Ulrich—Paleozoic Systems in Wisconsin, 
99 
Maysville) where Butts/® following the long-continued practice of 
the State geologists of New York, recently drew it. But it is im- 
possible to locate this plane in New York. There, as in Pennsyl¬ 
vania, the boundary must be placed above the Oswego.^^ 
The Ordovician-Canadian break .—The Ordovician system as de¬ 
fined and restricted by Ulrich and given in the preceding table 
begins in the Upper Mississippi Valley with the St. Peter sand¬ 
stone. That the surface upon which this sandstone was laid down 
is decidedly uneven because of preceding weathering and erosion 
has been long recognized in the geological reports of the State. 
The evidence in hand 40 years ago was already sufficient to estab¬ 
lish much of the importance of the break between the St. Peter and 
the underlying formation which then was supposed to be always 
some part of the ^‘Magnesian limestone.” However, in more re¬ 
cent years much information bearing on this break has been ac¬ 
quired. We now know, especially from the evidence of deep wells, 
reported by Thwaites, that the St. Peter may rest on any forma¬ 
tion between the top of the Shakopee and the eroded top of the 
Eau Claire formation. We have learned further that the Sf. Peter 
represents merely a late and perhaps the latest stage of an alter¬ 
nating series of limestones and sandstones that so far as known is 
best developed in northern Arkansas. There, moreover, it is clearlj 
shown^® that the deposition of this series was interrupted at least 
once by emergence and deep erosional wear of the Everton lime¬ 
stone. At the bottom of the series in Arkansas there is often a 
chert conglomerate which sometimes reaches a thickness of 8 or 10 
feet. Under this lies the Powell limestone, which corresponds ap¬ 
proximately in age to the Shakopee dolomite of Wisconsin and ad¬ 
joining States to the west and south. Both of these formations are 
assigned to the upper division of the Canadian system. 
But we have learned something more, too, about the occurrence 
of thick deposits of clastic material at' the base of the St. Peter in 
southern Wisconsin and northern Illinois. This information has 
been acquired in limited amount from study of outcrops by the 
writer and his associates and in greater amount and details through 
the study of well records by Thwaites. This clastic material or 
conglomerate consists of great accumulations of chert boulders 
Geologic section of Blair and Huntingdon counties, central Pennsylvania. Amer. 
Jour. Sci., vol. 46, p. 536, 1918. 
This systemic break is discussed at length by E. O. Ulrich, Ordovician-Silurian 
Boundary, Int. Geol. Cong., XII, Canada, 1913, C. R., pp. 593-667, 1914. 
Geol. Atlas U. S., Eureka Springs-Harrison Polio, 202, 1916, p. 7. 
