44Q 



HISTORICAL GEOLOGY 



through Idaho to Canada and another from Mexico into Arizona 

 and New Mexico. It was during this time that the great limestone 

 strata (Niagaran) were deposited. (3) The epicontinental seas 

 were again restricted in the Upper Silurian (Fig. 412), the most 

 important of them extending from Wisconsin and Illinois through 



New York and over 

 the Appalachian 

 trough. The three 

 subdivisions of the 

 period are therefore 

 characterized (1) by 

 constricted seas, (2) 

 by expanded seas, 

 and (3) by a later 

 shrinking and shift- 

 ing of the seas. It 

 should be pointed 

 out in this connec- 

 tion that Silurian 

 strata do not cover 

 all of the areas shown 

 in the maps (p. 403), 

 but often lie only in 

 widely separated 

 patcheswhich appear 

 to be remnants of a 

 once continuous 

 formation. The age 



and correlations of 

 FlO 4.0. -Map showing the probable outline of North h h d 



Amktii a during a portion of the Lower Silurian. (Modified . r 



after Schuchert.) termined by their 



contained fossils. 



I he sediments which were deposited in the Appalachian trough 

 \\tn d( n\ ed from the broad island or continent of Appalachia (p. 406). 



Character and Thickness of the Sediments. — Limestones are the 

 common strata of the Silurian, but in eastern North America con- 

 glomerates, sandstones, and shales predominate. These latter were 

 deposited In shallow se;is, as the ripple marks and cross-bedding show. 

 I 1 1( formation and distribution of these coarse sediments teach an 

 important lesson. Along the western boundaries of the eastern lands, 



