298 Searles V. Wood—The Cause of the Glacial Period. 
agreed, but J. D. Dana has acutely perceived that this ice-mass was 
due, not to any invasion of the area by ice from the Polar region, 
but to the fall of snow over the area itself,! proportionally to the 
existing precipitation in the different States, as shown by Schott’s rain- 
map; its greatest thickness being, in Dana’s view, ‘over the region 
of greatest precipitation somewhere between the line from Wisconsin 
to Lake Winnipeg, and beyond, and that of the Atlantic coast.” 
West of this glaciated area, the whole of the Central part of the 
Continent, i.e. “the great region between Western Iowa and the 
Sierra Nevada in California and the country north to an unknown 
distance,’ is unglaciated, the higher valleys only of the mountain 
portion of this region, and of the Sierra itself, where the peaks range 
between 12,000 and 14,500 feet of elevation, having during the 
Glacial period been occupied by ice. That ice was, however, in the 
condition of local glaciers, and unconnected with the great Lauren- 
tian mass to which the glaciated area just defined owes its features. 
Now the present precipitation of the Eastern side of North 
America does not diminish much from what it is in the area 
immediately surrounding the Gulf of Mexico, being between 40 and 
50 inches annually throughout the States of Indiana, Ohio, Penn- 
sylvania, New York, New Jersey, the New England States, and 
Maine, which, together with the corresponding part of Canada, was 
1 Otto Torrell, the Scandinavian glacialist, tried in a paper before the meeting 
of the American Association in 1877 (American Journal of Science, series il. 
vol. xiii. p. 76) to show the contrary of this, insisting that Greenland was the source 
of the American ice, and observing, ‘‘that if we bear in mind the certainty that 
during the Glacial period the glaciers moving from the heights of Greenland towards 
the sea could not have formed detached icebergs as now, but must have for the time 
blocked up all avenues except the one of easiest escape for the immense accumulations 
of ice, we may easily assume that this avenue was south-westwards across British 
America and the north-eastern part of the United States.” As Prof. Torrell supports 
this view with a statement that ‘‘the Scandinavian Glacier crossed the Baltic and 
German Ocean, and extended its moraines into the suburbs of London,’’ which 
(without trenching on the moot question whether the Scandinavian ice reached the 
Orkneys or Shetlands) my intimate knowledge of the Glacial beds of East Anglia 
tells me is utterly contrary to the fact, I do not attach any weight to his view ot 
the source of the American ice. Moreover, as explained in the text, I doubt the 
great volume of the Greenland ice during the Glacial period. 
2 Same Journal, vol. xv. p. 250. 
3 Dana in same Journal, vol. xv. p. 250. G. W. Dawson speaks (Q. J. G. 8S. 
vol. xxxi. p. 617) of a drift occurring over the region between this western edge of the 
glaciated area and the Rocky Mountains, but he explains that it is drift mainly 
derived from those mountains, and but subordinately from the glaciated area to the 
east and north-east. This he attributes to ice floating in water of some kind, and it 
has occurred to me that such water may have originated in part from the drainage 
from the Rocky Mountains, which now reaches Hudson’s Bay, through Lake Winnipeg, 
having been blocked from this escape by the Laurentian ice thus filling that and the 
other lakes, and in other part from the effluent water of this edge of the ice-mass ; the 
combined water thus resulting rising above the level of the low parting between the Red 
River (which now drains to Winnipeg and Hudson’s Bay) and the Minnesota River 
(which runs into the Mississippi), and so escaping by the Mississippi to the sea. Gen. 
Warren (Amer. Journ. of Sci. vol. xvi. p. 417) shows that the Red River once 
flowed this way, though he places the time of its doing so as subsequent to the 
Glacial period. I have, however, found phenomena in England which have been 
regarded as posterior to the Glacial period to be really coeval with it, and it may be 
the same in America. | 
