162 PALEOZOIC TIME. 



n. PALEOZOIC TIME. 



I. AGE OF INVERTEBRATES, or SILURIAN AGE. 



The term Silurian was first applied to the rocks of the Silurian age 

 by Murchison. It is derived from the ancient name Silures, the desig- 

 nation of a tribe inhabiting a portion of England and Wales where 

 the rocks abound. 



The subdivisions of the Silurian are not only widely different on 

 two continents, as America and Europe, but also on different parts of 

 the same continent. In American geological history, it has been found 

 most convenient to recognize in the main that subdivision into periods 

 and epochs which is derived from the succession of rocks in the State 

 of New York, where most of the strata are well displayed and have 

 been carefully studied. 



Some standard for the division of time must be adopted; and, whatever that stand- 

 ard, it is afterward easy to compare with it, and bring into parallelism, the successive 

 strata, or events, of other regions. The State of New York lies on the northeastern 

 border of the great interior, — a vast region stretching southward and westward from 

 the Appalachians to the Rocky Mountains, and beyond the head-waters of the Missis- 

 sippi to the Arctic Ocean, over which there were many common changes ; and, owing 

 apparently to this situation on the north against the Archaean, and near the head of the 

 Appalachian range, there are indicated a greater number of subordinate subdivisions in 

 the rocks, or of epochs in time, than are recognized to the west. It is, therefore, a 

 more detailed indicator than other regions, of the great series of changes and epochs in 

 the Paleozoic era. 



On pages 375 to 379, sections are presented of the Paleozoic strata in different parts 

 of the United States; and, by means of them, the diversities between the regions may 

 be studied. The general truth, above stated, is well exhibited, that the geological 

 structure of the great Interior basin is more simple than that of New York and the 

 Appalachian region. 



The order of succession in the Silurian periods and rocks is shown 

 in the section on page 142 (Fig. 201). The numbers affixed to the 

 subdivisions of the section are used for the same formations through- 

 out the work. 



The Silurian age is divided into the Lower Silurian and Upper Si- 

 lurian. In North America, the transition in the rocks and life of the 

 two eras is comparatively abrupt. In Great Britain, the two are 

 generally unconformable in stratification ; but as regards life there is a 

 gradual transition between them. In Bohemia, there is no break in 

 the rocks, but a somewhat abrupt change in the life. Thus, even the 

 grander divisions in Geological history are not set forth alike in all 

 countries ; each great region has carried forward independently its 

 making of rocks, and had often its independent disturbances. 



