774 T. C. BROWN — OOLITES AND OOLITIC TEXTURE 



When the circulating waters contained silica in solution the oolites 

 were frequently replaced by silica. If the oolite had an original nucleus 

 consisting of a quartz sand grain, this was frequently secondarily en- 

 larged by the deposition of silica around it and in optical continuity 

 with it. In other oolites the replacement was accomplished either by the 

 deposition of granular quartz or of finely fibrous chalcedony. In any 

 case minute rhombs of calcite are frequently inclosed in the replacing 

 silica, showing clearly that the aragonite had in part changed to calcite 

 before replacement by silica began. With the silica is often deposited 

 minute quantities of iron oxide intimately mixed with the fibrous chal- 

 cedony. 



At the Clinton horizon in the Silurian, in Pennsylvania, and in other 

 States along the Appalachian mountain belt, occurs one or more beds of 

 iron ore which are either fossiliferous or oolitic. Oolites are present to 

 some extent in all of these ores. They were evidently formed originally 

 as aragonite oolites and later changed, together with the associated fos- 

 sils, by ferruginous solutions, into oolites composed of iron oxide and 

 silica intimately mixed, but with the iron oxide greatly in excess. 



It is very probable that the silica replacing the oolites of the Ordo- 

 vician, and the iron and silica replacing the oolites of the Clinton, came 

 from material already in the beds and at no great distance from where 

 it now occurs. The silica probably came from sponge spicules and di- 

 atoms, and the iron (with its associated silica, perhaps) from green sand 

 or glauconite. 



BiBLIOGEAPHY 



The following list includes only the more important works consulted 

 by the writer in the preparation of the present paper. Many of these 

 papers contain further references, especially to the earlier literature on 

 the subject: 



1667. HooKE, Robeet: Micrography, or some physiological descriptions of 

 minute bodies made by magnifying glasses, with observations and 

 inquiries thereupon. 



1851. De La Beche, Sie Henry T. : The Geological Observer, pages 122-123. 



1857. Vielet-d'Aoust : Sur les oeufs d'insectes donnant lieu k la formation 



d'oolithes dans des calcaires lacustres au Mexique. Compt. rend. 45, 

 page 865. 



1858. RoGEEs, H. D. : Geology of Pennsylvania, Volume I, page 238. 

 1872. Dana, James D. : Corals and coral islands. 



1877. Chambeelin, T. G.: Clinton iron-ore deposits. Geological Survey of 

 Wisconsin, Volume II, pages 327-335. 



