EFFECTS OF WINDS, AND OF GLACIERS. 269 



their destructive marc.i when the burning winds awaken. History tells 

 of populous cities and fertile plains, where nothing but blown sands are 

 now to be seen, and geology easily leads us back to still more remote 

 periods, when the broad zones of sandy desert were but narrow stripes 

 if blown sand along the shores of the sea, or beds of comparatively loose 

 sand-sfone, which here and there came to the surface, and which the 

 winds have gradually removed from their original site, and wafted widely 

 over the land. 



Wherever these sand-drifts spread, it will also be clear to you, that 

 there may be no necessary similarity between the loose materials on 

 the surface and the kind of rock over which these materials are strewed. 



5°. Along with these I shall mention only one other great agent by 

 ^which loose materials are gradually transported to considerable dis- 

 tances. 



It is observed in elevated countries, where the snow never entirely 

 melts, and where glaciers or sheets of ice hang on the mountain sides, — 

 descending towards the plains as the winter's cold comes on, and again 

 retreating towards the mountain-tops at the approach of the summer's 

 heat — that the edges of the glaciers bear before them into the valleys, and 

 deposit along their edges, banks of conical ridges of sand and gravel 

 (Moraines). These con^st of the fragments of the rocky heights, worn 

 atid rounded by the friction of the sheets of ice beneath which they 

 have descended from above, and from the edges of which they finally 

 escape into the plain. 



These ridges of sand and gravel accumulate till some more sudden 

 thaw than usual, or greater summer's heat arrives, when they are more 

 or less completely broken up by the rush of water that ensues, and are 

 dispersed over the subjacent tracts of level land. 



When the rocks are of a kind to rub down so fine as to form much 

 mud as well as sand or gravel, the ridges are of a more clayey charac- 

 ter. And where the edges of the glaciers descend to the borders of lakes 

 or seas — as in the Tierra del Fuega — this mud is washed away and 

 widely spread by the waters, while the gravel and sand remain nearer 

 their original site ; or, finally, when the ice actually overhangs the wa- 

 ter, huge fragments break off' now and then — loaded with masses of gra- 

 vel and sand, or even with rocks of large size, — which fragments float 

 away often to great distances and drop their stony burdens here and 

 there, as they gradually melt and disappear. 



To these facts, let it be added, that recent geological researches, of a 

 very interesting kind, tend to show that nearly all the elevated tracts of 

 country in the temperate regions of Europe and America — in our own 

 island among other localities — have been covered with glaciers at a 

 ^comparatively recent period, (geologically speaking,) and that these gla- 

 ciers have gradually retreated step by step to their present altitudes, 

 halting here for a time, and lingering there ; — and we shall find reason 

 to believe that trsi^es of transported materials — moved from their origi- 

 nal site by this agent also — are to be looked for on almost every geolo- 

 gical formation. 



And such the geological observer finds to be in reality the case. 



