156 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.49. 



is usual in the recrystallization of carbonate rocks this secondary 

 material tended to approach the normal dolomite ratio, although as 

 the analysis shows, a slight isomorphous admixture of calcium and 

 • ferrous carbonates still remained. The secondary crystaUization 

 took place so slowly and quietly that the heaps of carbonaceous dust 

 were not disturbed, but merely enclosed by the crystal grains, and 

 their shapes preserved. 



The deposition of pyrite took place at still a later time. Migrating 

 solutions brought in ferrous sulphate, which was reduced to sulphide 

 by the carbon near the surface of the normal ooids, and around the 

 black part of the divided ones. Apparently the rearrangement of 

 the carbon particles in the latter rendered them more readily attacked, 

 for both microscopic examination and the analysis show them to be 

 the higher in limonite, which now takes the place of the pyrite. This 

 last change, which represents the latest chapter in the history of the 

 rock, was evidently brought about by the action of oxygen-bearing 

 rain water. 



Ooids exhibiting all stages of these various processes are shown in 

 the figures, examination of which will leave no doubt that this rock, 

 which at first sight appears so remarkable, has had the comparatively 

 simple origin here outlined. 



EXPLANATION OF PLATES. 



Plate 40. 



Fig. 1. A cross section of the oolite rock, natural size. Shows division of ooids into 

 black and white portions, and lens-shaped patch in which many are 

 unchanged, in the left-center. Cat. No. 88448, U.S.N.M. 

 2. The central portion of the same specimen, enlarged 5 times. Shows coarse 

 crystallization of ooids, variable though usually convex dividing line 

 between black and white portions, nuclei central in unchanged grains, but 

 dropped into black portions in the divided ones. 



Plate 41. 

 All enlarged 40 times. 

 Fig. 1. Ooid showing nucleus but little displaced from center. 



2. Nucleus, retaining several concentric coats, sunk in dark portion of ooid, the 



dark material being heaped up over it. 



3. Nucleus, deprived of concentric coats, sunk in dark portion of ooid with less 



heaping up of material; specks of limonite surrounding dark portion 

 prominent. 



4. Nucleus entirely dissolved away, and line between dark and light portions of 



ooid straight. 



5. An elongated ooid, with its nucleus of corresponding shape; the latter sunk in 



dark portion, which was, however, too viscous for it to fall flat. 



6. An ooid with two nuclei. 



