APPENDIX H: LYELL 437 



experience for the last three thousand years as an argument against 

 the probability of such occurrences in past ages ; but it is not pre- 

 tended that such a combination can be foreseen. 



In speculating on catastrophes by water, we may certainly antici- 

 pate great floods in future, and we may therefore presume that they 

 have happened again and again in past times. The existence of 

 enormous seas of fresh water such as the North American lakes, the 

 largest of which is elevated more than six hundred feet above the 

 level of the ocean, and is in parts twelve hundred feet deep, is alone 

 sufficient to assure us, that the time will come, however distant, when 

 a deluge will lay waste a considerable part of the American continent. 

 No hypothetical agency is required to cause the sudden escape of the 

 confined waters. Such changes of level, and opening of fissures, as 

 have accompanied earthquakes since the commencement of the 

 present century, or such excavation of ravines as the receding cataract 

 of Niagara is now effecting, might breach the barriers. Notwith- 

 standing, therefore, that we have not witnessed within the last three 

 thousand years the devastation by deluge of a large continent, yet, 

 as we may predict the future occurrence of such catastrophes, we are 

 authorized to regard them as part of the present order of Nature, and 

 they may be introduced into geological speculations respecting the 

 past, provided we do not imagine them to have been more frequent 

 or general than we expect them to be in time to come. 



The great contrast in the aspect of the older and newer rocks, in 

 their texture, structure, and in the derangement of the strata, ap- 

 peared formerly one of the strongest grounds for presuming that the 

 causes to which they owed their origin were perfectly dissimilar from 

 those now in operation. But this incongruity may now be regarded 

 as the natural result of subsequent modifications, since the difference 

 of the relative age is demonstrated to have been so immense, that, 

 however slow and insensible the change, it must have become im- 

 portant in the course of so many ages. In addition to the volcanic 

 heat, to which the Vulcanists formerly attributed too much influence, 

 we must allow for the effect of mechanical pressure, of chemical 

 affinity, of percolation by mineral waters, of permeation by elastic 

 fluids, and the action, perhaps, of many other forces less understood, 

 such as electricity and magnetism. In regard to the signs of up- 

 raising and sinking, of fracture and contortion in rocks, it is evident 

 that newer strata cannot be shaken by earthquakes, unless the sub- 



