.SEDIMENTATION, ENVIRONMENT, AND EVOLUTION. 7 



salts in sea-water,* proceeds pari passu with the deposition of 

 sand and silt or even fine shingle. For this reason, the majority 

 of sediments, both ancient and modern, are of mixed " grade " 

 and carry more or less clayey and silty material. Perfection 

 of grading (that is, the attainment of perfect evenness of size) 

 is a true phenomenon and is rarely met with in geological strata 

 or present-day sediments. Only by the long-continued action 

 of wind and water currents, particularly wind, are clastic 

 materials sorted effectively. 



In Fig. 1, representing ideal conditions, a, 6, c, and d 

 indicate in section the belts of shingle, sand, silt and mud 

 respectively. It is instructive to consider the effect on their 

 distribution of earth-movements such as are continually in 

 progress. Omitting from present consideration the change in 

 the proportion of the various river-borne constituents resulting 

 from increased or diminished denudation due to the con- 

 sequential greater or less elevation of the country (see p. 10), 

 it is evident that upon subsidence of the area, a corresponding 

 series of deposits will be laid down upon the new sea-bed 

 represented by XX. In consequence of the creep of the sea 

 over the land, each belt of detritus a\ 6', c', and d' will occur 

 slightly landward of the corresponding one below, and not 

 exactly vertically over it. A continuation of the subsidence 

 will result in a further series a", b" , c", and d" occupying the 

 position indicated. If the movement has been gradual and not 

 intermittent, the planes XX, YF, which are true time-planes, 

 may be obliterated, and the fact that the deposits a, 6, c, and d, 

 or a', 6', c', and d\ are contemporaneous, may not be obvious. 



Subsequent elevation and possibly denudation may result 

 in such a series of rocks forming the surface of the land. In 

 the event of a, 6, c, and d not containing fossils, or, what 

 provides equal difficulty, containing different fossil forms as a 



* River-water containing salts of calcium and magnesium in solution 

 also flocculates mud and causes its deposition. 



