THE DEVONIAN PERIOD 567 



beds may represent less than the full Upper Devonian, and in others 

 more. The Catskill formation is poor in fossils, and such as occur 

 are partly, if not wholly, of 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 conditions necessary for marine life. The 

 redness of the formation is a feature which marks many other forma- 

 tions made in inclosed or partially inclosed basins. 



The Catskill formation has a thickness of 3,000 feet in New York 

 and twice that amount in Pennsylvania. Local beds of sandstone 

 (the Oneonta of central New York), seemingly like the Catskill in 

 origin, occur outside the Catskill region, suggesting 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. 1 In Ohio the same series (Black, or Ohio shale 2 ) has 

 a maximum thickness of 2,600 feet, and thins notably to the north 

 and west, being but a few hundred, and often but a few score feet 

 thick in Indiana, Illinois, Iowa, southern Michigan, Kentucky, and 

 Tennessee. Diverse names are applied to the series and its sub- 

 divisions in various localities. 



Although the implication of the preceding paragraphs is that 

 the Devonian formations of the East are chiefly marine, it should 

 be added that some of the formations, as the Portage and some of 

 the beds of Maine, contain considerable numbers of land plants, 

 and are presumably, not altogether marine. 



West of New York and Pennsylvania the Upper Devonian beds 

 have few commonly recognized subdivisions, or, in most places, 

 none at all, and the eastern names are not in general use. 



Devonian of the West 



The Devonian system, so far as known, is absent from the 

 larger part of the Great Plains within the United States, and this 

 great expanse of territory was probably land during the period. 

 No Devonian beds are found about the older formations in Dakota 



1 Prosser, Jour. Geol., Vol. IX, pp. 415-442. This article is a concise sum- 

 mary of the Paleozoic systems of Maryland. 



2 Geol. Surv. of Ohio. 



