Shaler.] 362 



across the surface In a direction differing widely from tlie course of 

 the natural drainage. 



The water, relieved from its rigid condition and forced out of tlie 

 basin where it was produced, would necessarily have a certain effect 

 .in deepening the basin and in cutting out the lowest portion of the 

 rim. In addition to this, the waste of the ice within the depression 

 ■w^ould necessarily be supplied 'by the subsidence of the ice immedi- 

 ately over the basin, and the sliding toAvards the centre of the ice on 

 either side ; these actions would be attended with wearing of the 

 bottom and sides of the basin. If the thickness of the ice over the 

 basin remained the same, we would have with the increase in depth 

 of the cavity a constant augmentation in the intensity of the melting 

 action. If the glacial mass had no general movement, then the result 

 of these actions would be the formation of more or less circular de- 

 pressions ; if, however, the ice had motion in any direction, the result 

 would be the elongation of the basin in the direction of that move- 

 ment. This would arise from the j^reponderance of the erosion at 

 those points where the glacial stream entered and emerged from the 

 basin ; if the movement was sufficiently rapid, the waste by melting 

 might be sufficiently replaced by the material pushed into the basin 

 by the general motion alone, and the flow of ice from either side and 

 the lateral erosion arising therefrom destroyed. In this case, we 

 would expect to find the development of the basins producing very 

 much elongated and deep depressions, having their major axes corres- 

 ponding to the course of the glacier. Much would necessarily 

 depend upon the original contour of the basins, the trend of the val- 

 lies in w^hich they were placed, and the nature of the materials in 

 which the excavation went on. Notwithstanding all these obstacles in 

 the way of a uniform result, it is easily seen on the surface of New Eng- 

 land that some such laws in the formation of these basins exist, and 

 this view of the operation of the forces at work beneath the glacier 

 seems to be supported by the facts. 



There remains the question as to the origin of the basins ; a hy- 

 pothesis to account for their development has been advanced, but no 

 way shown in which they could originate. Something is without 

 doul)t to be attributed to the original irregularities of the rock surface 

 produced by unequal elevatory actions, and it must be recollected 

 that a few feet of depth might originate the actions which deepened 

 the basin. But it seems likely that the cause is to be sought ehv- 

 where, in actions which have received little attention from geologists, 

 yet which are capable of producing very important results, viz., in the 

 different conductive power to heat of the varied materials comi)osing 

 the earth's crust. This inequality of conductive jjower would cause 

 the isogeotheniial lines to rise with different rapidity at various points 



