4 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 76 



The nucleus here was a fragment of a small stalactite. In the same 

 way many of the oolites of phosphate contain a small tooth or some 

 other fragment as a nucleus. (PI. 4.) Other elongated oolites in 

 process of formation are shown in Plates 7 and 8. 



The rough surfaces on some of the oolites are probably due to a par- 

 tial drying up of the pools, when the calcite was deposited from a 

 film of water, as on the stalactites. Such oolites soon become at- 

 tached to the floor. Likewise whenever the drip of the water be- 

 comes too small to move the oolites with its splash, or whenever the 

 oolites grow too large to be moved by the splash of the drip that 

 formed them, they also become attached to the bottom, and gradually 

 lose their individual form, coalesce, and form a lumpy floor. Even 

 on the round oolites, which were presumably formed under conditions 

 most favorable for symmetrical growth, deposition did not take place 

 quite uniformly over the whole surface (see pi. 2) and probably 

 means that the asymmetrical deposition took place while the nuclei 

 were quiescent. 



Efforts have been made to show that oolites are formed as the 

 result of microbic or other unusual influences, but the oolites of nickel 

 and of sulphur, hailstones, and oolites formed in boiling sugar refuse 

 seem to make it unnecessary to assume other causes for their forma- 

 tion than moving supersaturated fluids. We may wonder why oolites 

 are not formed in more places where limestones of different kinds are 

 deposited, but most limestones are formed, not by precipitation from 

 solution, but through the accumulation of definite preexisting par- 

 ticles. Some limestones, like the Indiana rock, show a combination 

 of the two processes. The nonoolitic limestones, which have been 

 deposited from solution, have probably formed where there has been 

 insufficient motion or insufficient depth of water to make oolites. 



EXPLANATION OF PLATES 



Plate 1 



Fig. 1. Oolites or cave pearls from the Rookery in Carlsbad Cavern, New 

 Mexico. These sometimes have a surface resembling ground glass 

 and sometimes smooth as if polished. 

 2. A so-called bird's nest, a shallow depression with pearls like eggs in 

 a nest. 



Plate 2 



Sections through the centers of three spherical oolites ; note variation in bands 

 of growth. 



Plate 3 



Sections of oolitic sand from Great Salt Lake, Utah. Streams from the moun- 

 tain bring calcium carbonate in solution which is precipitated in the lake. 

 Small particles of sand often form the nuclei of the oolites. 



