406 GEOLOGY OF OHIO. 



The areas occupied by each division are indicated on the accompany- 

 ing map. The formations will be briefly treated in ascending order. 



1. The Blue Limestone or Cincinnati Group is principally shown in the 

 valleys of the county. Of these the valley of Twin Creek is the widest 

 and deepest, but it is so much obscured with drift, that it does not furnish 

 as satisfactory exposures of the rocky floor, as many of the shallower 

 valleys do. Seven Mile Creek gives, on the whole, the best exhibition 

 of this series. Prom Camden to Halderman's mill, the stream ruas 

 much of the way upon the rock, and excellent opportijnities are fur- 

 nished for studying the structure and collecting the fossils of this inter- 

 esting formation. The Blue Limestone is so homogeneous that a full 

 description of its beds at one point will cover almost every exposure of 

 the system. No general statements, therefore, will be added to those 

 that have already found place in previous reports. At Barnett's mills, 

 several specimens of a fossil, elsewhere quite rare, have been found, viz., 

 Trochoeeras f Baeri, M. and W., an early representative of a still existing 

 family of chambered shells. Rhynconella dentata is also found in the same 

 locality. 



The Blue Limestone yields a large quantity of building stone of fair 

 quality for local use. It was formerly the main dependence of that part 

 of the county in which it occurs as a source of lime, but the numerous 

 advantages of the Cliff Limestone for this purpose have brought it into 

 universal use as a substitute. 



2. The Clinton Limestone comes next in order. The line of junction 

 between the Lower and Upper Silurian is very distinct in Preble county. 

 The same line of facts observed in Montgomery county is met with here. 

 A series of springs and a very productive belt of country mark this 

 geological boundary. The explanation of each fact is obvious when the 

 nature of the formations that are in contact is taken into consideration. 

 The shales with which the Blue Limestone series is terminated are im- 

 pervious ; the Clinton Limestone that covers them is porous and is also 

 traversed with lines of fracture. Springs must, therefore, necessarily 

 appear along the outcrop of the two formations. Springs flowing over 

 the margin of shales will do something toward imparting fertility to 

 them, and this particular series of shales possesses the elements of fer- 

 tility in large measure in their natural constitution. 



Many of the finest farms of the county belong to this particular hori- 

 zon. All of the facts here noted can be very clearly seen in the Morning- 

 star neighborhood in Lanier township. 



The Clinton Limestone exhibits, in the county, all the usual charac- 

 ters of the formation as it is found in Southern Ohio. 



