THE ORDOVICIAN PERIOD 



81 



the Ordivician rocks as indicated on this map gives no adequate 

 idea of the former or present real extent of strata of this age, since 

 strata have either been removed from so many districts by erosion, 

 or are concealed under later formations, or are highly folded 

 so that outcropping edges only are at present visible. Some regions 

 can quite certainly be shown to have been formerly covered by 

 Ordovician strata, as, for instance, nearly all of the Adirondack 

 Mountain region, and a wide belt between the Great Lakes and 



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Fig. 43 



The Trenton (mid-Ordovician) limestone at its type locality, Trenton Falls, 



New York. (Photo by F. B. Guth, Utica, N. Y.) 



Hudson Bay. Also the distribution of outcrops, together with nu- 

 merous deep-well sections, conclusively prove that much, if not all, 

 of the Mississippi Basin contains concealed Ordovician rocks. In 

 the Appalachians, New England, and some of the western moun- 

 tains extensive Ordovician strata are actually exposed only along 

 comparatively narrow belts following the strike of the highly 

 folded rocks. 



Lower and Middle Ordovician Rocks. — Viewed in a broad 

 way, the Ordovician rocks (especially the Lower and Middle) are 



