296 PALEOZOIC SYSTEM OF ROCKS. 



SECTION 1. — SlLURIAN SYSTEM : AGE OF INVERTEBRATES. 



The Rock-System. — The rocks of this age have been carefully studied 

 in England, by Sedgwick and Murchison ; in Russia and Sweden, by 

 Murchison ; in Bohemia, by Barrande ; and in New York, by Hall. 

 The divisions and subdivisions established by these geologists have be- 

 come the standard of comparison elsewhere. The system was first 

 clearly defined by Murchison in Wales. The name Silurian was given 

 by Murchison to the rocks of the whole age. The name Cambrian was 

 given by Sedgwick to the lower part. We have called the w T hole age 

 Silurian, but the great thickness and exceptional importance of the 

 lower part have induced many geologists to erect this into a distinct 

 system equivalent to the Silurian, and to call it Cambrian, or Pri- 

 mordial. We shall, however, make Cambrian, or Primordial, one of 

 the primary divisions of the Silurian age. 



Subdivisions. — The following table gives the divisions and subdi- 

 visions of the rocks and the corresponding periods of the age in this 

 country : 



{Lower Helderberg Period. 

 Salina " 



Niagara " 



DiiumuAgeoiAgej , Trenton 



of Invertebrates. . ' Lower Silurian \ Canadian " 



Cambrian or Primordial, -j ££*^ 



The larger divisions, viz., Primordial, Lower Silurian, and Upper 

 Silurian, are generally recognized. The subdivisions are local, each 

 country having its own ; but they are synchronized, as far as possible, 

 by comparison of fossils. As we shall be compelled to treat the age 

 together as a whole, we shall use the word Silurian to express the rocks 

 of the whole age, although we freely admit the superior importance of 

 the Primordial, or Cambrian, as compared with other divisions. 



Character of the Rocks.— The Silurian, like nearly all rocks, are 

 greatly disturbed and metamorphosed in mountain-regions, though 

 less so than the Laurentian ; but in Sweden and Russia, and in the 

 valley of the Mississippi, they are found in their original horizontal 

 position, and not greatly changed from their original sedimentary con- 

 dition. 



Area ill America.— By turning to the map (page 291) it will be seen : 

 1. That the Silurian is attached to the Canadian Laurentian nucleus as 

 an irregular border on the outer side of the V-shaped area ; 2. Again, 

 the Appalachian Laurentian region is also bordered on the west side by 

 Silurian ; 3. Also we observe large patches in the interior— one about 

 Cincinnati, another occupying the southern portion of Missouri and 

 northeastern portion of Arkansas, and one in Middle Tennessee ; 4. 



