644 W. H. SHERZER RECOGNITION OF TYPES OF SAND GRAINS 



Under this head it is proposed to group those sands which owe their 

 origin and character of grain to animal or plant life, either during the 

 life of the organisms or subsequently. The composition of the material 

 is mainly calcareous (calcite or aragonite) or silicious, and was extracted 

 by the organisms from the water in which it was held in solution. Here 

 belong typically the accumulations of the finer varieties of molluscan 

 shells, along with the coarser forms of foraminifera and radiolaria.*^ 

 The so-called "green sand/' a composite of many minerals, chief of which 

 is glauconite, owes its composition, in part, as well as its form, to foram- 

 inifera, the granules having been moulded inside the empty shells. The 

 generally accepted view is that the glauconite results from the organic 

 reaction of the decaying life on muddy sediments derived from the land. 

 Much of this has formed in the past, especiall}^ during the Cretaceous, 

 and it is still forming in the ocean at depths of 100 to 200 fathoms. The 

 granules are rounded, sometimes mammillated and nodular, often coarse, 

 but mingled with much fine material. ^^ So far as shell marl is of suita- 

 ble texture or contains such portions, to be considered as sand it would 

 belong under this heading, whether derived from shells or from the 

 growth of lime-secreting plants, as shown by Davis. ^^ This author 

 points out that the upper portions of marl beds often have such sandy 

 textures, composed of the incrustations that form on the stems of Chara 

 (page 493). To the work of this plant and to that of blue-green algoe 

 {Zonotrichia and Schizothrix) , which secrete calcium carbonate, is as- 

 scribed the production of marl. The two latter algge give rise to incrus- 

 tations of spongy material over dead bivalve shells of inland Michigan 

 lakes and over those portions of live forms not embedded in the sand. 

 They also form ellipsoidal, calcareous pebbles of spongy nature, showing 

 both a concentric and radial structure, as described by Davis. These are 

 readily disintegrated by wave action into sand, assorted, and the frag- 

 ments often made into a beach. By this wave action the coarser shells, 

 the corals, bryozoa, echinoid and crinoid fragments, are broken up and 

 comminuted into sand, the edges more or less rounded, and there results 

 the aqueo-organic subtype of sand*^ (see figure 3, plate 46). 



3^ Hinde and Fox : On a well marked horizon of radiolarian rocks, etc. Quarterly 

 Journal of the Geological Society of London, vol. li, 1895, p. 609. Also Evans : Mechan- 

 ically formed limestones from Junagarh and other localities, vol. Ivi, 1900, p. 576. 



38 Murray and Renard : Challenger report on deep sea deposits, 1891, pp. 239 and 387. 

 Also 



Clark : Annual Report of the State Geologist of New Jersey for 1892, part ii, 1893, 

 p. 218. 



Diller : Bulletin No. 150, U. S. Geological Survey, 1898, p. 63. 



3» Davis : A contribution to the natural history of marl. Journal of Geology, vol. vili, 

 1900, p. 485. Also vol. ix, 1901, p. 491. Geological Survey of Michigan, vol. viii, part 

 iii, 1903, p. 65. 



^o See Sorby in Proceedings of the Geological Society of London, vol. xxxv, 1879, p. 69. 



