80 CHARACTERS OF STRATA 



deposits themselves, which present successions of 

 mud, of sand, and of gravel. There is no difficulty in 

 understanding the nature of this process ; since it is 

 the necessary consequence of the comparative sizes of 

 the materials, or, more correctly speaking, of the 

 differences of ratio between the contents and the 

 surfaces of the respective solids. The regular inter- 

 stratification of sand and mica on sea shores, is a 

 familiar example of this fact. 



Thus we provide, without difficulty, for one succes- 

 sion at least, of sand or gravel, and clay, the germs of 

 future sandstone and shale. By the assistance of 

 marine animals, whose colonies are distributed on the 

 bottom, we also obtain a calcareous stratum ; and we 

 are thus furnished, with the essence, at least, of every 

 stratum which nature has produced among the secon- 

 dary ones. Such of the causes of their variations as 

 we have been able to conjecture, are stated in their 

 proper place, as are those by which the primary and 

 secondary are distinguished. Whence those very 

 frequent alternations have arisen among them, geolo- 

 gists have not yet been able to discover ; but the 

 general principle is established, when it is shown that 

 a single series of three can be produced, and is ac- 

 tually formed, by these operations. It must indeed be 

 apparent, that in a series of revolutions, probably of a 

 very intricate nature, and through an incalculable 

 lapse of time during which the materials must have 

 often been changed, it is impossible to discover all the 

 circumstances by which these alternations have been 

 regulated. But it must still be remembered, that as 

 far as transportation and deposition alone are con- 

 cerned, we are bound to explain only the alternations 

 of sandstone and shale, or clay, in which there can be 

 no great difficulty. Those of limestones have been 



