376 THE ORDOVICIAN PERIOD 



land, and on the west islands of unknown size demarcated it 

 from the Pacific. The Ordovician rocks accumulated in the Inte- 

 rior Sea were principally limestones and dolomites, while at the 

 east extensive sandstones and slates were formed at the begin- 

 ning and toward the end of the period. Ordovician rocks have a 

 much wider extension than would appear from their surface dis- 

 tribution, for they are generally buried under sediments of a later 

 date. In the west, for example, they are exposed at the bottom 

 of many deep canons, lying beneath thousands of feet of younger 

 beds. In the southwest of the United States was a land area of 

 unknown extent. In the Grand Canon section no strata occur 

 between the Cambrian and the Carboniferous, while from Mexico 

 no rocks older than the Carboniferous have been reported. 



Around the northern pre-Cambrian land, in New York and 

 Canada, was formed the Calciferous, a limestone, generally magne- 

 sian and often sandy or cherty, which extends southward through 

 New Jersey and Pennsylvania, while equivalents of it are found in 

 the magnesian limestones of Iowa, Missouri, and Michigan. 



Deepening waters next gave opportunity for the formation of 

 limestones (C/iazy) which grew to a vast extension, especially the 

 great formation called the Trenton, which is developed in New 

 Brunswick, New York, Canada, the upper Mississippi valley, and 

 in the Rocky Mountains. Toward the end of the Ordovician 

 period there was a change in the eastern part of the Interior Sea, 

 whereby the clear waters became charged with fine mud and clay, 

 which now form a great mass of shales and slates (Utica and 

 Hudson). These rocks are thickest toward the east, extending 

 from the St. Lawrence along the Appalachian uplift into east 

 Tennessee, where they become much thinner and are in many 

 places represented by shaly limestones, and westward into Indiana. 

 The effects of the change were very widely felt. 



Ordovician rocks, prevailingly limestones, are also extensively 

 displayed in the western mountain ranges, the Rockies, Uintas, 

 Wasatch, and others ; they fringe the western side of the great 

 northern pre-Cambrian area and recur in the islands of the Arctic 

 Ocean. 



