PART I. CHAPTER VI. 



85 



Ancient Alluviums called Diluvium. 



nental areas, whether insensibly or by a repetition of sudden 

 shocks, was admitted as part of the actual course of nature, all 

 ancient alluviums were classed by some authors under the com- 

 mon title of " diluvium," and were said not to be due to existing 

 causes. To establish this proposition, it was thought sufficient to 

 demonstrate that the rivers which may now happen to drain a 

 given district, could never, in the course of thousands of ages, 

 have given rise to the valleys of denudation in which they now 

 flow, and that these same rivers could never have washed into 

 their present situations (often the summits of hills, and high table- 

 lands,) all the gravel and boulders evidently connected with for- 

 mer denuding operations. It was therefore usual to refer the 

 *' diluvium" to a deluge, or succession of deluges, which rolled 

 with tremendous violence over the land, after it had acquired its 

 present configuration, and its present height above the sea. Not 

 only small gravel, but large blocks of stone, were supposed to 

 have been transported from a distance by these devastating floods 

 or waves, and lodged upon the hill-tops. 



But rivers, as we have seen, are not the only existing causes, 

 nor even the most energetic agents, by which denudation may be 

 effected. If the upward movement of land be very slow, the 

 waves may easily clear away a stratum of yielding materials as 

 fast as they rise, and before they reach the surface. Thus, a 

 wide uninterrupted expanse of denudation may take place, and 

 masses, many hundreds of feet or yards in thickness, may waste 

 away by inches in the course of thousands of centuries. But if 

 reefs composed of a more refractory stone should at length rise 

 up, the breakers, as they foam over them, may still tear off" frag- 

 ments, and roll them along until the bottom of the sea becom.es 

 strewed over with blocks and pebbles. This alluvium of marine 

 origin will be uplifted when the reefs are ultimately converted 

 into land, and may then constitute the covering of the summits 

 of hills, or of elevated terraces, or table-lands. At the same 

 time, this gravel may be wanting in all valleys excavated either 

 during the rise of the land, by currents of the sea running be- 

 tween islands, or eaten out or deepened by rivers after the emer- 

 gence of the land. At the bottom of such more modern valleys 

 a distinct alluvium will be found, containing, perhaps, some peb- 

 bles washed out of the older or upland gravel, but principally 

 composed of the ruins of rocks removed during the erosion of 

 the newer valleys. 



It must be remembered, that when we introduce such an hy- 

 pothesis, and take for granted the rise of the land out of the sea, 

 we are merely supposing what we know, from the discovery of 

 marine fossils, to have happened again and again, at former periods. 



H 



