HISTORICAL REVIEW 749 



structures which he considered organic and referred to the Ehizopoda 

 under the generic name Girvanella. Later Wethered, working on the 

 British Carboniferous and Jurassic oolites, discovered similar structures, 

 and considered all of these oolites to be of organic origin.® 



In 1892 Eothpletz studied the oolites now forming along the shore of 

 Great Salt Lake, ITtah. He found that oolites obtained below the sur- 

 face of the water were coated with a bluish-green algal mass, and that 

 when the oolites from the shore were dissolved in very dilute hydro- 

 chloric acid there remained granules which he recognized as dead and 

 crumpled algal cells. Among the living algse he recognized both Gloeo- 

 capsa and Gloeothece, and he concluded that these forms were instru- 

 mental in separating the calcium carbonate and building up the oolites. 

 When he extended his observations to the oolites forming around the 

 Eed Sea, he was unable to find any living algae cells on the oolites, but 

 within the little spheres he often found wormlike, and often branching, 

 tubules which had been filled with calcite. These he concluded were 

 identical with the structures described by Wethered from the Paleozoic 

 oolites, and he believed that they were produced by threadlike algae. As 

 a result of his investigations, Eothpletz concluded that the majority of 

 marine calcareous oolites with zonal and radial structure were produced 

 by microscopic alg^ of very low rank, capable of secreting lime.^^ These 

 investigations have been widely quoted and accepted as conclusive by 

 many later workers on oolites. 



Another interesting suggestion as to the organic origin of oolites is 

 that made by Seeley in a paper read before the Bath meeting of the 

 British Association, in which he compared oolites to the inter-nodal 

 grains of nuUipores. These grains have a concentric and a radial struc- 

 ture very similar to that found in oolites. ^^ But in spite of this, Seeley 

 was inclined to believe that the great majority of the oolite grains were 

 formed by direct chemical precipitation, probably formed by evapora- 

 tion of sea-water at the surface. He thought that this took place about 

 shell fragments, and that these continued to increase until they attained 

 a certain size, when they would sink to the bottom. This, he thought, 

 would account for the uniformity of size of the grains in any one stratum. 



Even though these various arguments for an organic origin had been 

 advanced from time to time, the general consensus of opinion at the be- 

 ginning of the present decade seemed to be that calcareous oolites were 

 direct chemical precipitates formed about foreign nuclei kept in motion 



» E. B. Wethered : The formation of oolite. Quart. Jour. Geol. Soc, vol. 11, pp. 196-209. 

 " A. Rothpletz : On the formation of oolites. Am. GeoL, vol. x, pp. 279-282. 

 ^ H. G. Seeley : The oolitic texture in rocks. Brit. Assoc. Adv. Sci., Bath meeting, 

 1888, pp. 674-675. 



