Shaler.] 292 ' [December 16 , 



be that this represents the region of constant alternation between 

 land and sea conditions in the endless rise and fall of the continents ! 

 Suppose a dozen or more elevations and depressions within that 

 range ; we should then have during each recession a vast amount of 

 solid matter torn from the land carried to the seaward to a little be- 

 yond the limit of the upheaval, which would not be carried back dur- 

 ing the return of the sea in the next subsidence. As far as I have 

 been able to determine, this submarine shelf is wider, and on the 

 whole deeper submerged, in the boreal than in the equatorial sections. 

 If the circumpolar regions have been the areas of the greatest mechan- 

 ical erosion, which considering their frequent glaciation seems very 

 probable, then we should expect to find this erosion plane more ex- 

 tensive in those regions. 



I have elsewhere endeavored to show that the present shore line 

 of New England is almost exactly where it was before the glacial 

 period. If this be the case it is a confirmation of the idea that the 

 continents are. in a state of equilibrium, the stability of which is fre- 

 quently disturbed by various changes, and that when any of these 

 disturbances is temporary, as that of the glacial period was, they re- 

 turn on its disappearance to their original position. 



The other point bearing on this general question is connected with 

 the history of deltas. Nothing shows the essential instability of the 

 land better than the fact that of all the great deltas none have 

 accumulations of several different geological formations on the same 

 level. If the land was unchanging in its level for any great geologi- 

 cal time, deltas should show different formations in succession as we 

 descend towards their seaward face at the same level; but deltas are, 

 in fact, generally limited to one geological formation, and that when 

 new deposits of the different geological ages occur together there is 

 usually a considerable difference of level between them. 



If the erosion plane, above referred to, be really the result of the 

 shore erosion, then it is another and very striking evidence of the 

 perpetuity of the continental forms, and an equally strong argument 

 against the existence of other great continents in recent geological 

 times. 



Mr. J. A. Allen exhibited a specimen of the Sharp-tailed 

 Finch (Ammodromus caudacutus Sw.), collected the pres- 

 ent autumn in the Calumet Marshes near Chicago, 111., where 



