406 DEVONIAN PERIOD 



the interior sea), after standing low while the Onondaga limestone 

 was making, were elevated, or were less protected by vegetation, 

 or subjected to more concentrated or spasmodic precipitation 

 during the Hamilton epoch. The land formations might then have 

 been undergoing decay during the Onondaga epoch, though the 

 products of decay were not removed. Under the changed condi- 

 tions postulated, there would have been opportunity for their 

 transportation and deposition. 



In the east, where the series is mainly clastic, it reaches a thick- 

 ness of 1,500 to 5,000 feet (Pennsylvania); but in the interior, where 

 it contains more limestone, it is much thinner. 



The Upper Devonian. The Upper Devonian has a distribution 

 (Fig. 359) similar to that of the Middle, though somewhat more 

 widespread. The Upper Devonian is more distinct from the 

 Middle than the Middle is from the Lower, and is somewhat closely 

 connected with the lower part of the succeeding system. 1 An un- 

 comformity appears at the base of the Upper Devonian in some 

 places, and in others the series overlaps other Devonian formations, 

 resting on the Ordovician. 



The Senecan series of New York consists of various thin forma- 

 tions (p. 402), chiefly clastic, of shallow-water and terrestrial origin. 

 The Chemung formation of western New York is similar to the Sene- 

 can formation, though more sandy, or even conglomeratic. The 

 Catskill formation of the Catskill region consists of red shales and 

 sandstones, which appear to be, in a general way, the time-equiva- 

 lents of the Chemung. In some places the Catskill beds may 

 represent more than the Upper Devonian, and in others less. They 

 are poor in fossils, and those known are partly, if not wholly, fresh- 

 and brackish-water forms. Hence it is inferred that the Catskill 

 region was so far shut off from the ocean as not to afford the condi- 

 tions necessary for marine life. Redness characterizes many forma- 

 tions made in inclosed or partially inclosed basins. Outside the 

 Catskill region local beds of red sandstone suggest that similar 

 conditions of deposition existed now and then farther west. 



The thickness of the Upper Devonian in central and western 

 New York approaches 4,000 feet, and is even more in Pennsylvania 

 and Maryland. In Ohio the equivalent series (Black, or Ohio 



1 Ulrich has recently proposed grouping the Upper Devonian with the lower 

 part of the Mississippian, as a new system, under the name of Waverlyan. Bull. 

 Geol. Soc. Am., Vol. XXII. 



