2 PEOCEEDINGS OF THE NATIONAL MUSEUM vol. 76 



Their inner structure in general is similar, though differing in de- 

 tail. Sections were prepared for microscopic study of three round 

 oolites, each a little more than half an inch in diameter. These are 

 shown magnified 4V3 diameters in Plate 2. In each oolite there may 

 be observed a nuclear mass surrounded by a broader noticeably radial 

 zone of more coarsely crystalline material surrounded by circum- 

 ferential rings of very fine texture. The rings differ in number, 

 thickness, and continuity in the three specimens. 



The coarser part of the oolites is certainly calcite, and not arag- 

 onite, as might be supposed. The dense rings are apparently also 

 calcite, though their mineral composition is difficult to determine. 



The nuclei, too, are all floccules of calcite — aggregates that formed 

 within the little pools as these became supersaturated with calcium 

 carbonate. Drops collecting on the ends of the stalactites above 

 pools begin to lose carbon dioxide, and as they drip and splash 

 into the pool lose enough more to become supersaturated with 

 calcium carbonate, and thus to set free molecules, which begin a 

 separate existence. These molecules attaching themselves to other 

 molecules in the little eddies of a tiny pool form a spherical body. 

 They must have been kept in sufficient motion to maintain this form 

 as they increased in size. The movement of the water caused by the 

 splash of falling drops was probably sufficient. In much the same 

 way oolitic sands have formed on the bottom of Great Salt Lake 

 near the shore, though the grains are very much smaller than the 

 smallest collected in Carlsbad Caverns and average only about one 

 one-hundredth of an inch (0.4 mm.) in diameter. (See pi. 3.) The 

 streams that flow from the great limestone beds of the Wasatch 

 Mountains carry large quantities of calcium carbonate in solution to 

 the lake. Here calcite is precipitated from the supersaturated waters 

 as it is in the tiny pools of Carlsbad Cavern, but in many of the 

 oolites tiny grains of sand form the nuclei. As the lake waters move 

 over the bottom, calcium carbonate is added to the oolites already 

 formed or new ones are started. The size of the oolites is probably 

 closely related to the amount of their movement after they are 

 formed. 



Similarly the great oolitic limestone beds of the world have been 

 formed. They are of many geologic ages and are very widely dis- 

 tributed. In this country the oolitic limestone quarried at Bedford, 

 Ind., is one of our best-known building stones and is shipped to 

 all parts of the United States. 



Oolites are formed from supersaturated solutions other than those 

 containing calcium carbonate. The great phosphate deposits of 

 Utah, Idaho, Wyoming, and Montana contain abundant oolites of 

 phosphorite — a calcium phosphate containing more or less fluorine 



