576 GINSBURG, LLOYD, STOCKMAN AND MCCALLUM [CHAP. 22 



sediments produces the specific oolith form {op. cit., pp. 44-45). This conchision 

 is supported by the distribution of oohths elsewhere in the Bahamas, in Florida 

 and in the Mediterranean (Lucas, 1955, p. 53). In all of these occurrences the 

 oolitic particles are found in shallow water less than 50 ft deep and within 

 range of exchange with deeper-water areas nearby. All these occurrences are 

 in the paths of strong currents, as evidenced by the rippled surface of the 

 bottom sediments. Elevated salinity is not required for oohth formation. 



B. Cements 



Illing's classic study of some samples from the southeastern Bahamas 

 described two types of cement: (1) cryptocrystalline aragonite that hardens 

 and enlarges biological and physical aggregates of fine-grained material to form 

 sand-sized particles, and then cements these and sand-sized skeletal particles 

 together to form larger composite jjarticles {op. cit., pp. 24-31), and (2) fibrous 

 aragonite that grows within the interstices of sand-sized sediments and within 

 the empty chambers of shells and Foraminifera {op. cit., p. 30). 



The internal hardening of aggregates of fine sediment is most clearly demon- 

 strable in fecal pellets. These ellipsoidal aggregates of silt- and clay-sized 

 sediment particles are soft when produced by sediment-ingesting animals. The 

 hardened pellets are similar in size and shape to soft ones (Fig. 13B) and, 

 occasionally, faint sculpturing of the exterior can be observed. It is not possible 

 to determine the precise nature of the aragonite that hardens these pellets 

 because it is fine grained and thus indistinguishable in thin section from the 

 detrital sediment of which the pellets consist. This "cementing" aragonite may 

 be a purely physicochemical phenomenon occasioned by the trapping of water 

 within the minute pores of the fecal aggregates, or, perhaps more likely, it may 

 be of biochemical origin, a result of the local changes in water chemistry within 

 the pellets from bacterial decay of organic matter. 



Illing also proposed that the same fine-grained precipitate that hardens fecal 

 pellets forms the cement in aggregates of sand-sized particles for which he 

 suggested the term "grapestone lump", and for which Daetwyler and Kidwell 

 used "composite grains" {op. cit., p. 10). These composite grains consist of 

 sand-sized skeletal and non-skeletal particles that are, according to Illing and 

 Daetwyler and Kidwell, cemented around the points of contact by extremely 

 fine-grained aragonite (Figs. 13C, D). 



The fibrous aragonite, described by Illing, that occurs in the interstices of 

 composite grains and in the voids within the shells of mollusks and Foraminifera 

 and the pores of Halimeda is indistinguishable from the cement of some beach- 

 rock (Ginsburg, 1953, p. 88). Both consist of acicular aragonite that grows 

 perpendicular to the particles within the voids. It is this habit, which resembles 

 the growth of crystals in geodes, that suggests a precipitated origin. Illing 

 believes that the submarine examples of fibrous aragonite were, like the cement 

 of beachrock (Ginsburg, 1954), a chemical precipitate. Kaye (1959, j)j). 73-76) 

 has recently questioned this interpretation of the beachrock cement. Kaye 

 {ibid.) follows Cloud (1952, p. 29) and others in attributing the growth of fibrous 



