406 | GEOLOGY OF OHIO. 
The areas occupied by each division are indicated on the accom pany- 
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 1s 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. From Camden to Halderman’s mill, the stream runs 
much of the way upon the rock, and excellent opportunities 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., 
Trochoceras? Baeri, M. and W., an early representative of a still existing 
family of chambered shells. Hhynconella dentata is also found in the same 
locality. 
The Blue Limestone yields a large quantity of building stone of fair 
quality fer 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 ere 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, anc 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 ee hori- 
zon, Allof the facts here noted can be very clearly seen in the Morning- 
star neighborhood in Lanier township. i 
The Clinton Limestone exhibits, in the county, all the usual charac- 
ters of the formation as it is found in Southern Ohio. 
