INTRODUCTION. 
Although the geology of but a part of the continent of North America 
has yet been carefully studied, local surveys and general reconnaissances 
have made known all the principal features of its structure. They have 
shown that all the members of the geologic series are somewhere in its 
great area well represented and that the strata contain fossils which enable 
us to make satisfactory comparisons with the geology of other divisions of 
the earth’s surface, and to fix beyond question the relative position of all 
important groups of rocks. We have learned, further, that the order of suc- 
cession in the strata composing the geologic column is the same here as in 
other parts of the world, and that the progress of animal and plant life 
during the geologic ages was in North America essentially the same as that 
revealed by explorations in Europe, Asia, Australia, Africa, and South 
America. 
Here, as elsewhere, the oldest group of fossiliferous rocks is the Cam- 
brian, and it contains a fauna which, including representatives of all the in- 
vertebrate subkingdoms, yet consists chiefly of trilobites. These are not 
only more abundant relatively and absolutely, but are more varied and 
larger than in the rocks of later ages. Hence the Cambrian is not improp- 
erly designated the Age of Trilobites. 
In the Ordovician (Lower Silurian) all the invertebrate subkingdoms 
are well represented, 10,000 or 12,000 species having been already col- 
lected from these rocks But the Mollusca are by far the most numerous ; 
and the huge cuttle-fishes, of which the chambered shells known as Ortho- 
cerata are sometimes a foot and a half in diameter, were the ruling dynasty. 
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