48 A NATURALIST IN THE GREAT LAKES REGION 



they do 'now along the coral banks of the Gulf of Mexico. No 

 fish were swimming in the waters; they had not yet appeared 

 upon the earth. The land areas would have seemed strange, too, 

 for the vegetation did not consist of trees and grasses and flower- 

 ing herbs, but lowly plants like algae, liverworts, and ferns. 

 No insects were flying, no vertebrates, neither snakes nor frogs 

 nor birds, had yet evolved. 



Toward the close of the Ordovician period another extensive 

 change in level of the sea occurred. The central sea decreased in 

 size in this region though widening in the north (Fig. 32), coastal 

 plains emerged hereabouts, and from them and across them 

 great quantities of mud were washed so the shallow sea became 

 unfit for many animals. Many kinds were forced to migrate to 

 deeper seas, many perished, and their remains are very abundant 

 in the beds of shales and sandy limestones known as Richmond 

 shales. This stratum formed in the Chicago region to a depth of 

 100 feet and elsewhere in the interior sea became even thicker. 



The formation of the Richmond shales and sandstone was ter- 

 minated by a rise of the land so that the northern portion of the 

 interior sea including the Chicago region became dry land, subject 

 then to the destructive forces of erosion. So closed the Ordovician 

 period, and the Silurian period was ushered in. As it advanced 

 the sea deepened rapidly (Fig. 33) until conditions were favor- 

 able again for an abundant animal life along the shore and for 

 corals in the deeper waters. Many, many generations of animal 

 forms with calcareous skeletons lived and died and left their shells 

 or other hard parts to form the thick deposits that later trans- 

 formed into Niagara limestone to a depth of from 250-400 feet 

 in the Chicago region. 



This rock is full of fossils that help us gain a clear idea of the 

 animals that lived upon the sea bottom where today Chicago 

 stands and that contributed their skeletons and shells to help 

 form the rock out of which Chicago's inhabitants constructed 

 their buildings in the days before cement so largely replaced 

 building stone (Fig. 34). 



