Devonian System in North America. 57 



greater vertical range." (Pal. of the Eureka District p. 4). 

 Some cases mentioned by Mr. "Walcott are Orthis Tulliensis at 

 the top, Orthis impressa at the base, and several Corniferous 

 corals at the upper horizon (see pp. 4 and 5, etc). 



It is also noticed that the faunas in the higher shales show 

 combinations of Devonian and Carboniferous types (White 

 Pine Shales), but a careful study of the species reveals the 

 characteristic changes of the general fauna that are seen in the 

 eastern sections. 



For instance, the new type of Brachiopods belonging to the 

 genus Produetus (called Productella in the New York Re- 

 ports) begins in this western section with certain small forms 

 typical of the lower and middle Devonian of the east, and it is 

 only in the upper horizons that the larger Chemung types of 

 Produetus appear. The same thing is seen in the changes in 

 the types of ISpirifera; the characteristic upper Devonian Sp. 

 disjuncta appears only in the upper part of the section as in the 

 east. The peculiarities of this western section in its Paleontol- 

 ogy, are most readily explained by the assumption, supported 

 also by other facts, that throughout the whole age the deposits 

 ■of this area were made in a wide, open ocean, with islands, 

 perhaps, but with no great masses of land to disturb the general 

 uniformity of the conditions of life. 



The central area was, doubtless, at considerable distance from 

 land but in no great depth of depression. The eastern conti- 

 nental area from Michigan around through Canada, ~New York 

 and down the Appalachians, must have been during the Devo- 

 nian age, near enough to shores for the faunas, as well as the 

 nature of the deposits, to be affected by the ocean currents, and 

 to feel strongly the effects of relatively small amounts of change 

 of level between land and water. Here the faunas are both 

 more local and more limited in geologic range, changing more 

 suddenly and fully in their combinations and species. The 

 conditions of the eastern border were those of rough and tem- 

 pestuous coasts. 



Conclusions. 



There are thus, 1st, a northeastern border area, mainly com- 

 posed of coarse, arenaceous deposits, thick, and with little to 

 distinguish it into subordinate zones. 



2d. An eastern continental area, with sandstones, limestones, 

 shales and conglomerates alternating with each other, and pre- 

 senting a rich and varied series of faunas, marking a consider- 

 able number of distinct zones which follow in a constant order. 



3d. A central continental area, mainly limestone and soft 

 argillo-calcareous shales, and, compared with the more eastern 

 sections, very thin, and presenting a fauna which represents the 



