Vernon County censor. [volume] (Viroqua, Wis.) 1865-1955, 1909-09-22
GROUND PLAN OF
NORTH AMERICA
PREPARED BY PRESIDENT VAN
HIsE AND PROFESSOR
LEITH.
FEDERAL GOVERNMENT ISSUES
THE WORK IN FORM OF A
BULLETIN.

All work done and conclusions
reached by biologists in regard to
the older rocks of the North Amer
lean continent have been summarized
by President C. R Van HIse and
Professor C. K. Leith. in a bulletin
Just issued by the United States ge-
illogical survey under the title. Pre-
Cambrian Geology of North America.
The term pre- Cambrian is used to
include all rocks supposed to have
been in existence before those laid
down in the Cambrian period al
the beginning of the Paleozoic era.
In the early days of geologic study all
these pre-Cambrian rocks were sup.
posed to be of igneous origin and to
record a time before life on the globe
began. Later, when both sedimeHtary
beds and fossils were discovered in
them, they were subdivided, usually
Into two variously named systems. the
older of which, now generally known
as the Archean, was supposed to be
destitute of life and sediments, while
the younger, now called the Algonkian,
was supposed to mark the earliest he
ginnings of both. More recently sedi.
ments have been found and fossils sus-
pected in certain Archean rocks. so
that the distinction between thesc two
great systems now depends on other
points of difference, most of which are
quite so easily grasped or applied.
Nevertheless the subdivision is useful.
since the Archean and Algonkian are
separated from each other by a great
"unconformity" and differ from each
other physically as much as the Al
gonkian differs from the Cambrian.
Where so much doubt existed in re.
gard to even such enormous systems.
it was inevitable that greater doubt
should exist in regard to details.
Many writings on the subject have
proved to be erroneous very shortly
after their publication, and marked in
consisteneies and disagreements still
exist, The matter in the present vol
ume has been collected in the hope
that the setting of the ideas of all
writer's on the subject side by side
might help to clarity matters and to
show definitely the present state OF
knowledge on the subject.
In the discussion the aim OF the
authors has ben aot to call attention
to what seemed erroneous, but to
point out where there was harmony
between the different authors, often
yelled because terms are used with
different Specifications; and to note
the important conclusions which have
been clearly determined. Statements
and conclusions with which the writers
did not agree are in general not criti-
cized, nor is any REPUTATION attempted.
unless the point at issue was funda
mental and could not be overlooked.
The pre-Cambrian rocks are i'm.
portant because they are generally
considered TO indicate the basal archi-
tectural features in the ground plan
of the North American continent. The
largest area occupies the northeastern
portion Of the continent, extending
northwest. north. and northeast from
Minnesota, the Great Lakes, ana the
st Lawrence to the Arctic and Allan,
tic oceans. This has been called the
"pre-Cambrian shield" the 'Iaurentran
continent" and the "Arehean pro
taxis' OF Nor h America.
On the southeast the pre-Cambri-
an shield is separated BY a narrow
area of younger rocks from the pre-
Cambrian rocks of the northern END
.of the Appalachian system. Con
siderable areas begin in Newfound-
land and extend through New Eng.
land down the eastern side of the
Appalachian system 10 Alabama. In
cas- .ern Canada and in the Adiron-
daclys OF New York are outliers of
pre-Cambrian rocks surrounded by
younger sediments, which, while
close to the Appalachian system, do
not form any part of it. In western
United States ure-Cambrian rocks
form parts of some of the Cordil-
leran ranges, and in northern ZION
tana and Idaho and southern Brit
ish Columbia they occupy yer: large
areas. IN the Mississippi valley is
olated areas of them appear in Wis
eonsin. Minnesota, Iowa. Dakota,
Missouri, Oklahoma, and Texas.
The bulletin may be had BY APP
plying 10 the director of the United
States geological survey, at Wash
ington.