THE DEVONIAN PERIOD. 433 



sulphuretted hydrogen or ammonium sulphide derived from decaying 

 vegetable or animal matter. This layer of pyrites is remarkable for 

 its fauna of dwarfed species, 45 in number. The average size of the 

 species is only about one fifteenth of the normal. Not only are the 

 species dwarfed, but they represent species arrested in their develop- 

 ment, appearing like the young of species of earlier Devonian types. 

 The succeeding Genesee shales are black and bituminous, and range from 

 a few feet (25 on Lake Erie) to a few hundred feet (150 in central New 

 York, 300 in west central Pennsylvania) in thickness. The Portage 

 formation, composed of shales and sandstones, is much thicker, being 

 1000 to 1400 feet in western New York. To the southward, its separa- 

 tion from the Chemung is less distinct than in New York. All these clas- 

 tic beds bear evidence of shallow-water origin. Their stratigraphic 

 relations are suggested by Fig. 186. 



The Chemung and Catskill formations (Chautauquan series). — The 

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

 more arenaceous. It is locally conglomeratic, and everywhere bears 

 the marks of a shallow-water origin. Locally the conglomerate has 

 been eroded into fantastic forms, giving rise to "Rock Cities/' as near 

 Panama in western New York. The formation ranges in thickness 

 from 950 feet near Lake Erie, to 1500 feet in the vicinity of Cayuga 

 Lake. Much greater thicknesses, 8000 feet maximum, are attained 

 in Pennsylvania, but the formation thins rapidly westward. In 

 northern Pennsylvania, the Cattaraugus formation overlies the Che- 

 mung, and a part of the Oswayo formation above the Chemung is 

 also classed as Devonian. 1 



West of New York and Pennsylvania the Upper Devonian beds 

 have few commonly recognized subdivisions, or none at all, and the 

 eastern names are not in general use (see Appendix, Vol. III). 



It may be here remarked that the Devonian was probably more 

 extensively developed to the southwest than was formerly supposed. 

 Devonian beds are said to occur at the summit of the Ozark uplift 

 in Missouri, 2 though their present exposure in this region seems to be 

 limited. 



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

 which appear to be, in a general way, the time-equivalents of the Che- 



1 Fuller, Gaines (Pa.) folio, U. S. Geol. Surv. 



2 Keyes, Bull. Geol. Soc. Am., Vol. XIII, pp. 268-292. 



