Brigham.] 150 [Xovember 4, 



as stated by Mr. Dall, would certainly seem to be overwhelming^ 

 evidence against tlie assumption of the Ibrmer existence of any exten- 

 sive terrestrial sheet of ice similar to that which once covered New 

 England. All the positive evidence in favor of the glacial theory 

 must necessarily be confined to the presence of these two kinds of" ice- 

 marks," and, according to Mr. Dall, both the drift and the scratches 

 axe absent. Mr. Shaler's observations, that the detrital matter was 

 derived from the sides of hills rising above the upper siu'face of the 

 glacier, would not hold as a general rule. The driit material so plen- 

 tifully scattered throughout New England, has evidently been derived 

 in great part from rocks which have been broken off by the under 

 surface of the ice sheet, and transported often to considerable dis- 

 tances.^ The general formation of the hills, their gradual northern 

 and steep, rugged southern slopes and their rounded summits, as 

 well as slight elevation, would seem to susrain this view. 



The sui'tace drift of the vicinity of Salem and Cambridge has been 

 derived from, and in many instances the boulders may be traced to 

 locky ledges, at greater or less distances to the northward. And 

 these rocky ledges, which at the most are only a few hundred feet 

 high, must have underlain the: glacier, or else their summits would 

 not now present such an accurate copy of the roclirx moutoimc'es, but 

 would be ragged or serrated. 



',"Mr. W. T. Brigham called the attention of Mr. Shaler to the 

 actual motion of rocks imbedded in the bottom of a glacier. To 

 be reduced to mud, i-ocks or boulders must be dragged along; in 

 other words, glaciei-s tear up and transj^ort on their under surface as 

 well as on the upper surface or in the mass. It is difficult to imagine 

 a layer of ice a thousand feet thick, nielting in situ, as it must have 

 done according to Mr. Shaler's theory, without leaving some of the 

 so-called glaciation marks. If the sheet was foi-med by " re^Deated 

 congelation of aqueous precipitation," the summits of the hills must 

 have remained uncovered last, and when the melting commenced,, 

 have first appeared above the sea of ice, giyiiig ,an admirable opp(>r- 

 tunity for debris to be deposited, on the surface of the retreating ice- 

 sheet, and we should have had deposits in concentric circles around 

 'the conical hills. 



The j)resence or absence of striation does not determine the exist- 

 ence , of glaciers, for in valleys of the Hawaiian Islands^ striae i and 



^ Fraginents of rock from Lake Telos have been fourid on the sides of 3It. Katah- 

 4itt, fourteen miles distant, and three or four hundred feet above the generatsiir 

 iace. 



