446 PRINCIPLES OF STRATIGRAPHY 



Ballstone Reefs. 



In the Siluric (Clinton and Lockport ) horizons of western New 

 York and the Wenlock of western England, reef-like accumulations 

 of an impure argillaceous calcilutyte occur, of a lens-like or ball- 

 like form, and embedded in shales or fragmental limestones, which 

 arch more or less around them. They vary from two to fifty feet 

 in diameter, though one in the Lockport has a length of no feet. 

 When not spherical, they range in thickness from one to fifteen 

 feet. In England these have long been known as Ball Stones, but 

 their origin has not heretofore been described. While in large part 

 composed of fine, calcareous mud, the structural groundwork of 

 these lenses is formed by branching and fan-like types of Bryozoa 

 or other organisms which grew upon the sea bottom in colonies, 

 and which captured the fine sediment from the water and formed 

 a suitable medium for its retention. On these reefs, as rich feeding 

 grounds, lived a multitude of brachiopods, molluscs, trilobites and 

 other organisms, whose remains are now found in or around these 

 lenses, while many of them are comparatively rare or unknown in 

 the adjoining strata. In western New York (Grabau-39 :pp ; 

 Sarle-76) these ball reefs are found in the upper Clinton limestone 

 (Irondequoit limestone), and are either entirely embedded in this 

 otherwise clastic lime rock (calcarenyte) or lie in its upper portion, 

 partly enclosed in the overlying Rochester shales. More rarely are 

 reefs of this type found in the higher Lockport dolomite. They 

 are generally in the form of flattened lenses due to the cessation of 

 growth before they had reached their full size. These reefs are 

 often closely crowded, so that it has been possible to step from 

 one reef knoll to the others in places where erosion has exposed 

 their upper surfaces. The bryozoans forming the foundation of 

 these structures are several species of fistuliporoids, these some- 

 times being the only organic material found in the reefs. Tri- 

 lobites, especially the cephala and pygidia of Illaenus, abound near 

 the margin, and brachiopods are also common, most abundant 

 among them being WhMeldclla nitida. The silt captured by these 

 organisms was exceedingly fine, so that the rock is a typical argil- 

 laceous calcilutyte, and breaks with a conchoidal fracture. On fresh 

 surfaces the bryozoans are not visible, except when their epithecal 

 surface is exposed, but weathering generally brings out their struc- 

 ture. 



The Wenlock Ballstones are generally more spherical than those 

 of New York, in which the lens form prevails. They are found at 

 various levels in the thin-bedded limestones of Wenlock Edge in 



