LIMESTONES 125 



other marine and fresh-water animals. Many of them are but 

 consolidated beds of calcareous mud, full of more or less frag- 

 mentary shells or casts of shells, as shown in Fig. 1, PL 10. The 

 name coquina is given to such as that shown in Fig. 2, PL 10, 

 from St. Augustine, Florida. The rock is composed almost 

 wholly of very perfect shells of a bivalve mollusc, loosely ce- 

 mented by calcareous materials in a finely divided condition. 

 Special names are given these calcareous rocks, designating the 

 character of materials from which they are derived. Coral and 

 shell limestones, as the names denote, are composed mainly of the 

 debris from these organisms. In like manner such names as 

 cnnoidaly fusulina, etc., are applied. 



Nummulitic limestone contains fossil nummulites. Rocks of 

 this type were used in the construction of the pyramids of 

 Cheops. Chalk is a fine-grained, white, pulverulent rock, com- 

 posed of finely broken shells of marine molluscs, among which 

 minute foraminifera are abundant. Shell sand is a loose aggre- 

 gate of shell fragments, formed on sea-beaches by the action of 

 the winds and waves. On certain Hawaiian beaches, such sands 

 give out a distinct note, or peculiar crunching sound when 

 walked over, or even when shaken in a closed vessel, and are 

 popularly known as sounding^ or singing, sands. The property 

 is manifested only when the sand is dry and is assumed to be 

 due to the minute air cavities enclosed by the shells. Oolitic 

 and pisolitic limestones, as previously noted, are made up of 

 rounded concretionary masses of calcium carbonate, and are 

 in part of mechanical origin, and in part chemical deposits 

 (PL 11). 



The microscopic structure of an oolitic limestone from Prince- 

 ton, in Caldwell County, Kentucky, is shown in the accompany- 

 ing figure (p. 126). It will be noticed that the first step in the 

 formation of this stone was the deposition of concentric coat- 

 ings of lime about a nucleus which is sometimes nearly round, 

 but more frequently quite angular and irregular. After the 

 concretions were completed there were formed about each one, 

 narrow zones of minute radiating crystals of clear, colorless 

 calcite; then the larger crystals formed in the interstices. The 

 nuclei are composed in some eases of single fragments or, again, 

 of a group of fragments. Recent microscopic studies have tended 



