98 HARSHBERGER— COMPARATIVE LEAF STRUCTURE [April 24, 



coral and coralline (calcareous) sea-weeds which have grown on the 

 fringing coral reefs. Bermuda, geologically speaking, is an atoll, 

 a ring of coral reefs surrounding a central lagoon. The elevated 

 land was formed by the raising of the weather edge of the reef 

 above the level of the sea. The tops of the projecting corals were 

 broken off and along with calcareous sea-weeds and mollusk shells 

 were ground by surf action into a fine sand, which was formed into 

 a beach. As the top of the beach dried in the sun, the sand was blown 

 off and was deposited in the crevices of the coral breakwater, which 

 gradually widened. Ultimately, by wind action, sand hills were 

 formed. The limestone rock found throughout the islands was origi- 

 nally derived from broken-down coral and shells. These rocks vary 

 in texture from loose sand to compact limestone. The process by 

 which the coral sand was converted into limestone was very simple 

 and it involved no great lapse of time. As the sand consists almost 

 entirely of calcium carbonate, it was easily soluble in water contain- 

 ing carbon dioxide. The rain water took up a little of the calcium 

 carbonate in the form of bicarbonate, and as it percolated through 

 the sand, it lost its carbonic acid gas and evaporating left the dis- 

 solved calcium carbonate as a thin layer of cement uniting together 

 the grains of sand. The rocks remain permeable to water and 

 soluble, so that this process of solution and deposition goes on con- 

 stantly until even a marble-like limestone may result. The usual 

 building material consists of blocks of limestone sawed out of the 

 hillside. When built as a wall sufffcient solution takes place so 

 that the stones become united together into an almost solid piece. 

 The red soils of the islands represent the one per cent, residue of 

 solid material after the rain has leached out all of the other con- 

 stituents. When the solution, owing to wave action or constant 

 rain action, is excessive, caverns with stalactites and sinks are formed. 

 The honey-combed eolian rock of the shore line on which charac- 

 teristic Bermuda plants occur owed its origin to similar water erosion. 

 The sand dunes thus represent stages intermediate in the geologic 

 changes which have combined to give the present form to the islands. 

 They represent shifting masses of coral sand, forming flat surfaces 

 in some places, in other places heaped into conical dunes or raised 

 into long ridges. Frequently dune hollows exist as a result of wind 



