114 No Marine or Fresh-water Fossils in Drift. 



view that they have been accumulated by the agency of strictly 

 terrestrial glaciers ; such is, at least, the case everywhere 

 far from the sea-shore. But we may conclude that these 

 ancient glaciers reached, upon various points, the sea-shore 

 at the time of their greatest extension, just as they do at 

 present in Spitzbergen and other arctic shores ; and that 

 therefore, in such proximity, phenomena of contact should 

 be observed, indicating the onward movement of glacial 

 material into the ocean, such as the accumulation within 

 these materials of marine fossil remains, and also the influ- 

 ence of the tidal movements upon them. And now such is 

 really the case. Nearer the sea-shores we observe distinctly, 

 in some accumulations of the drift, faint indications of the 

 action of the tide, reaching the lower surface of glaciers, and 

 the remodelling to some extent of the materials which these 

 poured into the sea. A beautiful example of the kind may be 

 observed near Cambridge, along Charles River, not far from 

 Mount Auburn, where the unstratified glacial drift (a) pre- 

 sents in its upper masses strictly the characters of true ter- 

 restrial glacial accumulation, but shews underneath faint 

 indications {b) of the action of tides. Above, regular tidal 

 strata {c) are observed, formed probably after the masses 

 below had subsided. The surface of this accumulation is 

 covered with soil (d). 



The period at which these phenomena took place cannot 

 be fully determined," nor is it easy to ascertain whether all 



