THE CAMBRIAN
THE PALEOZOIC ERA. The second volume of the geological record,
called the Paleozoic (Greek, PALAIOS, ancient; ZOE, life), has
come down to us far less mutilated and defaced than has the first
volume, which contains the traces of the most ancient life of the
globe. Fossils are far more abundant in the Paleozoic than in the
earlier strata, while the sediments in which they were entombed
have suffered far less from metamorphism and other causes, and
have been less widely buried from view, than the strata of the
pre-Cambrian groups. By means of their fossils we can correlate
the formations of widely separated regions from the beginning of
the Paleozoic on, and can therefore trace some outline of the
history of the continents.
Paleozoic time, although shorter than the pre-Cambrian as measured
by the thickness of the strata, must still be reckoned in millions
of years. During this vast reach of time the changes in organisms
were very great. It is according to the successive stages in the
advance of life that the Paleozoic formations are arranged in five
systems,--the CAMBRIAN, the ORDOVICIAN, the SILURIAN, the
DEVONIAN, and the CARBONIFEROUS. On the same basis the first three
systems are grouped together as the older Paleozoic, because they
alike are characterized by the dominance of the invertebrates;
while the last two systems are united in the later Paleozoic, and
are characterized, the one by the dominance of fishes, and the
other by the appearance of amphibians and reptiles.
Each of these systems is world-wide in its distribution, and may
be recognized on any continent by its own peculiar fauna. The
names first given them in Great Britain have therefore come into
general use, while their subdivisions, which often cannot be
correlated in different countries and different regions, are
usually given local names.
The first three systems were named from the fact that their strata
are well displayed in Wales. The Cambrian carries the Roman name
of Wales, and the Ordovician and Silurian the names of tribes of
ancient Britons which inhabited the same country. The Devonian is
named from the English county Devon, where its rocks were early
studied. The Carboniferous was so called from the large amount of
coal which it was found to contain in Great Britain and
continental Europe.
THE CAMBRIAN
DISTRIBUTION OF STRATA. The Cambrian rocks outcrop in narrow belts
about the pre-Cambrian areas of eastern Canada and the Lake
Superior region, the Adirondacks and the Green Mountains. Strips
of Cambrian formations occupy troughs in the pre-Cambrian rocks of
New England and the maritime provinces of Canada; a long belt
borders on the west the crystalline rocks of the Blue Ridge; and
on the opposite side of the continent the Cambrian reappears in
the mountains of the Great Basin and the Canadian Rockies. In the
Mississippi valley it is exposed in small districts where uplift
has permitted the stripping off of younger rocks. Although the
areas of outcrop are small, we may infer that Cambrian rocks were
widely deposited over the continent of North America.
PHYSICAL GEOGRAPHY. The Cambrian system of North America comprises
three distinct series, the LOWER CAMBRIAN, the MIDDLE CAMBRIAN,
and the UPPER CAMBRIAN, each of which is characterized by its own
peculiar fauna. In sketching the outlines of the continent as it
was at the beginning of the Paleozoic, it must be remembered that
wherever the Lower Cambrian formations now are found was certainly
then sea bottom, and wherever the Lower Cambrian are wanting, and
the next formations rest directly on pre-Cambrian rocks, was
probably then land.
EARLY CAMBRIAN GEOGRAPHY. In this way we know that at the opening
of the Cambrian two long, narrow mediterranean seas stretched from
north to south across the continent. The eastern sea extended from
the Gulf of St. Lawrence down the Champlain-Hudson valley and
thence along the western base of the Blue Ridge south at least to
Alabama. The western sea stretched from the Canadian Rockies over
the Great Basin and at least as far south as the Grand Canyon of
the Colorado in Arizona.
Between these mediterraneans lay a great central land which
included the pre-Cambrian U-shaped area of the Laurentian
peneplain, and probably extended southward to the latitude of New
Orleans. To the east lay a land which we may designate as
APPALACHIA, whose western shore line was drawn along the site of
the present Blue Ridge, but whose other limits are quite unknown.
The land of Appalachia must have been large, for it furnished a
great amount of waste during the entire Paleozoic era, and its
eastern coast may possibly have lain even beyond the edge of the
present continental shelf. On the western side of the continent a
narrow land occupied the site of the Sierra Nevada Mountains.
Thus, even at the beginning of the Paleozoic, the continental
plateau of North America had already been left by crustal
movements in relief above the abysses of the great oceans on
either side. The mediterraneans which lay upon it were shallow, as
their sediments prove. They were EPICONTINENTAL SEAS; that is,
they rested UPON (Greek, EPI) the submerged portion of the
continental plateau. We have no proof that the deep ocean ever
occupied any part of where North America now is.
