Since the sands blew over the ancient desert of Florida, many lake basins have been formed because of the sinking of the ground. This sinking of the ground is a common occurrence in limestone regions, where a great amount of material is moved in solution, leaving caverns whose roofs often fall in. The great amount of sand blown upon Florida caused the marine strata to give way in the weaker places under its burden. The sinks thus formed, probably of frequent occurrence at one time, have now nearly ceased. Still, there are depressions to be seen to-day where the tops of large pine-trees, which grew on dry, sandy land, are barely above the surface of the water which partly fills the basins so recently formed. Yet I would not assert that all of the depressions where Florida lakes exist were caused by the sinking of the ground; for the winds may have caused shallow basins in the sand, where the decayed vegetation has formed mud sufficient to hold the water which now partly fills such basins. The mobility of Florida sands can be seen to good advantage when exposed to a strong, dry north-west wind, where the ground happens to be destitute of vegetation. An observer can then realize what the result would be, should the whole land be deprived of vegetation and laid bare to the action of the winds. Under such conditions, not only would the winds be much stronger than now, but the air near the ground would be filled with sand, moving like drifting snow in a Dakota blizzard. And, furthermore, it is probable that the rainfall was very light while Florida was void of vegetation; and, even if shallow basins were formed, there would be a lack of rain to supply them with water. The wide plains west of the Mississippi River, extending southward into Texas, during the frigid period must have been covered with a sheet of ice and snow. And it is probable that it was not wholly a product of more northern latitudes, but was mostly produced by the snow which fell on the plains during the long winters of that period, which could not be melted away during the cold summers of an ice age, when it is considered that an ice-sheet, with a temperature sufficiently low as to carry glacial drift, covered the lands of Missouri as far as latitude 38° south; and it may have been through the pressure from an ice-sheet in its south-eastern movement that we are to account for the numerous ore-bearing faulting fissures traversing the limestone strata.