162 FLORIDA GEOLOGICAL SURVEY-I3TH ANNUAL REPORT

sand any irregularities that might tend to be formed by erosion, burrowing animals, uprooted trees, etc.*

 Lake basins. The hills of the lake region are interspersed with many saucer-like basins of various sizes and depths, some dry and some containing water. just how these basins were formed is an
-unsolved problem. Some have ascribed them to solution and some to the action of strong ocean currents when the land was submerge.dt but neither explanation fits all the facts. Basins of somewhat similar outline but usually shallower are very common in the lime-sink region, and as some of those are known to have been formed by a sudden caving in of the roof of a subterranean passage and the subsequent smoothing of the sides by rain and wind, it may be assumed that most of them originated in some such way. But in the lake region sinks, caves, and other solution phenomena are very rare, and no one seems to have ever observed the beginning of one of the basins in question. They could hardly have been scooped out by the wind or the elevations around them piled up by
-vaves, either, for many of the hills have a hard clay substratum in them considerably above the bottom of the basins. And lakes a short distance. apart often differ considerably in elevation, showing that they rest on an irregular surface Tof clay or some other impervious material.

 Lime-sinks. This term is used for several different things. Some lime-sinks are small dry sandy basins of the kind just described, with no visible outlet, while others have rock outcropping in them and a hole at the bottom through which water escapes, and some have steep banks and are more or less permanently filled with water, which is usually bluish from dissolved limestone. The dry sandy type is most common in the lime-sink region and the

 *It seems probable that the wind has had a much larger share in shapin. the topography of the uplands of peninsular Florida than is commonly realized. Although the sand does not move noticeably on windy days, except in cultivated fields (and even there there is little evidence of drifting after the wind dies down), in the course of centuries any minor irregularities must be pryctt thoroughly smoothed out.
 tSee pages 150-156 of the paper on the topography of Florida by Prof. Shaler, cited on the preceding page.