Oh. XII.] CAUSES OF CHANGE OF CLIMATE. 175 



lapse of geological time lias been sufficiently protracted. But suppose 

 the submerged area to have been continually traversed by huge ice- 

 bergs like Baffin's Bay, for thousands of years before it became part 

 of the continent. In that case we should not only find on it a multi 

 tude of morainic lakes of various sizes, but probably many shallow 

 saucer-like cavities worn in the bed of the sea, out of rocks in situ, by 

 the reiterated impinging upon them of huge masses of ice, moving (as 

 before described, p. 148) in their lowest parts with a velocity of as many 

 miles as even the uppermost strata of a glacier move iuches. The 

 wiuds and currents might carry hundreds of such bergs during every 

 century toward the same tracts, and these might exert a great amount 

 of friction on the floor of the ocean. The mud and sand formed by 

 the abrasion of rock, or any stones which might be frozen into the 

 bottom of the iceberg, or driven into it when the mass impinged with 

 great force on the bed of the sea, may be removed as soon as the berg, 

 by melting in its upper part, becomes lighter, and rising floats away. 

 In this instance the conditions are more favorable, both for triturating 

 a rocky floor and clearing out earth and stones from the new-formed 

 cavity, than are conceivable in the case of a glacier descending a valley. 

 Causes of Change of Climate. — Submergence of the Sahara. — I 

 endeavored in 1 830, in the " Principles of Geology," chapters vii. and 

 viii., to point out the intimate connection of climate with the state of 

 the physical geography of the globe existing at any given period. If, 

 for example, at certain periods of the past, the antarctic land was less 

 elevated and less extensive than now, while that at the north pole was 

 higher and more continuous, the conditions of the northern and south- 

 ern hemispheres might have been to a great extent the reverse of what 

 we now witness in regard to climate. But if in both of the polar re- 

 gions a considerable area of elevated dry laud existed, such a concur- 

 rence of refrigerating conditions in both hemispheres might have 

 created for a time an intensity of cold never experienced since. Some 

 geologists have objected that the cold of the glacial period was so 

 general throughout the polar and temperate regions on both sides of 

 the equator, that mere local changes in the external configuration of 

 our planet cannot be imagined to afford an adequate cause for a 

 revolution in temperature of so modern a date. But the more we 

 compare the state of the earth's surface in pliocene, post-pliocene, and 

 recent times, the more evidence do we obtain of upward and down- 

 ward movement on such a scale as to convince us that in different parts 

 of the periods in question a map of the world would no more resem- 

 ble our present maps than Europe now resembles America or 

 Africa. A careful study of the distribution of the living species of 

 animals and plants in tertiary and recent times leads to similar con- 

 clusions as to the vastness of the changes which the physical geogra- 

 phy of the globe has undergone, so that the theory in question cannot 

 be impugned on the score of a want of universality in the movements 

 of the earth's crust. 



