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of the calcareous nature of all the rocks of Florida. In the Everglades this water is exposed in broad shallow sheets to active evaporation, agitation and variations of atmospheric temperature and pressure. Concentration of the solution and escape of carbonic acid, including some of that in the bicarbonate in solution, follow, and the neutral carbonate is produced in excess of the amount that can be retained in dissolved form. It is therefore precipitated in two forms—first, from the mass of the water as a flocculent mud; second, from the lower layers of the water in contact with limestone as crystals forming an integral part of the solid rock.

The alternation of dry and wet seasons is accompanied by concentration and sluggish flow, alternating with dilution and flood currents. Therefore there are seasons of more active precipitation interchanging with those of more vigorous transportation and, perhaps, partial re-solution. In these latter seasons the calcareous mud is swept beyond the shallow basin where it forms, and enters as a suspended sediment into the Gulf circulation. What part, if any, is dissolved, what is deposited as mud in the lagoons along the coast, and what is swept into the silt banks of the Atlantic, is not known.

Conditions which produced similar results are described by Gilbert as having existed in Lake Bonneville.^[1] Tufa was deposited on the shores of the lake at various stages, but most abundantly at the Provo stage, during which the water lingered longest at one level. The occurrences are thus described:

"The distribution of tufa along each shore is independent of the subjacent terrane......No deposit is found in sheltered bays, and on the open coast those points least protected from the fury of the waves seem to have received the most generous coating. These characters indicate, first, that the material did not have a local origin at the shore, but was derived from the normal lake water; second, that the surf afforded a determining condition of deposition."