68 ALLUVIAL AGENCIES. 



Dr. Hitchcock divides the deposits from diluvial 

 currents into " first, boulder-stones, formed gener- 

 ally of the least yielding rocks ; second, gravel or 

 sand mixed, or the more yielding rocks ; third, beds 

 of clay ; fourth, consolidated sand and pebbles ; and 

 fifth, beds of sand. The depositions are generally 

 horizontal, although some of them dip a little, and 

 lay according to the degree of their pulverization. 

 Sometimes, by an action which appears to have been 

 lateral pressure, the strata becomes somewhat con- 

 torted. 



" From alluvial agencies are produced — 



" 1. Soil, or disintegrated and decomposed rock, 

 with such an admixture of vegetable and animal 

 matter as will support the growth of plants. 



" 2. Those vast accumulations of sand which are 

 found in the beds of the ocean and of lakes, and 

 which, when driven upon the land by the wind, are 

 called downs. These are constantly shifting, and 

 are destitute wholly of organic matter, and are 

 therefore entirely barren. 



" 3. Peat. This consists almost wholly of organic 

 remains, which are deposited by the death of plants. 

 It is a powder when dry. These beds occur only in 

 the temperate zones. 



" 4. Alluvial marl. This varies somewhat in 

 composition, but consists of carbonate of lime, clay, 

 and soluble and insoluble geine. It often lies be- 



