24 New York State Museum 



Chemical sediments are sometimes precipitated in crys- 

 talline form and if the deposits are pure they are nearly 

 as hard as the mineral itself. When the deposit is un- 

 crystalline (colloidal) it may harden by the gradual loss 

 of water, as in the case of bog-iron ore, or it may finally 

 become crystalline. Organic sediments of mechanical 

 origin, such as coral sand, are consolidated as clastic 

 sediments. Organic precipitates are of a chemical nature 

 and consolidated in the manner of chemical sediments. 



Places of deposition. The clastic or mechanical sedi- 

 ments, particularly the marine deposits, are those which 

 are of the most importance to us here. As all sedimen- 

 tary rocks they may be classified, according to place of 

 deposition, into continental, littoral and marine deposits, 

 and this distinction is of great geologic importance. 



Continental deposits include lacustrine, fluviatile and 

 terrestrial deposits, the last-named group comprising 

 many important deposits of mineral matter. Among 

 clastic deposits of this group are residual rocks or those 

 in which the material was formed by disintegration and 

 decomposition with little or no transportation ; eolian 

 rocks or those derived from sediments deposited by the 

 wind upon the dry land; and rocks formed of all sedi- 

 ments deposited upon the land through the agency of 

 moving waters, such as deposits in lakes, ponds and 

 playas, river flood-plain deposits, alluvial fans and deltas. 

 Residual rocks produced by the destructive action of the 

 atmosphere are consolidated without sorting or rear- 

 rangement of material. A good example is consolidated 

 laterite. Laterite is a red soil or deposit produced in 

 tropical regions through subaerial decay of rocks such as 

 granite which leaves a mixture of quartz grains and a 

 claylike substance colored reddish by iron oxides. Eolian 



