a ae ee 
American Geology, by T. S. Hunt. 397 
palzozoic age, may have existed during the long Laurentian 
eri 
Al ong the northern rim of ros great palzeozoic basin of North 
America the Potsdam sandstone of the New York geologists is 
he 4 seman the lowest eateg from below Quebec to the Island 
eal, and thence passing up the valley of Lake Cham- 
sc and sweeping round the Adirondack mountains, until it 
reénters Canada and soon ‘ond ‘Black Bi to the north of Lake Onta- 
and furt 
_ great Lake Superior group of sla as rag sandstones, which re- 
posing on the unconformable Huronian system, constitute the 
upper penne aie rocks of this region. This Lake Superior 
group, as Sir William Logan remarks, may then include the 
Potsdam, Caleiferous and Chazy, and thus bé equivalent in part 
to the Quebec group hereafter to be described. 
Passing westward into the ee valley we again find a 
sandstone formation, which forms the base of the paleeozoic 
series, and is considered by Mr. Tall to be the equivalent of the 
Potsdam. Here it occasionally exhibits intercalate so 
silico-argillaceous limestone, in which occur abundant remains of 
trilobites of the gener a Dikellocephalus, Menocephalus, Arionellus, 
and Conocephalus. Pateing upwards this ae a is succeeded 
by the Lower Magnesian limestone, which is the mr sere of 
the Calciferous sand-rock of New York, and in Missouri, where 
it is the great metalliferous emai, alternates severe times 
with a sandstone, constituting the Magnesian imestone series, 
which in Missouri attains a thickness of 1300 feet. The same 
thing is observed to a less degree in Wisconsin and Iowa; 
throughout this region the higher beds of the Potsdam sandstone 
are often composed of rounded oolitic granules, and the of ° 
assage are frequently of such a character as to lead to the con- 
clusion that they have been deposited from silica in solution, 
and are not mechanical sediments.* For a discussion of some 
facts with regard to the chemical origin of many silicious rocks, 
see this Journal, [2], xviii, 
Kvidences of ‘disturbance during the period of its deposition 
are to be found in the brecciated beds, sometimes art feet in 
thickness, which pees in oe pire sandrock e north- 
west, and are made of the ruins of anearlier sandstone. In Mis- 
souri, the Birdseye and Black 1 River limestones directly 
upo Lower Magnesian limestone, while farther north, a 
Sacro ster oe occupying the class of the emben lime- 
ston 
* See Mr. Hall’s prt apieaes. to which we are indebted for many of these facts 
regarding the formation o west, and also the Reports of the Geological Survey 
of Missouri. 
