PHYTOGENIC OOLITES 469 



These are believed to be due to the presence of thread-Hke algse 

 living symbiotically on the lime-secreting species and being encased 

 by the deposits of lime. 



In the low coastal tract in the further environs of Suez, Quater- 

 nary oolites of this type are found, built up subaerially into beds 

 and dunes and frequently consolidated into hard oolitic limestones 

 (Bauermann). These oolites, however, generally contain a foreign 

 sand grain, but the lime surrounding them has the structure of the 

 oolites of Great Salt Lake and on solution leaves the organic resid- 

 uum found in these. 



Some observers contend that the oolites of the Red Sea region 

 have a radial structure, but this is denied by others. Linck, more- 

 over, holds that this radial structure, when it occurs, does not indi- 

 cate organic growth. He interprets the rods as minute crystals of 

 aragonite deposited in a purely hydrogenic manner. 



Oolites regarded as due to the growth of algae have been de- 

 scribed from the salt lakes of the Kalahari desert of Africa. 

 (Kalkowsky-26.) From his studies Rothpletz concludes that the 

 "majority of the marine calcareous oolites with regular zonal and 

 radial structure are of plant origin ; the product of microscopically 

 small algge of very low rank, capable of secreting lime." (Roth- 

 pletz-Cragin-36:i'7p.) On the other hand, Linck, who approached 

 the subject from the experimental point of view, is equally positive 

 that these oolites are wholly of chemical origin, deposited either 

 freely or upon a foreign nucleus, under the influence of chemical 

 reaction, as aragonite (really ktypeit) and that when organic re- 

 mains, such as algse, etc., were enclosed they lived upon the growing 

 shells of inorganic origin or were mechanically enclosed by the 

 chemical deposit. The algous rods {Algcnstabchcn) of Rothpletz 

 he regards as minute crystals of aragonite. The aragonite oolites 

 are subsequently altered to calcite, which is the condition of most 

 fossil oolites. (See further under fossil oolites, postea, p. 471, and 

 oolites of chemical origin, ante, p. 336, Chapter IX.) 



Order CJiloropliycecr, or Green Algce. 



In this division the chief lime-secreting alg?e of the modern 

 sea are the genera Halimeda and LMolea of the group Siphonales. 

 These grow in the shallower portion of the coral reefs connecting 

 dead coral masses. (Agassiz-i -.82.) Halimeda opiintia and H. fri- 

 dens form masses of lime on the shores of St. Thomas. (Challen- 

 ger Narrative-/ :7.?7.) It is a jointed plant and readily broken up 

 into small lime rods. An analysis of Halimeda gave CaCO.,, 90.16%, 



