Chap. XVIIL] LOWEE SILUEIAN— NOETH AMEEICA. 
435 
Lower Silurian group of the eastern basin presents only the Pre-Chazy forma- 
tion, unconformably overlain in parts by Upper Silurian and Devonian rocks. The 
group between the Potsdam and Chazy deposits in the eastern basin has been 
separated into three divisions j but in the western basin these subdivisions have 
not yet been defined. In the latter the strata are comparatively flat and undis- 
turbed, while in the eastern basin they are thrown into innumerable plications, a 
vast majority of which present anticlinal folds overturned on the north-western 
side. The general sinuous north-eastern and south-western axes of these folds 
are parallel with the great dislocation of the St. Lawrence, and the undulations 
are a part of those belonging to the Appalachian chain of mountains. It is in 
the western basin that we must look for the more regular succession of the 
Lower Silurian rocks from the time of the Chazy Limestone, and in the eastern, 
including Newfoundland, for that of the older deposits. It is to be remarked, 
however, that in the great northern peninsula of Newfoundland, instead of un- 
dulations, lines of fracture and dislocation are observed, while the strata are 
comparatively little tilted ; and it has not yet been precisely defined where the 
limit between the eastern and western basins of that island is to be placed. It 
seems probable, however, from the recent investigations of Mr. A. Murray, that 
it will run on the east side of the Laurentian mountains, forming the chief mass 
of the peninsula just mentioned. 
" The succession of the Lower Silurian deposits in North America may be 
thus tabulated, the palaeontological evidence as to the relations of the Upper 
Calciferous and the Quebec Groups to the formations above and below having 
been determined by Mr. Billings : — 
Complete Series. 
Western Basin. 
Eastern Basin. 
Newfoundland. 
Caradoc - 
Llandeilo - 
' Primor- 
dial'../ 
12. 
11. 
10. 
9. 
8. 
7. 
6. 
5. 
4. 
3. 
2. 
1. 
Hudson-river Group 
Trent 11 , Bird's-eye, &c 
Chazy Limestone . . . 
Sillery. ] 
Lauzon. 1 Quebec Gr. 
Levis. J 
UpperCalciferous Gr. 
LowerCalciferous Gr. 
Upper Potsdam Gr. 
Hudson-river Gr. 
Utica Slate. 
Trenton,B's-eye,&c 
Chazy Limestone. 
Quebec Group. 
Sillery. ] 8 
Lauzon. 1 'S 
Levis. J <~? 
Sillery. ] 8 ^ 
Lauzon. | o 
Levis. J 
U. Calciferous G. 
L. Calciferous G. 
U. Potsdam Gr. 
L. Potsdam Gr. 
St. John'sGroup. 
L. Calciferous Gr. 
U. PotsdamGroup. 
L.Potsdam G. 
St. John's Gr. 
a It thus appears that the lower portion of the series is complete in Newfound- 
land, the upper in New York, and that the divisions 3, 4, and 5 have not yet 
been recognized in the eastern continental region. The St. John's Group (1) is 
represented at St. John's, New Brunswick by three thousand feet of black slates 
and sandstones, whose fauna, described by Mr. Hartt, was correctly referred by 
him to Barrande's ' Stage c ' or 1 Primordial ' Silurian zone. It there reposes on 
rocks regarded as Cambrian. The slates of St. John's, Newfoundland, and the 
Paradoxides-bed at Braintree, Massachusetts, also probably belong to the same 
horizon. 
u The Lower Potsdam Group (2) is represented by several hundred feet of 
limestones and sandstones on the Strait of Bellisle, by a much greater thickness 
in White Bay, Newfoundland, and by the slates of St. Alban's and Georgia, 
Vermont. The Upper Potsdam (3) is'that of Wisconsin and Minnesota, repre- 
sented by the typical Potsdam rock of New York, and is overlain by the 
2f2 
