324 SURFACE GEOLOGY. 
The facts seem to bear out the theory presented above of radial dis- 
persion from the Labrador peninsula in all directions except west and 
north-west. Observations are wanting for that part of British America. 
There seem to have been three centres of dispersion for our continent 
when the ice possessed its maximum development: Greenland, Labra- 
dor, and the Rocky Mountains. There seems, also, to have been a move- 
ment of icebergs up the St. Lawrence valley, possibly coeval with the 
Orange sand deposit. 
CAUSE OF THE GLacIAL Comp. 
Various theories have been proposed to account for the reason of the 
very severe climate of the glacial period. First, was the view that 
earthquake-paroxysmal waves passed southerly over both continents. 
Second, came the iceberg theory, involving a submergence of over 6000 
feet to explain all the phenomena. Third, the extreme glacial theory 
received much favor, where an elevation of the land was relied upon to 
produce the cold. If the land were extensively elevated, extreme cold 
would result, both because of increased cold in mountainous regions, and 
the deflection southerly of the warm oceanic currents. Both the iceberg 
and extreme glacial theories involve more extensive earth-movements 
than have appeared before in geological history, and hence do not fully 
commend themselves to general acceptance. There is no objection to 
the adoption of a modified glacial theory, such as has been advocated by 
A. S. Packard, Jr., where only 600 feet of elevation is called for to ex- 
plain the phenomena.* Prof. Dana, in the last edition of his Manual, 
seems to accept a modification of the enormous elevation—sooo feet— 
advocated in the first edition, though he does not say how great an up- 
rising of the land is called for. 
The less extreme our theories, and the less the variation from existing 
conditions required by our suppositions, the nearer will be their approxi- 
mation to truth. The following conditions probably existed, and by 
their combination brought about the extreme cold. 1. Elevation of land 
in the northern part of the continent for a few hundred feet, accom- 
panied by the necessary partial withdrawal of the warm oceanic currents 
in both the Atlantic and Pacific oceans. 2. Coincidence of longer 
* Memoirs Boston Society of Natural History, vol. i, p. 260. 
