414 CAMBRIAN AND SILURIAN IN NORTH AMERICA. [XV. 



by the stratigraphical break and discordance in Herkimer 

 County, New York ; and by the fact that beyond the limits 

 of the Ottawa basin, on either side, the limestone of the Tren- 

 ton group rests directly on the crystalline rocks ; the older 

 members of the New York system being altogether absent at 

 the northern outcrop, as well as in the outliers of Trenton 

 limestone seen at the north of Lake Ontario, and as far to the 

 northeast as Lake St. John on the Saguenay. This distribu- 

 tion shows that a considerable movement, just previous to the 

 Trenton period, took place both to the west and' the east of 

 Adirondack region, which formed the southern boundary of the 

 Ottawa basin. 



[Lesley observes, "There are certainly evidences of some 

 obscure unconformability between the limestones of II. and 

 the slates of III.," which immediately overlie them in the great 

 Appalachian valley in Pennsylvania. This horizon corresponds 

 to the base of the Trenton (see, further, page 421), and the 

 evidence consists in the different strike of the rocks of the two 

 divisions, that of the overlying slates being alwaj's with the 

 valley, while the limestone-outcrops often cross it at various 

 angles. Lesley appears to regard the discordance of no great 

 importance ; but it deserves further study in connection with 

 the evidences of a similar want of conformity farther north- 

 ward. (Proc. American Philosophical Society, December, 1864, 

 page 469.) 



[There are, as we have seen, two breaks in the succession of 

 life in the Ottawa basin, the one at the base of the Trenton 

 group, and the other at the base of the Chazy ; and in this 

 connection, besides the fact of the absence of the latter between 

 the Calciferous and Trenton, observed by Hall in Herkimer 

 County, New York, should be noticed the remarkable section 

 near Grenville on the Ottawa, described by Logan in the 

 Geology of Canada. Here, at what is regarded as the base of 

 the Chazy, a conglomerate layer of seven feet, made up of lime- 

 stone pebbles, rests upon beds of yellow-weathering limestone, 

 supposed to be magnesian, and holding obscure fossils ; while 

 above it are fifty feet of sandstones, sometimes conglomerate, 



