UPPER SILURIAN. 241 



sylvania. It is worthy of note that this limestone formation, of the 

 later Upper Silurian, was the first limestone that was produced over 

 the Appalachian region after the Lower Silurian. But the Trenton 

 beds spread through the west as well as the east, while the Helderberg 

 occur less extensively at the west ; and in this the two periods are in 

 contrast, the older limestone having the widest distribution. 



It has been stated that the Lower Helderberg limestone occurs even 

 east of the Hudson, overlying unconformably the Lower Silurian 

 slates, its nearly horizontal beds constituting the summit of Be- 

 craft's Mountain and Mount Bob, near Hudson ; and also that other 

 patches of it exist near Montreal. Logan suggests that a conglomer- 

 ate limestone filling a break in the rocks near Burlington, Vermont, 

 may be Lower Helderberg, as the conglomerate closely resembles that 

 near Montreal. Whatever the doubt with regard to the last men- 

 tioned locality, the other isolated beds are proofs of a former wide dis- 

 tribution of the Lower Helderberg limestone over Canada, and along 

 the lower part of the western slopes of the Green Mountain chain. 



4. ORISKANY PERIOD (8). 



I. Rocks: kinds and distribution. 



The Oriskany sandstone extends from central New York (the region 

 of Oriskany, Oneida County) southwestward along the Appalachians, 

 and spreads westward through Upper Canada and Ohio, into Indiana, 

 Illinois, and Missouri. Unlike the Lower Helderberg beds, it thins 

 out toward the Hudson River, becoming barely recognizable. The 

 rock over these regions is mostly sandstone, often rough in aspect, but 

 is partly limestone in the Mississippi basin. 



In the Eastern-border region the rock is mainly limestone. It 

 constitutes, in many places, the upper portion of the Silurian forma- 

 tion, lying between northern Vermont and Moosehead Lake in Maine, 

 and between the latter and Gaspe on the Gulf of St. Lawrence, its 

 characteristic fossils occurring at several localities over the region. 



The Oriskany sandstone strata are passage-beds between the Silurian 

 and Devonian. 



The Oriskany sandstone was made the commencement of the Devonian by De Ver- 

 neuil; but Hall has since referred it to the Upper Silurian, on the ground of the rela- 

 tions of its fossils. In New York, it consists either of pure siliceous sands, or of argil- 

 laceous sands. In the former case, it is usually yellowish or bluish, and sometimes 

 crumbles into sand suitable for making glass. The argillaceous sandstone is of a dark 

 brown or reddish color, and was once evidently a sandy or pebbly mud. In some 

 places, it contains nodules of hornstone. The beds are often distinguished by their rough 

 and hard dirty look (especially after weathering), and by the large coarse calcareous 

 fossil shells, — species of Brachiopods. In some regions they are cherty. The sand- 

 stone appears on Lake Erie near Buffalo, and enters Canada at Waterloo, on the Niagara 

 16 



