THE DEVONIAN PERIOD 565 



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

 or subjected to more concentrated or spasmodic precipitation 

 during the Hamilton epoch. Under the earlier conditions, the 

 land formations would have been undergoing decay, but the prod- 

 ucts of the decay might not have been removed; under the latter, 

 there would have been opportunity for the transportation of the 

 products of decay. Even during the general period of shale forma- 

 tion, however, limestone (often shaly) was making in some places, 

 as in the Mississippi basin. 



In the east, the Hamilton (including Marcellus) formation is 

 very thick, being 1,500 to 5,000 feet thick in Pennsylvania, where it 

 is mainly clastic. Its thickness over the interior, where it contains 

 more limestone, is much less. 



The Upper Devonian. The Upper Devonian series has a distri- 

 bution (Fig. 410) similar to that of the Middle, though it is more 

 wide-spread, especially west of the Mississippi. On the whole, 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. An uncomformity appears 

 at the base of the Upper Devonian in some places, and the series 

 overlaps all other Devonian formations, resting on Ordovician beds 

 in others. 1 



The Senecan series of New York consists of various thin marine 

 formations (p. 559), chiefly clastic, all of which bear evidence of 

 shallow-water origin. 



The Chautauquan series possesses some exceptional features. 

 The Chemung formation of New York is very like the Portage, 

 though more sandy, and even conglomeratic locally. It ranges in 

 thickness from 950 feet near Lake Erie, to 1,500 feet in the vicinity 

 of Cayuga Lake. Much greater thicknesses are attained in Penn- 

 sylvania, but to the westward the formation thins rapidly. 



In the Catskill region, there is a series of red shales and sand- 

 stones, the Catskill formation, which appear to be, in a general way, 

 the time-equivalents of the Chemung. In some places the Catskill 



1 Ulrich has recently proposed grouping the Upper Devonian with the 

 lower part of the Mississippian, as a new system, under the name of Tennes- 

 seean. Geol. Soc. Am., Dec., '08. 



