1 82 CONSOLIDATION OF SEDIMENTS 



if the rise and fall of the tide be great. On these flats, exposed 

 during low tide to the sun and air, sun-cracks are formed on the dry- 

 ing surface, and these, together with the prints of raindrops and the 

 tracks of land animals, will be preserved when the incoming tide, 

 advancing too gently to scour the slightly hardened surface of the 

 flat, deposits a fresh layer of sediment upon it. If the estuary be 

 the opening of a large river, considerable deposits of river sedi- 

 ment will, in times of flood, be laid down upon the other beds, 

 producing an alternation of fresh and brackish water beds. On 

 the coast of North Carolina somewhat peculiar conditions obtain ; 

 the low sand-spits thrown up by the waves enclose extensive shal- 

 low sounds, into which the tide enters by only narrow openings, 

 but which have numerous streams flowing into them. At high 

 water the incoming tide acts as a barrier, damming back the 

 river waters, checking their velocity, and causing them to deposit 

 their burdens of sediment. In course of time, the sounds must 

 be silted up by the rivers. 



For reasons that we have already discussed, estuaries are not 

 favourable to either fresh-water or marine organisms, and hence 

 estuarine deposits do not contain any great variety of remains of 

 either group. These remains may, however, represent numerous 

 individuals, sufficient sometimes to form limestone layers. Diatoms 

 may also accumulate in great quantities, as in one of the Baltic 

 harbours, where they form 1 8,000 cubic feet of deposit annually. 

 On the other hand, estuaries are often favourably situated for the 

 reception and preservation of the remains of land animals and 

 plants which are swept into them by streams. 



The Consolidation of Sediments 



The processes of deposition upon the land and beneath the 

 water, which we have so far been studying, result, for the most 

 part, only in the bringing together of great masses of loose and 

 incoherent material. If 'such masses are to be properly com- 

 pared with the hard rocks of the earth's crust, it will be necessary 

 to show that loose sediments may be consolidated and rendered 



