Ch. XII.] AMERICAN GLACIAL FORMATIONS. 163 



Afterward the heel of this sea was converted into dry land, and 

 underwent several oscillations of level, so as to he first land, support- 

 ing a forest, then an estuary, then again land, and finally a sea near 

 the mouth of a river, till the downward movement hecame so great 

 as to convert the whole area into a sea of considerable depth, in 

 which much floating ice carrying mud, sand, and boulders melted and 

 let fall its burthen to the bottom. Finally, over the till, with boul- 

 ders, stratified drift was formed, after which, but not until the total 

 subsidence had amounted to more than 400 feet, an upward move- 

 ment began, which reelevated the country, so that the lowest of the 

 terrestrial formations, or the forest bed, was brought up to nearly its 

 pristine level in such a manner as to be exposed at low tide. Both the 

 descending and ascending movements seem to have been very gradual. 



GLACIAL FORMATIONS IN NORTH AMERICA. 



In the Western Hemisphere, both in Canada and as far south as 

 the 40th and even 38th parallel of latitude in the United States, we 

 meet with a repetition of all the peculiarities which distinguish the 

 European boulder formation. Fragments of rock have travelled for 

 great distances, especially from north to south : the surface of the 

 subjacent rock is smoothed, striated, and fluted ; unstratified mud or 

 till containing boulders- is associated with strata of loam, sand, and 

 clay, usually devoid of fossils. Where shells are present, they are 

 of species still living in northern seas, and half of them identical 

 with those already enumerated as belonging to European drift. The 

 fauna also of the glacial epoch in North America is less rich in 

 species than that now inhabiting the adjacent sea, whether in the 

 Gulf of St. Lawrence, or off the shores of Maine, or in the Bay of 

 Massachusetts. 



The extension on the American continent of the range of erratics 

 during the Post-pliocene period to lower latitudes than they reached 

 in Europe, agrees well with the present southward deflection of the 

 isothermal lines, or rather the lines of equal winter temperature. 

 It seems that formerly, as now, a more extreme climate and a more 

 abundant supply of floating ice prevailed on the western side of the 

 Atlantic. 



Another resemblance between the distribution of the drift fossils 

 in Europe and North America has yet to be pointed out. In Canada 

 and the United States, as in Norway, Sweden, Scotland, and Europe 

 generally, the marine shells are confined to very moderate elevations 

 above the sea (between 100 and 700 feet), while the erratic blocks 

 and the grooved and polished surfaces of rock extend to elevations 

 of several thousand feet, 



I described in 1839 the fossil shells collected by Captain Bayfield, 

 from strata of drift at Beauport, near Quebec, in lat. 47°, and drew 



