DEPOSITS BY RIVERS. 59 



low peat ill limestone countries, and is sometimes 

 found at the bottom of ponds. 



" 5. Calcareous tufa^ or travertine. This is a de- 

 posit of the carbonate of lime from springs, and 

 forms a substance like stalactites. 



" 6. Coral reefs. 



" 7. Silicious sinter, which is a deposit of silica 

 from thermal springs. 



" 8. Silicioiis marl, much resembling calcareous 

 marl, but composed almost entirely of silica, de- 

 posited from the skeletons of minute infusoria, which 

 at death sink to the bottom of the pond. 



" 9. Bitumen, deposited from springs in the form 

 of naphtha and asphaltum. 



" 10. Sulphate of lime, very rarely deposited by 

 springs. 



"11. Hydrate of iron. 



" 12. Hydrate of manganese. 



" 13. Chloride of sodium. 



"14. Sandstones,conglomerates, and breccias. Sand 

 cemented, is called sandstone ; when it contains 

 rounded pebbles, it is said to be conglomerate ; and 

 when it has angular fragments, it is called breccia." 



The substance deposited by rivers dijQFers with 

 the distance from the mouth. When it comes forth 

 from the mountain, coarse stones and boulders are 

 rolled along, the finer materials are carried further, 

 and the finest are found near the mouth of the 

 stream. Alluvial deposits are made from currents^ 



