U2 THE BOOK OF NATURE STUDY 



mechanical deposit are connected with the relative stillness, and 

 hence with the depth of water. If the ocean were to become deeper, 

 organic deposits would encroach upon the mud and sand of a 

 previous epoch. On the other hand, a shallowing of the sea, such as 

 accumulating sediment causes, would result in mechanical deposits 

 encroaching on and covering organic deposits, and in the coarser 

 mechanical sediments overlapping the finer grained. At the end 

 of a long period of such changes we should expect to find an 

 alternation of large sheets of deposits of these kinds, a phenomenon 

 precisely similar to that known to geologists as bedding or strati- 

 fication in the rocks. It will be obvious that the material of any 

 bed may be laminated, and that the plains of lamination and 

 bedding will be approximately parallel, except in the case of 

 false-bedded deposits. This is also the character of rock strati- 

 fication, which may be defined as the occurrence of the rocks in 

 successive sheets, inches or feet in thickness, which differ from one 

 another in a marked degree in colour, character, or composition 

 (Fig. 18). Evidently this stratification is only a magnified form 

 of lamination, and it is really not possible to draw an exact line 

 between the two (Fig. 19). Both laminae and strata may vary 

 considerably in thickness ; in the case of laminae to the extent 

 of fractions of an inch, while strata may be a few inches or, in 

 extreme cases, hundreds of feet in thickness. 



The deeper water strata are the most regular in thickness and 

 extent, as is the case with laminae, a character also borne out 

 in the rocks. A comparison 6f the structures in sediments and 

 rocks brings out the fact that they are identical in all vital 

 particulars, the containing and position of organisms, the lie of 

 the constituent fragments, and the parallelism of strata to the 

 laminae they are made of in relation to the coarseness or fineness 

 of deposit. 



Shallow-water deposits, whether formed in the sea, in lakes, or 

 in rivers, are likely to show peculiarities worthy of observation. 

 Current or wave action will give rise to a ribbed or ripple-marked 

 surface of the sand, surfaces exposed in shallow-water or between 

 tides are likely to show tracks of animals (Fig. 20), casts or burrows 

 of worms, and there may even be present in exceptional cases the 

 cracks formed when mud or sand dries under the sun at low tide, 



