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 

 attaches to g, 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 f^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 w^th 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 he 

 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- 

 cretionary phase, amasses 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 Stromatopora 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 northern 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 



