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Show 132 ISLAND LIFE. [PART r. elevated borders; while the polar area is, with the exception of Greenland and a few other considerable islands, almost all water. In the southern hemisphere the temperate zone is almost all water, while the polar area is almost all land, or is at least inclosed by a rinct of hicth and mountainous land. The result is that in 0 0 the north the polar area is free from any accumulation of per-manent ice (except on the highlands of Greenland and Grinnell's Land), while in the south a complete barrier of ice of enormous thickness appears to surround the pole. Dr. Croll shows, from the measured height of numerous Antarctic icebergs (often miles in length) that the ice-sheet from which they are the broken outer fragments must be from a mile to a mile and a half in thickness.1 As this is the thickness of the outer edge of the ice it must be far thicker inland; and we thus find that the Autarctic continent is at this very time suffering glaciation to quite as great an extent as we have reason to believe occurred in the same latitudes of the northern hemisphere during the last glacial epoch. The accompanying diagrams show the com pamtive state of the two polar areas both as regards the distribution of land and sea, and the extent of the icc-sheet and floating icebergs. The much greater quantity of ice at the south pole is undoubtedly due to the presence of a large extent of high land, which acts as a condenser, and an unbroken surrounding ocean, which affords a constant supply of vapour; and the effect is intensified by winter being there in aphelion, and thus several days longer than with us, while the whole southern hemisphere is at that time farther from the sun, and therefore receives less heat. We see, however, that with less favourable conditions for the production aml accumulation of ice, Greenland is glaciated down to Lat. 61°, What, then, woulcl be the effect if the Antarctic continent, instead of being confined almost wholly within the south polar circle, were to extend in one or two great mount<1inous p-romontories far into the temperate zone 1 The comparatively small Heard Island in S. Lat. 53° is even now glaciated down to the sea. What would be its condition 1 "On the Glacial Epoch,'' by James Croll. Geol. },fag. July, August, 1874. CHAP, Yilt.) 'l'IIE CAUSER OF GLACIAL EPOCHS. ~"' s ~ ~ ~ -<l H 0 "" ~ !-t p:; 0 "" .,; I« ~ ~ j 0 "" ~ E-< ~"' 133 ~ "' ~ ~ ~ ~ gj 0 p, :S - ~ 0 w ~ "0 § ~ ~ :9 "0 § ~ .."<.J'. t:o I': ~ 0 &: "0 ~ ..".', ~ <I> § ~ .~.... ..0. , ~ .25 ~ ~ ·e~ p. p. d :"S' ..... 0 s ~ .~0 A |