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.
[Illustration: Fig. 264. Hypothetical Map of Eastern North
America at the Beginning of Cambrian Time
Unshaded areas, probable 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.
[Illustration: Fig. 265. Sponge Spicules as seen in Flint under
the Microscope]
=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.
[Illustration: Fig. 266. Graptolites]
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.
[Illustration: Fig. 267. Cystoids, one showing Two Rudimentary
Arms]
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.
=Arthropods.= 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.
[Illustration: Fig. 268. Trilobites
A, a Cambrian species; B, a Devonian species showing eyes;
C, restoration of an Ordovician species]
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.
[Illustration: Fig. 269. A Phyllopod]
=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
(Fig. 270). 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 (Fig.272).
[Illustration: Fig. 270. A Cambrian Articulate Brachiopod, Orthis]
[Illustration: Fig. 271. Cambrian Inarticulate Brachiopods
A, Lingulella; B, Discina]
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 (Fig. 271). 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.
[Illustration: Fig. 272. A Cambrian Pelecypod]
[Illustration: Fig. 273. Gastropods]
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.
[Illustration: Fig. 274. Cambrian Pteropods]
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_ (_s_, Fig. 275, _B_), passed through from the body
chamber to the closed tip of the cone.
[Illustration: Fig. 275. Orthoceras
A, fossil; B, restoration]
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.