540 THE POPULAR SCIENCE MONTHLY. 



As facts accumulated, however, it was noticed tbat the sides and bot- 

 toms of such lakes are smoothed, in many cases polished, and almost 

 always covered with grooves and scratches ; and also that in their 

 vicinity beds of clay are usually found, intermixed with pebbles and 

 large bowlders which, like the rocky basins, are also smoothed and 

 frequently scratched. It was noticed, too, that the rock from which 

 these bowlders and pebbles had been formed commonly differed from 

 the rocks in place on the shores of the lakes. Thus, throughout New 

 York and Ohio, huge bowlders are common, composed of crystalline 

 rock found in place nowhere nearer than the Canadian Highlands, a 

 hundred miles to the northward ; while the peculiar native copper 

 of Northern Michigan is sometimes found mingled with the bowlders 

 and striated stones of the drift far southward in Ohio. 



The problem now was to discover what forces in Nature could 

 polish and scratch both rock-surfaces and detached stones, and could 

 also transport masses of rock, tons in weight, far from their native 

 home. 



It is well known that the loose stones and pebbles along the sea- 

 shore are made very smooth and round, and often polished, by the ac- 

 tion of the waves. It might be thought from this that the pebbles 

 found on the shores of the lakes, and imbedded in the clays, were 

 fashioned in the same manner. On one occasion, at the Cape of Good 

 Hope, the writer, after wandering for a time along the sloping sandy 

 beach of Table Bay, came suddenly to a little rocky cove exposed to 

 the full swell of the South Atlantic. As each wave broke on the 

 steep, rocky beach and retreated, it was followed by a sharp, rattling 

 sound that could be distinctly heard above the roar of the waves ; we 

 noticed, too, that the stones all along the shore were in motion, rolling 

 down the beach, only to be caught up by the next white-capped wave 

 that came in from the ocean, and again carried up the beach, and 

 rolled and pounded against each other by the untiring waters, that 

 were fast reducing them to sand and dust. On examining these 

 water-worn stones, we found them all smoothed and rounded, and 

 often beautifully polished; but in no case could we discover, even 

 with a magnifying-glass, any that were scratched, or in any way 

 marked similarly to the stones which we have so often examined in 

 the clays and hard-pans that cover so great a portion of our Northern 

 States. From this fact, and also from watching the action of the 

 waves on many other coasts, we conclude that the sea tends to 

 smooth and wear away the stones and rocks along its shores, but has 

 no power to cover them with grooves and scratches ; and that, instead 

 of wearing the coast into pockets and basins, it tends only to griud 

 down the islands and continents to one uniform level. 



Again, we have traversed the deep, picturesque valleys of the 

 southern Alps, where we could see the glaciers glittering on the 

 mountain-sides far up at the head of the valley, and have noticed as 







