78 ON THE REVOLUTIONS OF 



parts, we clearly see that the last revolution, and consequently the 

 establishment of present society, cannot be very ancient. It is one of 

 those results, though most clearly proved, is the least regarded in sound 

 geology; a result the most valuable, as it unites, in an unbroken chain, 

 natural and civil history. 



In measuring the effects produced in a certain time by actual 

 causes, and in comparing them with those which they have produced 

 since the commencement of their operations, we can determine 

 nearly the very moment whence their action may be dated, which is 

 necessarily the same as that when our continents received their present 

 form, or that of the last sudden retreat of the waters. 



It is in fact from this retreat that we must begin to calculate the 

 wearing away of our steep eminences, and the formation of hilly 

 remains at their bases ; that our present rivers began to flow and to 

 deposit their alluvial spoils ; that our present vegetation began to 

 extend itself and to produce mould ; that our present cliffs began to 

 be worn away by the sea, and that our present downs began to be 

 accumulated by the wind. Also from this epoch must we calculate 

 that colonies of the human race commenced or re-commenced to 

 spread themselves abroad, and to form establishments in places which 

 nature had assigned to them. I do not speak of volcanoes, not only 

 because of their irregular irruptions, but because nothing proves that 

 they could have existed beneath the sea, and therefore they are of no 

 service in proving what lapse of time has occurred since the last 

 retreat of it. 



Lands Gained by the Perpetual Deposit of Alluvial Matter by Rivers. 

 MM. Deluc and Dolomieu have most attentively examined the pro- 

 gress of the lands formed by the deposites of the rivers; and, although 

 at issue on a great number of points relating to the theory of the 

 earth, they agree to this : that these alluvial accumulations increase 

 very rapidly, and must have augmented much more quickly at first, 

 when mountains afforded more materials for streams, and yet their 

 extent is but very limited. 



The memoir of Dolomieu on Egypt * tends to prove, that, in the 

 time of Homer, the tongue of land on which Alexander built his city 

 was not then in existence ; that they were able to navigate from the 

 island of Pharos into the gulf since called lake Mareotis ; and that 

 this gulf was then from fifteen to twenty leagues long, as stated by 

 Menelaus. The nine centuries then between Homer and Strabo were 

 sufficient to bring matters to the state described by the latter, and to 



* Journal de Physique, tome xlii, p. 40, &c. 



