124 HISTORICAL GEOLOGY 



to 5000 feet in Pennsylvania, while in the upper Mississippi Basin, 

 where much limestone still formed, its thickness is notably less. 

 A good idea of the distribution (surface and concealed) of the 

 Middle Devonian rocks is afforded by noting the water areas on the 

 paleogeographic map (Fig. 70), though from these areas some 

 Devonian strata have been removed by erosion. 



Upper Devonian Rocks. — These show a distribution very 

 similar to the Middle Devonian, except that the southern Appa- 

 lachians and region immediately westward also contain them. 

 Leaving out the area of Onondaga at the south end of Hudson Bay, 

 a good conception of the distribution of the Upper Devonian rocks 

 may be gained by examining the map (Fig. 68), because almost 

 everywhere that any Devonian is present, the Upper Devonian 

 also occurs. 



The Senecan series, except for the comparatively thin and local 

 Tully limestone, consists of the Genesee shales, and Portage sand- 

 stones and shales. The Genesee ranges in thickness from a few 

 feet in western New York, to several hundred feet in central 

 Pennsylvania, while the Portage is over 1000 feet thick in western 

 New York. 



The Chautauquan (Catskill x and Chemung) series of sandstones 

 have a thickness of 1000 to 1500 feet in western New York; 3000 

 feet in eastern New York; and a maximum of 8000 feet in eastern 

 Pennsylvania. The Catskill was quite certainly mostly a fresh or 

 brackish water deposit. 



In the Mississippi Valley, westward from New York and the 

 Appalachians, the Upper Devonian is much thinner; subdivisions 

 are not so well represented, or recognized; and the New York 

 names have not been applied. Also in western America, the LTpper 

 Devonian subdivisions have not been well made out. 



Comparison of Ordovician, Silurian, and Devonian Systems. — 

 " Comparing the rocks of the Ordovician, Silurian, and Devonian, 

 as these are developed in the Appalachian and adjoining regions, 

 a certain rhythmic or periodic recurrence of events may be dis- 

 covered among them. Each system is characterized by a great 

 and very widespread limestone formation, the Trenton, Lockport- 

 Guelph (Niagara), and Onondaga, respectively, and in each the 

 limestone is succeeded by shales or other clastic rocks, the Utica 



1 The Catskill is essentially an eastern pha^e of the Chemung in New 

 York. 



