Jan., 1912.] The Arnheim Formation. 431 



2. Eastern Kentucky, from Maysville to Stanford. 



Platystrophia ponderosa ranges from the middle Fairmount to 

 the base of the Richmond. Alone ,it does not designate any special 

 horizon within this large vertical range, unless advantage be taken 

 of some of the slight variations in form which may be recognized 

 at certain horizons. However, associated with any of the other 

 fossils mentioned above, it at once designates the Oregonia division 

 of the Arnheim. 



At weathered exposures, the upper or Oregonia division usually 

 is represented by a limestone rubble. This term is used to desig- 

 nate a mass of small, irregular limestone fragments. The lime- 

 stones from which the fragments are derived are thin, their upper 

 and lower surfaces frequently are irregular, they are more or less 

 penetrated by argillaceous material, and they break readily, 

 especially along the surfaces of the included fossils. Some layers 

 consist chiefly of entire shells and large fragments of fossils em- 

 bedded in a matrix of clay which is somewhat more indurated than 

 the clays immediately above and below. This induration is due 

 to a greater lime content, probably owing to the imbedded fossils 

 which may have given up part of their lime to the infiltrating 

 waters. On weathering, these layers are reduced to a mass of 

 fossils, partly free, but largely attached to one another more or less 

 irregularly at their surfaces of contact. These masses of free 

 fossils, of fossils partly cemented together by lime or indurated 

 clay, and of irregular fragments of limestone are very character- 

 istic of the upper or Oregonia division of the Arnheim. 



The lower or Sunset division of the Arnheim presents a very 

 different lithological appearance, but this appearance varies along 

 the line of exposure. 



At the deep railroad cut three miles southeast of Maysville, in 

 Kentucky, the Sunset division, 16 feet thick, consists chiefly of 

 comparatively unfossiliferous argillaceous limestone layers inter- 

 bedded with clay. The limestone layers usually are several inches 

 thick, they are of rather even texture, and their upper and lower 

 surfaces are not conspicuously irregular. They, therefore, do not 

 wear into a rubble, as in the case of the upper division of the 

 Arnheim. 



Northward, in Ohio, the quantity of clay interbedded with the 

 limestones of the lower division of the Arnheim increases, fossils 

 become fairly numerous, some of the limestone layers are distinctly 

 less argillaceous, and the strata forming the upper and lower divi- 

 sions of the Arnheim are less readily distinguishable, except by 

 means of their fossil content, the species of Platystrophia, Leptaena^ 

 Rhynchotrema, and Dinorthis, mentioned above, occurring at the 

 base or in the lower part of the upper or Oregonia division of 

 the Arnheim. 



