GEOLOGY: A. G. MAYER 
29 
Shell No. 3 was placed for 364 days in a 15 liter glass bottle which 
was enclosed in a wooden dark chamber to prevent the growth of plants. 
The water was mechanically changed by each rise and fall of the tide, 
flowing in and out of small glass tubes, thus preventing the formation 
of strong currents within the bottle. This bottle was placed in the 
marine moat surrounding Fort Jefferson, Tortugas. This moat-water 
supports an abundant animal life and contains more CO2 than the open 
ocean, but remains always alkaline to phenolphthalein. At the begin- 
ning of the experiment the shell weighed 10.5435 grams, and at the 
end of the year it had lost 0.0115 gram. Thus it would have taken 917 
years to dissolve the entire shell and a superficial thickness of 0.00069 
mm. was removed in one year. 
Shell No. 4 was placed for 364 days in a similar glass bottle which 
remained between tides off the western wharf of Loggerhead Key, 
Tortugas, thus being surrounded by open sea-water. Considerable silt 
was drawn in through the glass tubes, and the shell was found buried 
beneath about 8 mm. of limestone mud which was charged with H2S. 
A number of tunicates, shrimps, molluscs and worms were living within 
the bottle at the end of the year, but the circulation had become inter- 
rupted by the growth, within the bottle, of a tunicate across the opening 
of one of the tubes. The shell originally weighed 15.22 grams and it 
had lost 0.047 gram, amounting to a superficial thickness of 0.0019 
mm. in a year. Thus it would have required 324 years to dissolve 
the shell. 
Taking the results of this last experiment as a maximum rate of 
solution of calcium carbonate by sea-water as such, it appears that 
19,250,000 years would be required to dissolve out a layer of calcium 
carbonate to a depth of 20 fathoms, this being about the average depth 
of atoll lagoons; and as many if not all atoll lagoons have been formed 
since the beginning of Tertiary times it appears that they owe their 
development to agencies other than that of solution by sea-water, for 
even if the rate of solution of reef limestone is 100 times as rapid as 
that of the Cassis shell it would require 192,500 years to dissolve out 
an average lagoon. 
Holothurians, Echini, boring algas, certain sponges, worms, dead 
organisms, and probably most important rain water washing out from 
forested shores are agencies which dissolve limestone but their effects 
have not yet been quantitatively evaluated. Thus the lagoons of 
barrier reefs surrounding volcanic islands may have been formed in 
some appreciable measure by solution, but this cannot apply in the same 
degree to the lagoons of atolls where the land area is insignificant in 
