I GENERAL SUMMARY OF SCIENTIFIC AND 



pletely decayed, tlie chauge, extending from natural joints, 

 produces a concentric exfoliation, leaving rounded nuclear 

 masses of unclianged rock, like the boulders of decomposition 

 described bv llartt in Brazil. lie concludes that the bould- 

 ers of our northern glacial drift are due to such a process, 

 and that the glacial action which displaced the already soft- 

 ened and disintegrated rocks did not produce the great mass 

 of glacial drift by mechanical abrasion of hard rocks. As 

 regards glacial action, the extreme views of those who assert 

 the existence of immense continental glaciers, or of an ice- 

 cap covering the greater part of the northern and southern 

 hemispheres, while strongly defended in some quarters, are 

 rejected by many. Foremost among its opponents is Daw- 

 son, who maintains that the glacial phenomena seen over 

 northeastern America are to be ascribed in great part to 

 the action of polar ice borne over the submerged land by the 

 polar current. He, however, at the same time admits the ex- 

 istence of local glaciers in the mountainous regions, which 

 were the cause of some of the phenomena observed. He has 

 shown that a portion, at least, of the so-called glacial drift is 

 clearly a submarine accumulation. Similar views are held 

 by most of the English geologists, in opposition to those of 

 Ramsay and Giekie, who are partisans of the hypothesis of 

 land-olaciation. The careful studies of Searles V.Wood on the 

 glacial deposits of southern England seem to be conclusive 

 in favor of its submarine origin, and of the agency of floating 

 ice from local glaciers in the distribution of the glacial drift. 

 As regards the supposed power of land-glaciers to excavate 

 valleys and lake-basins, Phillips of Oxford, in his address be-, 

 fore the British Association in August, IS^S, remarks that it 

 is " a proposition which can not be accepted until we possess 

 more knowledge than has yet been attained regarding the 

 resistance offered by ice to a crushing force, its tensile 

 strength, the measure of its resistance to shearing, and other 

 data requisite for a just estimate of the problem, which is dis- 

 tinctly a mechanical one. At present it would appear that 

 under a column of its own substance 1000 feet high, ice 

 would not retain its solidity ; if so, it could not propagate a 

 greater pressure in any direction." 



A small but valuable geological map of the United States, 

 prepared for General AYalker, the superintendent of the Ninth 



