CONTINENTAL TERRACES AND SUBMARINE VALLEYS 133 



strated in the continental terraces, we have no evidence that 

 they are abundant enough or continuous enough to match the 

 number of submarine valleys already discovered. Then, too, it 

 is highly improbable that there can be aquifers outcropping 

 near the fall-ofT to the continental slope, in number sufficient 

 to explain the heading of so many furrows at the line of fall- 

 off. In the third place, the published maps do not represent the 

 valleys as of the box-head kind; their heads are more sharply 

 bitten into the continental terrace. A fourth objection is still 

 more cogent. By hypothesis the fresh underground water must 

 continue to flow oceanward, though opposed by the pressure of 

 the denser sea water. Professor Johnson recognizes this diffi- 

 culty and tries to meet it by supposing that the aquifers have 

 been fed from rain-gathering surfaces many hundreds of feet 

 above sealevel. In this way the subterranean streams are 

 thought to have been supplied with pressure sufficient to dis- 

 place the heavier sea water at the submarine outcrop of the 

 aquifer. In the case of the Atlantic terrace between Long 

 Island and Cape May, a fraction of the required "hydraulic 

 head" might be conceived as given, if the landward limit of 

 each aquifer were located on the coastal plain, well above sea- 

 level. Compare Figure 2. However, the amount of such coun- 

 teracting pressure is far from enough, even under the most 

 favorable conditions, in the existing coastal plain. Professor 

 Johnson therefore adds an additional speculative premise: the 

 needed hydraulic head was supplied when the coastal plain of 

 the mid-Atlantic States extended far beyond its present limit 

 and to a height far greater than its present maximum height. 

 This ad hoc hypothesis has no definite backing from geological 

 observations; it increases the number of un verifiable premises; 

 and it leads to new troubles. Among these a few may be noted. 

 As fully developed, the spring-sapping hypothesis implies that 

 the submarine valleys off the eastern United States were exca- 



