Cause of the Glacial Period 
Effect of the Conversion of the Isotherm of 42° into that of 32°.—We are safe in supposing that if the 
isotherm of 32° once occupied the position of 42° of the present day, the climate along this line was very 
different from that which now prevails ; let us endeavour to define its conditions in outline. 
As regards America, it may be inferred that the Great Lakes were in their northern portions 
permanently frozen over like the waters of Hudson Bay throughout eight months in the year, while 
Labrador and the lands lying along the western shore of Hudson Bay and extending to the shores of the 
Great Lakes were covered by snow which the sun of summer would be unable to melt. There would 
not be much change in the condition of Greenland or of the neighbouring seas except in the direction of 
increase of cold and greater accumulation of snow and ice. As regards Scandinavia, we may safely infer 
that, owing to the increase of cold and the enormous precipitation of snow on the western slopes of 
the mountains, the snow-line would descend far below its present limits, and glaciers would enter the sea 
north of the Arctic Circle, where the ocean would resemble that of Davis Strait at the present day. 
That the highlands of the British Islands would be sufficiently cold to support perennial snow and glaciers 
may also be assumed. At the present day some of the highest parts of the Grampians are not much 
below the snow-line, and snow often lies on Ben Nevis and Ben MacDhui all the year round. But we 
need not follow the subject further except to observe that the additional accumulations of snow on the 
higher regions would tend to intensify the cold throughout all the adjoining tracts of Western and 
Northern Europe and Asia. 
Parr Il].—Errecrs or Erevation or Lanp 
But we must not forget that, as shown by Professor Spencer, and more recently by Mr. Warren 
Upham, the submerged platforms and river-valleys occur along the American coast at least as far north 
as the Susquehanna in lat. 42° N., while other drowned “ fjords” have been determined by Lindenkohl 
in connection with the Hudson—descending to 2250 and 2844 feet below the surface of the sea.!| These. 
features indicate elevation of the American Continent along the Atlantic coast, but though not to the 
extent which was indicated in the case of the Antilles, still sufficient to have produced very marked 
effects on the climate of Eastern America. If this be so, then to the cold produced by the lowering of 
the temperature of the Gulf Stream must be added that due to increased elevation of the continental land 
itself. The combined effect of these two factors would, as it seems to me, suffice to call into existence a 
glacial climate of great severity over the region lying to the north of the St. Lawrence and the Great 
Lakes. 
As regards the area of the British Isles and Western Europe a few words may be added to those on a 
previous page. It has been established, as above stated, by the observations of Mr. Godwin-Austen,? 
Prestwich,? Delesse,t and Rupert Jones,’ that the platform upon which they are built was elevated to the 
extent of the 100-fathom line, owing to which Great Britain was united to Europe on the east and Ireland 
on the west. The distribution of the land fauna and flora requires such an hypothesis ; as does the 
extension of the glaciers and sheets of ice over the area of the Irish Sea and the isles which border the 
western coasts of Ireland and Scotland from their centres of dispersion. At the time of this elevation, 
which, according to Mr. Godwin-Austen, was the close of the Pliocene period, Snowdon would have 
reached an elevation of 4200 feet, Ben Nevis and Ben MacDhui about 5000 feet each, and the Reeks, 
4014 feet. The whole region would have suffered a considerable decrease of temperature as compared 
1 Report U.S. Coast Survey for 1884, pp. 435-841 ; J. D. Dana, dm. Fourn. Sci. iii. vol. xl. pp. 425-437- 
2 Quart. Fourn. Geol. Soc. vol. vi. p. 69. 
8 Geology, vol. i. p. 118, and vol. vii. p. 118. 
4 Lithologie des mers de France, 1871. 
5 “ Antiquity of Man,” Rep. Croydon Micros. Club, 1877, p. 2. This paper is accompanied by a map showing the land area produced by an 
uprise of 600 feet (100 fathoms) above the present sea-level. It is remarkable that this platform corresponds in position to the Continental Shelf 
of Eastern America above described. The descent from the 100-fathom plateau to that of 1000 fathoms is remarkably steep along the western 
margin off the coasts of the British Isles, France, and Spain ; see Professor C. Wyville Thomson’s Zhe Depths of the Sea, Plate VII. Pp. 362 (1873): 
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