FORMATIONS AND PHYSICAL HISTORY 393 



clastic sediments in the Appalachian belt, while in the interior it 

 largely of limestone. The site of sedimentation in the east was a 

 rt of trough (the Appalachian trough) shut off from free communi- 

 . -at ion with the interior sea, but connected narrowly with the 

 Atlantic, perhaps by way of the present Chesapeake region. Since 

 most of the sediments of this trough were deposited in shallow-water, 

 they are thought to indicate that the trough was sinking at a rate 

 i-omparable to that at which the sediments accumulated. With the 

 do\vn-\\arping of the trough, there may have been up-warping of 

 tin- adjacent lands to the east, which supplied the sediments. 



The history of the Silurian period, as now understood, involves, 

 (i) a general submergence of the eastern part of the United States 

 irest of Appalachia, by which the sea became more and more wide- 

 >read until the close of the Niagaran epoch; (2) a partial with- 

 Irawal of the sea from the same area in the Salina epoch; and 

 5) an extension of the sea at the close of that epoch. 



Silurian of the West 



At various points in the West, there are sedimentary beds, poor 

 fossils, between the known Ordovician below and the Devonian 

 )ve. The character of the fossils being indecisive in many places, 

 the age of the beds is open to question. Some of them may be 

 Silurian. If the Silurian is really absent from all the areas where 

 its presence is not now known, it would appear that a large part of 

 western North America was land during the Silurian period. Silu- 

 rian beds are however known in Southern California, Nevada, Utah, 

 and Alaska, and perhaps in the Canadian Rockies, and their dis- 

 tribution may be more widespread than has been supposed. 



General Considerations 



Igneous rocks. At few points in North America have igneous 

 rocks of Silurian age been identified. Silurian formations are locally 

 affected by intrusions, but their dates are generally uncertain, 

 some of the igneous rocks of New Brunswick are thought to be of 

 Silurian age, and perhaps some of those of Nova Scotia and Maine. 



Close of the period. The geographic changes at the close of the 

 Silurian were less than those at the close of the Ordovician, and 

 the system is less distinctly separated from the Devonian above than 

 from the Ordovician below. 



Climate and duration. There is nothing to indicate great 



