404 LAKE SUPERIOR. 



very same phenomena, with precisely. the same characters, to which, 

 therefore, a sound philosophy should ascribe, at least conditionally, 

 the northern and Alpine polished surfaces, and scratched and grooved 

 rocks, or at least acknowledge that the effect produced by the ac- 

 tion of glaciers more nearly resembles these erratic phenomena than 

 does that which results from the action of currents. But such is the 

 prejudice of many geologists, that those keen faculties of distinction 

 and generalization, that power of superior perception and discrimina- 

 tion which have led them to make such brilliant discoveries in geology 

 in general, seem to abandon them at once as soon as they look at the 

 erratics. The objection made by a venerable geologist, that the cold 

 required to form and preserve such glaciers, for any length of time, 

 would freeze him to death, is as childish as the apprehension that the 

 heavy ocean currents, the action of which he sees everywhere, might 

 have swept him away.* 



Now that these phenomena have been observed extensively, we 

 may derive also some instruction from the limits of their geographi- 

 cal extent. Let us see, therefore, where these polished, scratched 

 and furrowed rocks have been observed. 



In the first place they occur everywhere in the north within cer- 

 tain limits of the arctics, and through the colder parts of the tem- 

 perate zone. They occur also in the southern hemisphere, within 

 parallel limits, but ' in the plains of the tropics, and even in the 

 warmer parts of the temperate zone we find no trace of these phe- 

 nomena, and nevertheless the action of currents could not be less 

 there, and could not at any time have been less there than in the 

 colder climates. It is true, similar phenomena occur in Central 

 Europe and have been noticed in Central Asia, and even in the 

 Andes of South America, but these always in higher regions, at 

 definite levels above the surface of the sea, everywhere indicating a 

 connection between their extent and the colder temperature of the 

 places over which they are traced. 



More recently, a step towards the views I entertain of this subject, 

 has been made by those geologists who would ascribe them to the 

 agency of icebergs. Here, as in my glacial theory, ice is made 



* Berlin Academy, 1846. 



