FRANKLIN COUNTY. 601 
About fifteen feet of this formation are shown at the point first men- 
tioned, viz., in the bank of Big Darby below Georgesville. It is im- 
mediately overlain by the heavy and easily recognized ledges of the Corni- 
-ferous limestone. This point, then, possesses the interest that always 
att ches to a well marked boundary in a geological series. In fact, the 
junction of two great divisions of geological time is found here, the Hel- 
derberg limestone belonging, as will be remembered, to the Upper Silu- 
rian +ystem, while the Corniferous is a Devonian formation. As this is 
the only point in all this portion of the State where the line of junction 
between these limestones is distinctly shown, it will be well to note 
with care the facts that are here met. 
The Lower Helderberg limestone as found here presents the same 
general appearances that its outcrops, both to the north and south, ex- 
hibit. The greater portion of it is a very thin-bedded, buff colored, 
magnesian limestone, which could be confidently identified at once by 
any one acquainted with the formation as shown either in Highland 
county or in the islands of Lake Erie. It contains also, here as well 
elsewhere, so notable a quantity of bituminous matter that it can be 
recognized by the odor of a freshly broken surface almost as readily as 
by its appearance; unless carefully examined, the limestone will be pro- 
nounced non-fossiliferous, for there are considerable portions in which 
no traces of life remain. Occasional layers are found, however, that con- 
tain indistinct casts of the characteristic fossil, Leperditia alta, of two or 
three small brachiopods and of a small number of corals. 
Another phase that the formation here exhibits may be styled the con- 
crelionary phase, masses rudely spheroidal in shape and which show in 
section something of a concentric structure, varying from six inches to 
two or three feet in diameter, are met with, especially in the lower part 
of the limestone that is here exposed. The smaller masses are in ap- 
pearance not unlike sponges of the S/rumatopora group, but there is no 
good reason to believe that any of them are of organic origin. These 
concretions seem entirely destitute of fossils. Some of the beds towards 
the upper portion of the series are distinctly brecciated, i. e. composed 
of angular fragments that have been re-cemented by the infiltration of 
water holding carbonate of lime in solution. This same peculiarity 
of structure is reported in rocks of this formation in the n».t. ern part 
of the State. 
These seams of clay are sometimes found in the uppermost beds of the 
section, a fact not elsewhere reported in the Lower Helderberg rocks of 
the State. A question may be entertained as to whether the clay occurs 
in seams or in pockets. If the latter term is the proper one, it might be 
