of Shells in the Till near Boston. 369 



beaches or of Pleistocene marine formations above the present 

 sea level are found in the Shetland and Orkney islands.* 



The occurrence of transported marine fossils in the till near 

 Boston shows that during the epoch preceding the latest glacia- 

 tion the North American coast in this latitude was not higher 

 than now in relation to the sea ; for in that case no marine de- 

 posits and shells would have existed here to be eroded by the 

 southeasterly moving ice-sheet and incorporated in its drift ac- 

 cumulations. Conversely, we know that the land then was not 

 appreciably lower than now, in other words, that there was no 

 considerable submergence of the border of our present land 

 area ; for this would have led to the intermingling of such 

 broken sea-shells with the glacial drift farther inland, where 

 no trace of them is found. So it appears that the relative levels 

 of land and sea here were closely the same before the last 

 glacial epoch as at the present time. 



The chief element of my interest in this subject has been a 

 hope that its bearing thus on the oscillations of land and sea 

 during the Quaternary period would contribute to the solution 

 of the question whether the northward ascent of the beaches 

 of the glacial Lake Agassiz, assigned to me for investigation 

 in the United States Geological Survey, is to be explained 

 mainly by northward attraction of the water of that lake in 

 gravitation toward the ice-sheet, or mainly by a depression of 

 the earth's crust beneath the vast weight of the ice and its re- 

 elevation when the weight was removed. In this study of our 

 Atlantic coast, I have therefore sought to connect these obser- 

 vations near Boston with the allied evidence supplied by other 

 Pleistocene marine fossils both south and north of our latitude. 

 Some of the conclusions to which this correlation seems to 

 lead I will endeavor to state briefly. 



As before noted, it is only toward the south that we find 

 Pleistocene fossiliferous beds antedating the last epoch of 

 glaciation, when an ice-sheet covered all New England. They 

 occur in Sankoty Head and on Gardiner's island at elevations 

 respectively about 30 and 15 feet above the sea, and in numer- 

 ous localities on Long Island from the sea level up to eleva- 

 tions of about 200 feet. But at least the higher of these beds 

 appear to have been " upheaved by the lateral pressure of the 

 ice-sheet and thrown into a series of marked folds at right 

 angles to the line of glacial advance," as shown by Merrill ;f 

 and he finds that this uplifting and folding is also very distinctly 

 seen in the strata underlying the glacial drift on Gardiner's 

 island, so that the fossiliferous layer there, though raised little 

 above the sea level, is probably higher than its original position. 



* Quart. Journ. Geol. Soc, xxxv, 1879, p. 810 ; xxxvi, 1880, p. 663. 

 f Annals of the New York Academy of Natural Sciences, vol. iii, 1886, pp. 

 341-364, with sections and map. 



