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



of irregular shape^ with tubercular surfaces (see figure 4, plate 46). 

 Eothpletz reached the conclusion that 



"The oolites of the Great Salt Lake are, therefore, indubitably the product of 

 lime-secreting fissiou-algse, and their formation is proceeding day by day. . . . 

 According to the present stage of my researches, I am inclined to believe that 

 at least the majority of the marine calcareous oolites with regular zonal and 

 radial structure are of plant origin : the product of microscopically small algae 

 of very low rank, capable of secreting lime."*® 



Wethered had ascribed a similar origin to pisolitic granules from near 

 Weymouth, England, some three years earlier, attributing them to the 

 action of a tubular organism Girvanella}^ In 1895 this same author 

 presented to the Geological Society of London a paper entitled "The for- 

 mation of oolite/^^^ in which the organic origin of oolite was advocated. 

 Weed suggests, in the wa}^ of explanation of the deposition of calcium 

 carbonate by green plants (loc. cit., page 642), that their withdrawal of 

 carbon dioxide (for photosynthesis) from the water would reduce its 

 ability to retain the carbonate in solution and lead to its deposition. 

 This explanation seems plausible, but does not explain the silica-secreting 

 power of other plant forms, and leaves the question in doubt. Oolitic 

 granules occur in which the calcium carbonate has been replaced by silica, 

 dolomite, or iron oxide,*^ and all belong under this subdivision of sands, 

 since the form of the granules, aside from the composition, is still due to 

 the original organic agency. 



When any of the various varieties of organically produced sands are 

 subjected to wind action, geolian deposits may be formed similar to those 

 already described. Furthermore, the same kind of changes would occur 

 in the component granules, so far as the nature of the material would 

 permit. The oolitic granules, already so perfectly rounded, would lose 

 their polish and simply suffer reduction in size. Such sands would con- 

 stitute the subtype of geolo-organic, interesting examples of which are 

 known — as those of the Bermuda and Bahama coasts, described by Nel- 

 son. ^^ On the former island great dunes occur, some 200 feet in height, 

 composed of fragments of corals, shells, etcetera, and the products of 



46 American Geologist, loc. cit., pages 280 and 282. 



*'^ Wethered : On the microscopic structure of the .Jurassic pisolite. Geological Maga- 

 zine, 1889, new series, vol. vi, p. 196. 



*8 Quarterly Journal of the Geological Society, vol. li, 1895, p. 196. 



^^ Goodchild : The origin of hematite. Transactions of the Cumberland and West- 

 moi'eland Association, No. viii, 1883, p. 117. 



^0 Nelson : On the geology of the Bahamas and on coral formations generally. Quar- 

 terly Journal of the Geological Society of London, vol. ix, 1853, p. 200. Bermudas de- 

 scribed in Transactions, ser. 2, vol. v, part i, 1837, p. 103. See also Darwin's "Corals 

 and coral islands," 1872, pp. 152 to 156. 



