48 FLORIDA GEOLOGICAL SURVEY SIXTH ANNUAL REPORT. 



face area becomes better coated than a fine sand. While a coarse 

 sand is preferable to a fine sand, for certain purposes it may be 

 desirable to have a mixture of coarse and fine grains. The cement 

 used in the mortar must be sufficient to completely fill all voids 

 existing in the sand. The relative proportion of voids may be 

 reduced by the use of a mixture of coarse and fine sands. Such a 

 mixture of sands permits, the use of a relatively smaller amount of 

 cement, and is apparently without detriment to the resulting mor- 

 tar. Sand is used along with lime in the manufacture of sand-lime 

 brick ; and with cement in the preparation of artificial stone or build- 

 ing blocks. 



The sands most frequently used for mortar are siliceous. It 

 appears, however, ■ from various tests that calcareous sands are in 

 no way inferior.* In addition to the natural sands ground rock 

 may also be used. A small amount of clay, not exceeding eight 

 per cent, is said not to weaken the cement. The presence of humus 

 or peaty matter, or an excess of clay as well as mineral particles 

 of any kind subject to decay, must be guarded against. 



The accompanying illustrations show the prevailing shape of 

 the sand grains from a number of localities in Florida. The rela- 

 tive size of the sand grains is also indicated, all of the illustrations 

 having been drawn to the same scale. 



CONCRETE. 



The materials in Florida suitable for concrete consist chiefly 

 of shell deposits and some compact limestones and of flint rock 

 which may be crushed for the purpose. 



Shell deposits, both recent and fossil, are numerous in the State. 

 The use of shell from the coquina rock for building purposes has 

 already been mentioned. Among notable buildings from these shells 

 may be mentioned the Ponce de Leon hotel at St. Augustine. The 

 calcareous shell mass as found at this locality may contain a small 

 admixture of siliceous sand blown in by the winds. Recent shell 

 deposits occur at many places along both the Atlantic and the Gulf 

 coasts. Shell mounds piled up bv the Indians are likewise numerous 

 on and near the coast. Some occur inland also, those of the St. 

 Johns River from Jacksonville to Sanford being notable examples. 



*Sabin, Cement and Concrete, p. 170, 1907. 



