Searles V. Wood—The Cause of the Glacial Period. 299 
the part of the glaciated area where the ice was thickest, diminishing 
gradually westwards through Illinois, lowa, and Minnesota, to 12 
inches as the unglaciated region is reached; the line of small 
precipitation thus marking the beginning of the unglaciated region 
bending northward with the isothermal. 
It is evident that the precipitation to which the glaciation of the 
Eastern side of North America was thus due could have arisen only 
from the Gulf Stream, which follows the American coast up to 
Newfoundland, because the dryness of the central region shows that 
the precipitation on this side cannot be due to vapour coming across 
the continent from the Pacific, and the Atlantic as far south as the 
Gulf Stream must during the Glacial period have been, as Hudson’s 
Bay and Davis’ Straits now are, frozen over during most of the year ; 
so that the only sea which could supply much vapour to this side 
of the continent was that occupied by the Gulf Stream. 
The Pacific coast west of this great unglaciated area of the 
United States has, south of latitude 40°, a small rainfall only, and 
is unglaciated except in the mountain valleys; but northwards of 
this the precipitation increases panel so that from being only 
9 inches at St. Diego (in lat. 31° 30’), and 20 inches at St. Francisco 
(in lat. 85°), it ranges, save at some exceptionally dry stations, from 
55 to 123 inches between lat. 40° and 49°;' and as this precipitation 
becomes great, the evidences of former glaciation present themselves. 
The glaciation ‘of North America therefore so far from being con- 
nected with the diversion into the southern hemisphere of the ocean 
currents of the northern, was the direct result of the precipitation 
engendered by these currents having taken place under a refrigera- 
tion of climate produced by some cosmical cause that did not 
interfere with them; and so far from the theory of the late Mr. 
Hopkins (which thirty years ago found as much favour as Dr. 
Croll’s has at a later date), that the Glacial period was caused by 
a diversion of the current from the Gulf of Mexico up the valley of 
the Mississippi, and “into the Arctic sea along the eastern base of 
the Rocky Mountains,” by the submergence of that valley, having 
any foundation, geological research since then has shown the exact 
contrary to have been the case; for if such diversion had occurred, 
the regions of greatest precipitation and glaciation, and the regions 
of dessication and non-glaciation would have changed places, and the 
unglaciated centre of the United States been glaciated down to the 
edge of this supposititious strait, while the north-easternmost States 
and Hastern Canada would have been much less glaciated than they 
are, if not altogether unglaciated. So far also from the glaciation of 
Europe having been due to this diversion, as Hopkins suggested was 
the case,” the evidence of the land-ice in both Britain and Scandinavia 
shows that the glaciation of both these countries was the result of 
the vapour of the Gulf Stream precipitated ape them under the 
conditions of this cosmical refrigeration. 
It is to my mind therefore clear that the Glacial period was in no 
1 Schott’s tables, pp. 77-79. 
2 See Hopkins in Q. J. G. S. vol. vill. pp. 89 to 92. 
