surface, and by the forming of caves in the rock under- 
neath, the bottom afterwards sunk down, leaving the lakes 
thus with their precipitous sides. It is, perhaps, not out 
of place to mention in this connection that there are sev- 
eral large caves in the surrounding limestone rock; some 
of these have never been thoroughly explored. 
In bringing these introductory remarks to a close, we 
conclude, not because we have exhausted the list of the 
natural advantages of our county, but because enough of 
them have been mentioned to prove the statement with 
which we started. Hence, we refrain from enumerating 
others, and pass on to that portion of the work which will 
treat more particularly about its geological facts and phe- 
nomena. 
THE SILURIAN AGE. 
The Age of Invertebrates, as it is sometimes called, has 
been subdivided into two parts: the Lower and Upper 
Silurian Ages. Since the first of these divisions 1s nowhere 
found exposed in our county, we will leave it without 
further notice and say a few words concerning the second. 
The Upper Silurian Age is introduced with a limestone 
formation, has some courses of shale and a large distribu- 
tion of limestone. It is divided into three periods: the 
Niagara, Salina and Lower Helderberg. As a whole, the 
character of these different formations indicates an epoch 
when there were shallow waters, or even swamps and 
marshes. The mud-cracks and ripple-marks of the Medina 
Sandstone, or the immense hopper-shaped pseudomorphs 
of the Salina Period clearly indicate this. 
This age is marked by an abundance of life, especially 
the marine; land animals probably did not exist, although 
a few terrestrial plants grew in the swamps and marshes. 
Vertebrates, however, were still unknown, none of them 
being found until we come to the Devonian Age. 
9 
