288 



PALAEOZOIC TIME — DEVONIAN AGE. 



Chemung epoch.— The Chemung group extends widely over the 

 southern tier of counties of New York, and consists of sandstones 

 and coarse shales in various alternations. The thickness has been 

 estimated at 1500 feet near the longitude of Cayuga Lake, and less 

 towards Lake Erie and beyond 



Fig. 501. 



lib 



11a 



10 c 



10 & 



Section of rocks of the Hamilton and Chemung Periods. 



In this section, from one by Hall, taken in Yates co., N.Y., 10 a, 10 b, 10 c, are 

 rocks of the Hamilton period ; a, the Marcellus shale; b, the Hamilton group; 

 c, the Genesee slate ; and in the Hamilton group, 2 is the Encrinal limestone, 

 and 4 the Tully limestone; 11 a is the Portage group, 11 b the Chemung group. 



Westward of New York the Portage and Chemung groups have been supposed 

 to be represented in Ohio by a sandstone called "Waverly sandstone," three to 

 four hundred feet in thickness. Many facts, however, point rather to the conclu- 

 sion that the larger portion at least of this series really belongs to the Subcar- 

 boniferous, or is at any rate newer than the Chemung. Late investigations 

 also render it more than probable that beds referred to the Chemung in Missouri 

 and Iowa are really more recent. 



To the south and southwest of New York, in Pennsylvania, and 

 beyond along the Appalachian region, the corresponding beds have 

 great thickness, amounting in some places to more than 3000 feet. 

 The rocks have the same sandstone character as in New York. 

 This remark applies to the beds in the northeast towards Gaspe ; 

 for the Upper Devonian is represented in that part of the continent. 

 The beds at St. John's, New Brunswick, have been specially referred 

 to the Upper Devonian, but their precise parallelism with the New 

 York Devonian has not been determined. 



The beds of both the Chemung and Portage groups in New York 

 and Ohio abound in ripple-marks, obliquely-laminated layers, mud- 

 marks, and cracks from sun-drying,— evidences of the existence of 

 extensive exposed mud-flats, of sandy or muddy areas swept by 

 the waves, and of tidal currents in contrary movement through the 

 shallow waters. 