The Middle and Upper Cambrian strata are found together with the
Lower Cambrian over the area of both the eastern and the western
mediterraneans, so that here the sea continued during the entire
period. The sediments throughout are those of shoal water. Coarse
cross-bedded sandstones record the action of strong shifting
currents which spread coarse waste near shore and winnowed it of
finer stuff. Frequent ripple marks on the bedding planes of the
strata prove that the loose sands of the sea floor were near
enough to the surface to be agitated by waves and tidal currents.
Sun cracks show that often the outgoing tide exposed large muddy
flats to the drying action of the sun. The fossils, also, of the
strata are of kinds related to those which now live in shallow
waters near the shore.
The sediments which gathered in the mediterranean seas were very
thick, reaching in places the enormous depth of ten thousand feet.
Hence the bottoms of these seas were sinking troughs, ever filling
with waste from the adjacent land as fast as they subsided.
LATE CAMBRIAN GEOGRAPHY. The formations of the Middle and Upper
Cambrian are found resting unconformably on the pre-Cambrian rocks
from New York westward into Minnesota and at various points in the
interior, as in Missouri and in Texas. Hence after earlier
Cambrian time the central land subsided, with much the same effect
as if the Mississippi valley were now to lower gradually, and the
Gulf of Mexico to spread northward until it entered Lake Superior.
The Cambrian seas transgressed the central land and strewed far
and wide behind their advancing beaches the sediments of the later
Cambrian upon an eroded surface of pre-Cambrian rocks.
The succession of the Cambrian formations in North America records
many minor oscillations and varying conditions of physical
geography; yet on the whole it tells of widening seas and lowering
lands. Basal conglomerates and coarse sandstones which must have
been laid near shore are succeeded by shaly sandstones, sandy
shales, and shales. Toward the top of the series heavy beds of
limestone, extending from the Blue Ridge to Missouri, speak of
clear water, and either of more distant shores or of neighboring
lands which were worn or sunk so low that for the most part their
waste was carried to the sea in solution.
In brief, the Cambrian was a period of submergence. It began with
the larger part of North America emerged as great land masses. It
closed with most of the interior of the continental plateau
covered with a shallow sea.
THE LIFE OF THE CAMBRIAN PERIOD
It is now for the first time that we find preserved in the
offshore deposits of the Cambrian seas enough remains of animal
life to be properly called a fauna. Doubtless these remains are
only the most fragmentary representation of the life of the time,
for the Cambrian rocks are very old and have been widely
metamorphosed. Yet the five hundred and more species already
discovered embrace all the leading types of invertebrate life, and
are so varied that we must believe that their lines of descent
stretch far back into the pre-Cambrian past.
PLANTS. No remains of plants have been found in Cambrian strata,
except some doubtful markings, as of seaweed.
SPONGES. The sponges, the lowest of the multicellular animals,
were represented by several orders. Their fossils are recognized
by the siliceous spicules, which, as in modern sponges, either
were scattered through a mass of horny fibers or were connected in
a flinty framework.
COELENTERATES. This subkingdom includes two classes of interest to
the geologist,--the HYDROZOA, such as the fresh-water hydra and
the jellyfish, and the CORALS. Both classes existed in the
Cambrian.
The Hydrozoa were represented not only by jellyfish but also by
the GRAPTOLITE, which takes its name from a fancied resemblance of
some of its forms to a quill pen. It was a composite animal with a
horny framework, the individuals of the colony living in cells
strung on one or both sides along a hollow stem, and communicating
by means of a common flesh in this central tube. Some graptolites
were straight, and some curved or spiral; some were single
stemmed, and others consisted of several radial stems united.
Graptolites occur but rarely in the Upper Cambrian. In the
Ordovician and Silurian they are very plentiful, and at the close
of the Silurian they pass out of existence, never to return.
CORALS are very rarely found in the Cambrian, and the description
of their primitive types is postponed to later chapters treating
of periods when they became more numerous.
ECHINODERMS. This subkingdom comprises at present such familiar
forms as the crinoid, the starfish, and the sea urchin. The
structure of echinoderms is radiate. Their integument is hardened
with plates or particles of carbonate of lime.
Of the free echinoderms, such as the starfish and the sea urchin,
the former has been found in the Cambrian rocks of Europe, but
neither have so far been discovered in the strata of this period
in North America. The stemmed and lower division of the
echinoderms was represented by a primitive type, the CYSTOID, so
called from its saclike form, A small globular or ovate "calyx" of
calcareous plates, with an aperture at the top for the mouth,
inclosed the body of the animal, and was attached to the sea
bottom by a short flexible stalk consisting of disks of carbonate
of lime held together by a central ligament.
