CLINl^ON OOtlTiC IRON OHE 773 



more likely that both the silica and the iron were originally deposited in 

 these beds, as suggested by McCallie, in the form of glanconite. If this 

 be true, the iron is secondary in respect to the oolites, but not necessarily 

 secondary in respect to the Clinton beds, although in all probability 

 some lateral movement has taken place during the interchange of ma- 

 terials and replacements of the oolites and fossils. 



Summary and Conclusions 



From the observations here recorded it seems evident, therefore, that 

 the recent oolites now forming in Great Salt Lake and Pyramid Lake 

 are like those now forming in hot springs and in the open ocean, com- 

 posed of calcium carbonate in the mineral form aragonite, and they very 

 frequently contain sand grains or nuclei of other foreign material. They 

 probably are due to direct chemical precipitation caused by a reaction of 

 sodium carbonate on the calcium sulphate held in solution by the water. 

 The sodium carbonate in Great Salt Lake may be due to the decay of the 

 dead minute alg^ found so closely associated with these oolites below the 

 limit of wave action. The argument in favor of these oolites having been 

 derived from solutions of calcium sulphate is greatly strengthened by 

 the fact that the great deposits of the oolitic sand along the shore of the 

 lake are confined to a stretch near the mouth of the Jordan Eiver, and 

 this river until two or three decades ago carried calcium sulphate in solu- 

 tion in excess of any other salt. This salt is now exceeded in amount by 

 sodium chloride, due to the effects, of irrigation along the course of the 

 river, but it is still the most prominent calcium salt. 



The older oolitic beds of Pennsylvania were probably all originally 

 ■laid down as beds of calcareous oolites composed of the mineral aragonite. 

 This mineral, being unstable under ordinary rock-forming conditions, 

 soon began to change. Where the beds were not affected by waters bear- 

 ing other minerals in solution they simply changed to the; more stable 

 mineral calcite, and in doing so they often retained their original fibrous 

 radiated and concentric structure. Sometimes, however, they became 

 more or less completely changed into crystalline calcite, occasionally with 

 twinned lamellae. 



When the circulating waters contained magnesium salts in solution 

 the oolites became altered to dolomite, and in doing so lost their con- 

 centric and fibrous structure. The oolitic structure is then only shown 

 by the faint outlines of the original oolites, sometimes accentuated by a 

 ring of rhombohedral crystals around their margin, developed when the 

 process of alteration first commenced. 



