SIGNS OF PAST GLAGIATION. 57 



Gulf Stream which is slowly covering the area between 

 Northern America and Europe. Northern streams like 

 the St. Lawrence, which are deeply frozen over with ice 

 in the winter, and are heavily flooded as the ice breaks up 

 in the spring, afford opportunity for much transportation 

 of boulders in the direction of their current. In attribut- 

 ing the transportation of a boulder to glacial ice, it is 

 necessary, therefore, to examine the contour of the coun- 

 try, so as to eliminate from the problem the possibility of 

 the effects having been produced by floating ice. 



Another source of error against which one has to be 

 on his guard arises from the close resemblance of boulders 

 resulting from disintegration to those which have been 

 transported by ice from distant places. Owing to the 

 fact that large masses of rocks, especially those which are 

 crystalline, are seldom homogeneous in their structure, it 

 results that, under the slow action of disintegrating and 

 erosive agencies, the softer parts often are completely re- 

 moved before the harder nodules are sensibly affected, 

 and these may remain as a collection of boulders dotting 

 the surface. Such boulders are frequent in the granitic 

 regions of North Carolina and vicinity, where there has 

 been no glacial transportation. Several localities in Penn- 

 sylvania, also, south of the line of glacial action as de- 

 lineated by Professor Lewis and myself, had previously 

 been supposed to contain transported boulders of large 

 size, but on examination they proved in all cases to be 

 resting upon undisturbed strata of the parent rock, and 

 were evidently the harder portions of the rock left in loco 

 by the processes of erosion spoken of. In New England, 

 also, it is possible that some boulders heretofore attributed 

 to ice-action may be simply the results of these processes 

 of disintegration and erosion. Whether they are or not 

 can usually be determined by their likeness or unlikeness 

 to the rocks on which they rest ; but oftentimes, where a 

 particular variety of rock is exposed over a broad area, it 



