374 ON THE ALLUVIAL DEPOSITS. 



dency is to remove, at certain times or places, what it 

 had before deposited ; from variations in the forms 

 of the land itself, the nature and effects of which I 

 formerly discussed. 



I may rank the transference of sand with the mari- 

 time alluvia ; though there is one conspicuous case of 

 A different nature. Being generated under the water 

 from quartz or shells, and deposited at high water- 

 mark, it is dispersed by the winds ; often overwhelm- 

 ing large tracts, and forming such sand hills as those 

 of Holland, the Baltic, Vera Cruz, and so forth. In 

 favourable circumstances, it may also generate land ; 

 and thus Tirey appears to have been almost entirely 

 produced in this manner, from reefs of bare rock. In 

 such cases, it is consolidated by vegetation, as its pro- 

 gress may be arrested by the well-known plants cre- 

 ated for this especial end. But it is not always inju- 

 rious ; as, when calcareous and widely dispersed, it 

 operates as a manure to rude pastures, and thus ex- 

 tends a valuable vegetation. 



Though the origin of quartz sand is thus obvious 

 and simple, another view has been sought, in an ima- 

 ginary precipitation from solutions in water : an argu- 

 ment which has been supported by the defined crystals 

 of the sand of Neuilly. But the reasoning formerly 

 adduced against the production of rocks from solution 

 is here equally applicable ; while this case is easily 

 explained by the occurrence of crystallized quartz in 

 some granites, and probably in many other rocks. It 

 is a common case of decomposition : while, in every 

 other instance, the forms of the sand are too imperfect 

 to have been thus produced. 



The following cases belong strictly to the alluvia of 

 disintegration ; while the foregoing hypothesis has 

 been especially applied to these deposits. I have for- 



