Jan., 1912.] 
The Arnheim Formation. 
43 1 
2. Eastern Kentucky, from Maysville to Stanford. 
Platystrophia ponder osa 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. 
