CONTINUITY OF DEPOSITION. 



89 



xcnange or alternation of Beds, 

 Coralline Oolite. 



mechanical and chemical agencies operating in waters under similar 

 conditions. 



Alternation of Beds. "When sets of strata are in contact as, for 

 instance, limestone lying upon sandstone it often happens that while 

 the limestone above and the sand- 

 stone below are unmixed with other 

 matter, there is a middle set of beds 

 composed of alternate layers of the 

 sandstone and limestone. Thus, let 

 a be the Coralline Oolite of England, 

 and b calcareous sandstone beneath ; 

 the middle beds a' a" b f b" are alter- 

 nately oolite and sandstone. 



In such a case, therefore, the two strata are said to exchange beds, 

 or to be subject to alternation at their junction, and the phenomenon 

 seems to have been occasioned by temporary cessations of the deposit 

 of sandstone allowing the limestone which would normally have been 

 only a cement to the sand to accumulate and form a limestone deposit. 



Calcareous Grit. 



Fig. 39. 





Gradation of 

 into one another 

 ceptible gradation ; as for 

 instance, the Oxford Clay of 

 the Yorkshire coast graduates 

 into the Calcareous Grit above 

 so completely, that the bluish 

 colour of the crumbling shale 

 below is shaded off without 

 any hard line into the yellow 

 solid beds of grit above, See 

 fig. 40. 



Beds. In other instances, the two strata pass 

 by imper- 



Gradation of Strata. 



Oxford Clay. 



Fig. 40. 



In either case it seems quite evident that no considerable break 

 or interval of time happened between the different contiguous de- 

 posits; one bed was no sooner formed than another was laid down 

 and deposited upon it. By careful study of these phenomena it 

 appears that, bed by bed, and rock after rock, the whole series of 

 strata, even to miles in thickness, were successively and almost un- 

 remittingly accumulated, and buried and covered up the shells and 

 other organic beings which were then living in the water, or on the 

 shore, or drifted into it from the land. The strata are, therefore, the 

 best witnesses of the lapse of time, and of the changing conditions of 

 land and water during their deposition. 



Proportions of Chemical, Organic, and Mechanical Deposits. 

 Assuming limestones to be of chemical or vital origin, and sand- 

 stones, clays, &c., to be mechanical deposits, and putting for the 

 present out of consideration the detached organic remains which 

 abound, especially in calcareous strata, we shall be able by comparison 

 of the thickness of the several rocks to present a tolerably accurate 

 notion of the relative proportions of chemical, organic, and mechanical 

 deposits. 