ARTHOPODS. These segmented animals with "jointed feet," as their
name suggests, may be divided in a general way into water
breathers and air breathers. The first-named and lower division
comprises the class of the CRUSTACEA,--arthropods protected by a
hard exterior skeleton, or "crust,"--of which crabs, crayfish, and
lobsters are familiar examples. The higher division, that of the
air breathers, includes the following classes: spiders, scorpions,
centipedes, and insects.
THE TRILOBITE. The aquatic arthropods, the Crustacea, culminated
before the air breathers; and while none of the latter are found
in the Cambrian, the former were the dominant life of the time in
numbers, in size, and in the variety of their forms. The leading
crustacean type is the TRILOBITE, which takes its name from the
three lobes into which its shell is divided longitudinally. There
are also three cross divisions,--the head shield, the tail shield,
and between the two the thorax, consisting of a number of distinct
and unconsolidated segments. The head shield carries a pair of
large, crescentic, compound eyes, like those of the insect. The
eye varies greatly in the number of its lenses, ranging from
fourteen in some species to fifteen thousand in others. Figure
268, C, is a restoration of the trilobite, and shows the
appendages, which are found preserved only in the rarest cases.
During the long ages of the Cambrian the trilobite varied greatly.
Again and again new species and genera appeared, while the older
types became extinct. For this reason and because of their
abundance, trilobites are used in the classification of the
Cambrian system. The Lower Cambrian is characterized by the
presence of a trilobitic fauna in which the genus Olenellus is
predominant. This, the OLENELLUS ZONE, is one of the most
important platforms in the entire geological series; for, the
world over, it marks the beginning of Paleozoic time, while all
underlying strata are classified as pre-Cambrian. The Middle
Cambrian is marked by the genus Paradoxides, and the Upper
Cambrian by the genus Olenus. Some of the Cambrian trilobites were
giants, measuring as much as two feet long, while others were the
smallest of their kind, a fraction of an inch in length.
Another type of crustacean which lived in the Cambrian and whose
order is still living is illustrated in Figure 269.
WORMS. Trails and burrows of worms have been left on the sea
beaches and mud flats of all geological times from the Algonkian
to the present.
BRACHIOPODS. These soft-bodied animals, with bivalve shells and
two interior armlike processes which served for breathing,
appeared in the Algonkian, and had now become very abundant. The
two valves of the brachiopod shell are unequal in size, and in
each valve a line drawn from the beak to the base divides the
valve into two equal parts. It may thus be told from the pelecypod
mollusk, such as the clam, whose two valves are not far from equal
in size, each being divided into unequal parts by a line dropped
from the beak.
Brachiopods include two orders. In the most primitive order--that
of the INARTICULATE brachiopods--the two valves are held together
only by muscles of the animal, and the shell is horny or is
composed of phosphate of lime. The DISCINA, which began in the
Algonkian, is of this type, as is also the LINGULELLA of the
Cambrian. Both of these genera have lived on during the millions
of years of geological time since their introduction, handing down
from generation to generation with hardly any change to their
descendants now living off our shores the characters impressed
upon them at the beginning.
The more highly organized ARTICULATE brachiopods have valves of
carbonate of lime more securely joined by a hinge with teeth and
sockets (Fig. 270). In the Cambrian the inarticulates predominate,
though the articulates grow common toward the end of the period.
MOLLUSKS. The three chief classes of mollusks--the PELECYPODS
(represented by the oyster and clam of to-day), the GASTROPODS
(represented now by snails, conches, and periwinkles), and the
CEPHALOPODS (such as the nautilus, cuttlefish, and squids)--were
all represented in the Cambrian, although very sparingly.
Pteropods, a suborder of the gastropods, appeared in this age.
Their papery shells of carbonate of lime are found in great
numbers from this time on.
Cephalopods, the most highly organized of the mollusks, started
into existence, so far as the record shows, toward, the end of the
Cambrian, with the long extinct ORTHOCERAS (STRAIGHTHORN) and the
allied genera of its family. The Orthoceras had a long, straight,
and tapering shell, divided by cross partitions into chambers. The
animal lived in the "body chamber" at the larger end, and walled
off the other chambers from it in succession during the growth of
the shell. A central tube, the SIPHUNCLE, passed through from the
body chamber to the closed tip of the cone.
The seashells, both brachiopods and mollusks, are in some respects
the most important to the geologist of all fossils. They have been
so numerous, so widely distributed, and so well preserved because
of their durable shells and their station in growing sediments,
that better than any other group of organisms they can be used to
correlate the strata of different regions and to mark by their
slow changes the advance of geological time.
CLIMATE. The life of Cambrian times in different countries
contains no suggestion of any marked climatic zones, and as in
later periods a warm climate probably reached to the polar
regions.