472 PRINCIPLES OF STRATIGRAPHY 



show evidence of wind transportation, as in the case of modern 

 oolite dunes of the Florida coast, the shores of the Great Salt Lake 

 of L'tah, and the coast of the Arabian Sea. Such transported ma- 

 terial generally shows eolian cross-bedding, a feature very com- 

 monly associated with fossil oolites. This is well shown in the 

 Silufic oolites of Gotland and better -still in the Jurassic oolites 

 of England, where rema:rkably fine examples of wind bedding are 

 seen. 



The oolites of the Upper Siluric (Monroan) of Michigan have 

 been referred by Sherzer (43) to an origin similar to that of the 

 Great Salt Lake oolites. Rothpletz regards those from the Lias and 

 the Triassic Wetterstein Kalk of the northern Alps as due to algous 

 growths. From the Carbonic of England, oolites have been de- 

 scribed and referred to algous origin. The same is true of the 

 well-known oolites of the Jurassic of. England which have given 

 many of the formations their local names (Superior oolite, Great 

 oolite, etc.) These oolites show, according to Wethered (58; 59), 

 organic structures similar to those described by Nicholson and 

 Etheridge as Girvanella prohlematica from the upper Siluric of the 

 Girvan district of Ayrshire. Both oolites and pisolites of these hori- 

 zons show often a concentric encrustation of organic fragments by 

 organisms forming flexuous or contorted tubules. Several species 

 have been distinguished by Wethered (58), among them Girvanella 

 pisolithica, which forms the Peagrit of the Inferior oolite and 

 coralline oolite. 



Rogensteine of the Bunter Sandstein. The remarkable ooliths 

 of the Bunt Sandstein of North Germany known as Rogensteine 

 have been described in great detail by Kalkowsky {2^). This 

 author restricts the name oolith to the rock having an oolitic struc- 

 ture, and proposes the term "ooid" for the individual grains. The 

 diameter of the ooids never falls below o.i mm., though smaller 

 grains occur (0.1-0.5 mm.), which he regards as ooids in the mak- 

 ing — "embryonic ooids.'' The maximum size is 7 mm., only one 

 case exceeding that having been known. He finds in all cases for- 

 eign bodies forming the nuclei of the ooids, these bodies being 

 commonly small crystals of calcite, or again scales and rods of a 

 fine-grained clay slate. Kalkowsky finds the following structures 

 represented : 



1. Concentric structure, layers of clear calcite of varying width, 

 alternating with more or less opaque calcite layers carrying im- 

 purities of clay. 



2. Fine radial structure, produced by radial arrangement of 

 fine tiireads of CaCOg. 



