GLACIO-AQUEOUS ACTION IN NORTH AMERICA. 179 



their course at a lower level. But the change, so far as I have 

 noticed, is just the reverse of what, by this theory, we should ex- 

 pect ; that is, the course of the striae at the common levels is more 

 oblique or cross-wise to the mountain ridges than at their top. 



The only remaining theory that suggests itself, supposes that 

 the striag at the lowest as well as at the highest levels, were pro- 

 duced by tlie same icebergs, which were large enough to reach 

 the bottom of the valleys, and which, notwithstanding, have been 

 urged over the mountains. The latter point appears to present 

 the chief difficulty ; for some modern icebergs rise three hundred 

 feet above the sea, and consequently reach to the depth of tsvo 

 thousand four hundred feet; and Capt. Ross saw one sti-anded 

 in Baffin's Bay, in water one thousand five hundred feet deep. 

 How much deeper may we suppose such an iceberg would 

 reach, as Capt. Parry saw, on which he travelled for some time, 

 supposing it to be fixed, but found afterwards that it was floating 

 him southerly as fast as he advanced northerly ? 



What proportion of the surface of New England, if swept of 

 soil, would be found thus worn down and striated, it would be 

 presumption to say definitely. But from all that I have seen, for 

 the last fifteen years, I should not be surprised to find one third, 

 or even one half of the surface, exhibiting traces of glacio-aqueous 

 agency. 



The direction of the striae coincides, so far as I know, precisely 

 with that taken by the transported bowlders. And in order to 

 bring at once under the eye the facts on this subject, I have 

 constructed a Map of tlie north part of America, Plate VII. 

 The red stripes upon it represent the direction of the striae, as 

 they have been measured in different places ; and the green 

 stripes show the course taken by the bowlders. It will be seen, 

 however, that in some parts of the country, there is an apparent 

 disagreement between the green and red stripes. This results 

 from the fact, that observers, in speaking of the direction taken 

 by bowlders, use general terms, such as northerly and northwest- 

 erly ; whereas the exact course by the compass is usually given 

 when striae are described. Hence the discrepancy is only appa- 

 rent. But it is evident that the striee are most to be rehed upon 



