76 CHARACTERS OF STRATA 



received the materials that have formed the differed! 

 secondary strata, it is plain that the furthest limit of 

 identity among these strata thus deposited beneath 

 it, will be bounded by the extent of any one ; while, 

 in different seas, we must expect different, although 

 analogous, collections of strata. It is not however 

 neccessary that, even in one submarine cavity, or 

 within the limits of one sea, the strata should be every 

 where identical. Different rivers may have entered 

 by different aestuaries, and the depositions from these 

 must have varied in character and manner, according 

 to the materials which they deposited, or according 

 to the nature of the mountains in which their sources 

 lay. This supposition is confirmed by examining the 

 English series, of which an account is given in a future 

 chapter. There is a certain general order in that part 

 of it which lies about the coal strata, such as may 

 make us conclude that it was deposited in one period 

 of repose and in one cavity; but if we examine the 

 proportions and positions of the different members 

 throughout the whole space, there are differences 

 which bespeak analogous variations in the deposited 

 materials. 



This is confirmed in another way, by examining 

 those submarine depositions, now within our reach, 

 which are probably destined to form future strata, 

 and which are, to all appearance, the copies of those 

 from which the consolidated strata have originated. 

 It is so well known to mariners that the alluvia which 

 cover the bottom of the English Channel differ in 

 different places, that these variations are used as a 

 guide in navigating that sea. It is by the quality of 

 the soundings, as these may consist of sand, of mud, 

 of shells of particular character, or of various mixtures 

 of those, that the pilot determines his place when the 



