HYDROCLASTICS 295 



h. Hydrargilliitytes, or pure clay beds and varieties due to 

 the presence in small amounts of silica, lime, iron, or 

 carbon. 



i. Hydrosilicilittvtcs, or pure quartz-mud rocks, with their 

 varieties due to a slight admixture of argillaceous, 

 calcareous, carbonaceous, glauconitic, or ferruginous 

 matter. 



j. Hydrocalciliitytcs, or pure lime-mud rocks, with their 

 varieties due to a slight admixture of argillaceous, 

 siliceous, carbonaceous or ferruginous matter. 



The prefix "hydro" in all these cases is omitted when it is 

 understood that the rocks are water-laid deposits. This is the class 

 of rocks with which the stratigrapher has most to deal, for they 

 comprise by far the largest portion of the sedimentary rocks, and 

 they most commonly contain organic remains in greater or less 

 abundance. In general, hy^rorudytes have their pebbles rounded, 

 the degree of rounding depending on the length of time that the 

 pebbles have been subject to water wear, the character of the mate- 

 rial, the intensity of the abrading force, etc. Extensive and pro- 

 longed wave or current action will further result in eliminating 

 much if not all of the perishable mineral matter, so that a much- 

 worked-over conglomerate will consist largely or wholly of cjuartz 

 pebbles. The same is true of the arenytes, although the sorting 

 here is not so pronounced as that by the wind. Pure hydroclastic 

 quartz sands do occur, however, as at Escambia, on the Gulf coast 

 of Florida, in which scarcely a fragment of mineral other than 

 quartz is found. The granules of this sand range in size from o. i 

 to i.o mm. (fine to coarse sand, averaging 0.25 to 0.50 mm. (me- 

 dium sand). At West Palm Beach, on the Atlantic coast of Flor- 

 ida, a similar pure sand occurs which apparently has been trans- 

 ported along the shore from the Piedmont region to the north. In 

 both of these cases the grains are subangular. The finest sand and 

 the rock flour and clays will retain their angular outlines unim- 

 paired, for, as Daubree has shown, quartz and other mineral par- 

 ticles of 0.1 mm. or less diameter will float in faintly agitated 

 water. 



In the broader considerations of hydroclastic rocks it is impor- 

 tant that current or river-worn elastics be distinguished from 

 marine or other wave- formed elastics. The former may be spoken 

 of as fluvio-clastic (potamoclastic*) deposits, and the latter as kymo- 

 clastic.f So far as grain is concerned, no marked distinction exists 



* From woTa/Mos, a river and KXacrrds, broken. 

 fFrom Kvna, a wave and KXavrds, broken. 



