363 [Shaler. 



in the same region, and consequently the temperature capable of pro- 

 ducing melting would attain the surface of places of different heights 

 at the same time. Assuming a horizontal surface with varied con- 

 ductive power, melting would, from this cause, have the same tend- 

 ency to begin at some points much sooner than at others, as would 

 arise from vertical inequalities. There being no regions absolutely 

 level, and no considerable areas of the cnist of the same conductive 

 power to heat the points where melting would begin at the base of a 

 glacial accumulation, it would be determined by both of these fea- 

 tures together. The flow of heat from the interior during the latter 

 geological epochs has been of such small amount that the rise of the 

 isogeothermal lines would be necessarily very gradual. This would 

 admit of a considerable excavation before the lines had passed 

 tlirough the materials of slower conductive power, so as to produce 

 melting over the whole of the base of the ice. It is most probable 

 that on our hypothetical plain, melting would never take place over 

 the whole surface, but that the local melting would more than equal 

 the accumulation, long before any considerable portion of the surface 

 had been elevated above the melting point. The diversity in the rate 

 of increase of temperature as we descend in different parts of the 

 crust, is sufficient evidence of the varying conductive power. It could 

 be shown by a discussion of the circumstances attendant on these va- 

 riations, that it is eminently probable that veins of injection conduct 

 with much more facility than stratified materials, a conclusion which 

 harmonizes with the most familiar laAvs of conduction. 



It is not improbable that the extremely varied conductive power of 

 the earth's crust within New England, may be the main reason for the 

 great number of the glacial basins in that region. 



The efiect of this melting of the glacial base from the heat received 

 fi'om below, on the excavation of river valleys and the transportation 

 of drift materials, afTord interesting questions, but which it is not pos- 

 sible to discuss in the present state of our knowledge of the facts. 



These views are not presented by the author as altogether satisfac- 

 tory explanations of the facts, nor is it to be supposed that these notes 

 are meant to present anything like a discussion of the hypotheses 

 brought forward. We have unfortunately far too little accurate in- 

 formation on the subject to warrant such an attempt. It is hoped 

 however, that they may contain suggestions which will afTord the basis 

 of profitable investigation by geologists and physicists. It may be 

 remarked that some of the greatest objections which could be urged 

 to the views herein presented, are lessened in value when we recollect 

 that all the glacial erosion visible in drift regions can not properly be 

 attributed to the glaciation from which the hemispiiere has just 

 emerged. It is more than probable that these lake basins have aga;n 



