FOSSIL OOLITES 473 



3. Spindle structure, the concentric layers are traversed by 

 elongated spindles which become pointed toward the periphery and 

 the center of the ooid, the structure of the spindles being an ir- 

 regular mass of calcite grains. 



4. Cone shaped, conical bodies traverse the concentric layers, 

 their points at the center ©f the ooid. These cones are built up of 

 concentric layers of a radius often somewhat smaller than that of 

 the ooid itself, and separated by interradii containing clay and 

 sand and having a somewhat radial structure. 



By a combination of these structural groups, eight or nine differ- 

 ent ooid types were produced. Special types among these are the 

 roller form, the hemiooids, the polyooids, and the cystiform ooids. 

 These ooids are held together by a cement of CaCO,. Kalkowsky 

 regards these structures as due to secretion of lime^by minute algae 

 of low organization, all traces of which are destroyed by recrystal- 

 lization. 



Walther (sS'So) suggests that the Rogensteine of the Bunt 

 Sandstein of North Germany may have been formed in salt seas 

 in the desert, similar to those now forming through algous growth 

 in the Great Salt Lake of Utah, and in the Salt Lakes of the Kala- 

 hari desert. 



The stromatoliths of Kalkowsky are large masses of calcareous 

 material having a stratified as well as fibrous structure. He re- 

 gards them as structures similar to the ooids, only vastly larger 

 and growing chiefly in one direction, i. e., upward. In size they 

 are found up to one meter in diameter, and they often form beds 

 of considerable extent. 



Alteration of Oolites. Linck (Chapter IX) finds that recent 

 oolites consist or have consisted of aragonite, and that fossil oolites, 

 so far as investigated, are calcite, from which he concludes that al- 

 teration to calcite is a change which all oolites undergo with lapse of 

 time. It may also be briefly noted that oolites of chemical as well as 

 organic (phytogenic) origin may be altered by replacement by other 

 minerals. Thus siliceous as well as iron oolites are known, both 

 probably due to replacement of calcareous oolites. Of the former 

 a good example is found in silicified oolites of the salt seas of the 

 Kalahari desert in Africa (Kalkowsky-26), and in the Ordovicic 

 limestones of central Pennsylvania (Ziegler-6i). Of the latter a 

 typical example occurs in the basal iron ores of the Siluric in Wis- 

 consin where the grains are regular pellets of uniform character, 

 probably of phytogenic origin. Linck holds that original oolites 

 (chemically formed) of aragonite are saturated with iron solutions, 



