470 J. W. SPENCER — PLEISTOCENE SUBMERGENCE. 



The difficulties which the sea-level theory present to some minds may be 

 stated as: (1) a great regional depression of the continent; (2) the absence 

 of absolute continuity of the beaches; (3) the absence of marine organisms 

 in the beaches; and (4) the personal equation of theoretical views. On the 

 other hand, the theory of glacial dams presents such obstacles that their 

 value will be considered at length. 



The idea of the hydrostatic stability of the continents must not be too 

 strongly relied upou, for the evidence adduced, which shows that the continent 

 lately stood 3,000 or temporarily even 6,000 feet higher than now, appears 

 conclusive. Such mobility of the earth's crust being established, there ap- 

 pears no reason why the terrestrial pendulum could not have moved equally 

 in the opposite direction, and carried down the highlands of Pennsylvania 

 to nearly 3,000 feet, or those of New England to twice this depth. The ob- 

 jections to such subsidence could only be based upon its magnitude, which 

 observations must settle. 



The absence of the continuity of the shore-markings is an objection only 

 to a limited extent. Part of the reported absence arises from the imper- 

 fection in the explorations, OAving to their changing character ; to the local 

 non-formation of beaches as described in a previous paper ;^ to the failure 

 of identification of separated points, owing to subsequent terrestrial deforma- 

 tion ; and to the interruptions occasioned by topographic features and sub- 

 sequent obliteration by erosion. All of these difficulties are greatest in the 

 higher regions, for there the beaches must be looked for among islands and 

 detached mountain knobs. 



The absence of marine remains seems perhaps the greatest obstacle to the 

 acceptance of a sea-level formation of the beaches, as marine organisms are 

 found only up to 520 feet.j But the Pleistocene gravels oc-cur in Georgia 

 and Alabama, in positions facing the. sea, at altitudes of 700 or 800 feet, and 

 higher up the greater valleys at 1,500 feet, J without containing any marine 

 remains. Even where marine Pleistocene beaches occur on the coast of 

 Norway there are very few localities wdiere shells are found. How many 

 of the older geological formations are unfossiliferous ? How many of those 

 ancient beach deposits, now represented by conglomerates, porous sandstones, 

 and indeed many clays, are entirely barren ? Under such conditions have 

 we a right to pronounce judgment on the freshness of waters based on the 

 absence of aqueous organic remains? This question will be referred to again 

 in considering the glacial dam theory. 



As to the personal equation, it ought not to pass beyond the limit of con- 

 servatism, but it is quite proper that it should be considered ; for, as Pro- 



* Ancient Shores, Bowlder Pavements and High-Level Gravel Deposits in the Region of the Great 

 Lakes, by J. W. Spencer: Bull. Geo!. Soc. Ana., vol. 1, 1889, p. 77. 

 t At Montreal. 

 jOn the upper Etowah river of Georgia. 



