains may owe their relief to certain elements 

 of resistance to decay which their materials afford that may not be ap- 

 parent on inspection or even after careful laboratory study. Various 

 parts of the valley of the Mississippi abound in buttes or knobs Avhich I 

 at one time regarded as evidences of marine action. A more careful 

 study of these steep faced eminences has led me to the conviction that in 

 many cases they are due to differential erosion, soft beds at their base 

 dissolving away, and the firmer overlying rocks, when broken to pieces 

 by their downfall so as to expose a large surface to decay, likewise being 

 borne away by the surface water. In yet other cases, where the frost 

 breaks a portion of the material into the form of sand, the grains are 

 driven by the wind against the escarpment in such a manner as to bring 

 about a rapid process of erosion, such as is plainly visible along the cliffs 

 of the Millstone grit in eastern Kentucky. 



In the effort to interpret the deserted shorelines of the Atlantic coast 

 by the study of the ancient marine cliff I have been so far baffled by the 

 process of their ruining, combined, it may be, with dislocations of position 

 due to differential movements of the surface, that only here and there 

 does the evidence seem to me of a conclusive or even of a probable nature. 



Persistence of the submarine Shelf. — The innermost part of the submarine 

 shelf, that which is composed in most cases mainly if not altogether of 

 debris from the neighboring steeps, is in many cases, though originally a 

 less conspicuous, a much more enduring feature than the scarfs. Owing 

 to its approximately horizontal position, this comparatively inconspicuous 

 feature is often less effectively attacked by the agents of decay than the 

 firmer rocks of the ancient cliffs. It yields readily, of course, to stream- 

 action. It is apt to be covered over by the talus derived from the decay 

 of the cliffs, but the topographic indication, as well as that which may 

 be had from the form and disposition of its debris, long remain of value. 



Barrier-heaches untrustworthy as Criteria. — Along an old shore, one where 

 the sea for a considerable time has lain against the same part of the land, 

 and therefore has been able to bring its construction bench to near the 

 surface of the water for a considerable distance from the shore, Ave are 

 likely to find barrier-beaches with their concomitant hooks, tidal deltas 

 and other peculiar topographic forms due to migrating sands. Although 



