Precisely the same thing happens in the actual crust of the earth, except that, in the formations usually met with, the strata are so irregularly permeable that no such uniform percolation occurs, and most of the water, instead of oozing out near the sea-level, meets with obstructions which cause it to issue, sometimes below the sea-level and sometimes above it, in the form of concentrated springs.
The typical continental form is triangular as regards its sea-level outline.
It is derived from springs which burst through the beds below sea-level with such force as to retain their freshness in the midst of the surrounding salt water.
The geological sequence of events appears to have been the following: - After the deposition of the Eocene (or Oligocene) limestone - which reposes upon a floor of basalts and trachytes - basalts and basic tuffs were ejected, over which, during a period of very slow depression, orbitoidal limestones of Miocene age - which seem to make up the great mass of the island - were deposited; then elapsed a long period of rest, during which the atoll condition existed and the guano deposit was formed; from then down to the present time there has succeeded a series of sea-level subsidences, resulting in the formation of the terraces and the accummulation of the detritus now seen on the first inland cliff, the old submarine slope of the island.
The mean sea-level is shown by the horizontal line aa, dotted where it passes through the land, and the natural mean level of saturation bb, above the sea-level, by a curved dot and dash line.
The actual position of sea-level lies so near the summit of the crust-heap that the varied relief of the upper portion leads to the formation of a complicated coast- The con- line and a great number of detached portions of land.
In Pleistocene times, then, when there were prolonged glacial ages, the sea-level was lowered and at the same time there was a reduction in sea temperature, so that the rate of reproduction of the coral polypes, and so the growth of reefs, was diminished.
Above sea-level and is surrounded by high hills; its picturesque situation and its nearness to famous mineral springs make it a health resort.
Fresh water, rising and falling with the tide, is found in certain large caverns in Lifu, and by sinking to the sea-level a supply may be obtained in any part of the island.
Above sea-level the sun is seen continuously above the horizon from the 26th of May to the 18th of July; at Haparanda for 23 hours, at Stockholm for 181 hours and at Lund for 171 hours at the summer solstice.