teropods, Pteropods, Cephalopods, begin in the Lower Silurian, and
the Molluscan type is thus unfolded at the outset, while the Articu-
lates are represented by only the inferior marine divisions, and the
Radiates, besides being an inferior type, are present only in the
lower tribes of its three classes. The age is eminently, therefore,
the age of Mollusks. Any fishes discovered in the Silurian would
foreshadow, in the manner explained, the age of Fishes, as the
Reptiles of the Carboniferous age foreshadowed the Reptilian age,
and the Mammals of the age of Reptiles the Mammalian age.
In the Devonian age, the Fishes, the lowest of Vertebrates, are
the dominant type. The Reptilian age is still more eminently an
age of Reptiles, and the Mammalian age an age of Mammals, as is
shown beyond in the survey of these ages.
On botanical data, the ages would he— first, the age of Sea-weeds,
covering the ages of Mollusks and Fishes ; second, the age of Coal-
Plants or Acrogens, or the Carboniferous age ; third, the age of Cycads,
correspondmg to the age of Reptiles ; and, fourth, the age of Palms
and Dicotyledoyis, corresponding to the Mammalian age. The only
addition to the preceding divisions based on the animal tribes
which is thence suggested by the vegetable kingdom is the Carboni-
ferous. In the zoological series this might be called the age of Am-
phibians, as it is characterized prominently by the amphibian division
of Reptiles.
10
130 HISTORICAL GEOLOGY.
The ages recognized are, then, Age of Mollusks, or Silurian ; Age
of Fishes, or Devonian; Carboniferous Age; Age of Heptiles; Age of Mam-
mals; Age of 2 fan.
Preceding these, there is the Azoic era, — the name being derived
from the Grreek a and I^utj, life, and signifying the absence of life.
The Azoic rocks are mostly crystalline.
The Silurian, Devonian, and Carboniferous ages naturally stand
somewhat apart from the following in the j^eculiar ancient forms of
the great portion of their living tribes, and to the whole collectively
the term Palaeozoic era is appropriately afjplied, — the word "palaeo-
zoic" being from the Greek Tralawg, ancient, and ^aov. The following
age, or age of Eeptiles, is correspondingly termed the Mesozoic, from
necor, middle, and ^wov, it being the mediaeval era in geological history.
The Mammalian age is termed the Cenozoic, from Kaivoq, recent, and
Cwov. {The words Eocene, Miocene, etc., subdivisions of the age, are
in part from the same root.)
The subdivisions of geological tiine are, then, —
I. Azoic Time or Age.
11. Palaeozoic Time.
1. The Age of Mollusks, or Silurian.
2. The Aire of Fishes, or Devonian.
3. The Age of Coal-Plants, or Carboniferous.
III. Mesozoic Time.
4. The Age of Reptiles.
IV. Cexozoic Time.
5. The Age of Mammals.
V. Era of Mixd.
G. The Age of Man.
8id)divisiorisinto Periods and Epochs. ^^ri\Q'&\x\idAyv&\oxi% under the ages,
the periods and epochs, vary, as has been said, in different countries.
The following table (fig. 134) presents a general view of those of
eastern North America, as far as the Palaeozoic is concerned, — the
Silurian, Devonian, and Carboniferous being well represented on the
North American continent. The rest of the series is from European
geology, in which the later ages are far better represented than in
America. In this manual, American geology is in general first con-
sidered, and afterwards such further illustrations are drawn from
other continents as are necessary for comprehensive views and
generalizations. Where America is deficient in its records, the
European are taken as the standard.
The names of the periods and ei:)Ochs/or the Palaeozoic of Ame-
rica are the same that have been applied to the rocks by the New
York geologists.
SUBDIVISIONS IN THE HISTORY.
lai
Periods.
Fis. 134.
H
<
CO
&
O
»-<
>5
O
P
<J
o
O
o
\S
o
>
P
O
o
o
1-1
t«
Permian.
LMd^kilii|liw
.1 ^IJ,J 1^1 L_, ... I . . . . .
MMMiMiHUMMiiai
15
Carboniferous.
Epochs.
Permian.
Upper Coal Measures.
Sub-carboni fer-
ous.
Catskill.
Chemung:.
a >> ^'^ t a <> '
Hamilton.
Upper Helder-
berg.
Oriskany.
13b
13n
12
lib
11a
10c
10b
10a
9c
9b
9a
Lower Ilelder-
berg.
Salina.
Niagara.
^^3 S^ o'^"^ '^o'^.->'^ ^'a'^ o =L o -o O
Hudson.
tn
o
Trenton.
Potsdam.
Azoic.
^~J.-^^'•'^l' vl
I 1 1 ,'â– >'/ -/N < / A ^^/^ /~,T /^ !.-''/<"â– /' 1 Azoic.
6b
6a
6d
5c
5b
5a
4b
4a
3b
3a
2b
2a
Lower Coal Measures.
Millstone Grit.
Upper.
Lower.
Chemung.
Portage.
Genesee.
Hamilton.
Marccllus.
Upper Helderberg.
Schoharie.
Cauda-Galli.
Oriskany.
Lower Helderberg.
Saliferous.
Loclaire.
Niagara.
Clinton.
Medina.
Oneida.
Hudson River.
Utica.
[ Trenton.
Black River.
Birdseye.
Chazy.
Calciferous.
Potsdam.
132
HISTORICAL GEOLOGY.
Periods.
Fig. 134 [continued).
Age of Man,
! Post-tertiary
<
Tertiary. L ^:;:^•Vv:v ^^^^ j5^=tH iV■r-■^- -^ ^;-^; ^;;^^
Epochs and Sub-Epochs.
Pleistocene, or Post-tertiary.
Pliocene.
Miocene.
Eocene.
("Upper or White
Upper Cretaceous. < Chalk.
(^ Lower or Gray.
Middle Cretaceous (Upper Green-Sand).
Lower Cretaceous (Lower Green-Sand).
Wealden.
Upper Oolite. I P'"'!'^^''- Pf tland, and
( Kmimendge Clay.
Middle Oolite, j^^^^-f^;
( Oxford Clay.
f Stonesfield.
^ym\ Lower Oolite.
'• ' '- ' : Upper Lias.
Marlstone.
Lower Lias.
Keuper.
Muschelkalk.
Bunter-sandstein.
\ Inferior Oolite.
In the figures and maps introduced bej-ond, the numbers are used
as in the above tables : 1 standing for the Azoic ; 2 for the rocks of
the Potsdam period, 2 a for the Potsdam epoch, 2 h for the Calci-
ferous sand-rock ; 3 for rocks of the Trenton period, Z a, Zb, for the
epochs of this period ; and so on.
The following map of the United States east of the Rocky Moun-
tains exhibits the geographical distribution of the rocks of the several
ages, — that is, the regions over which they are severally the surface-
rocks.
The Silurian is distinguished by heavy horizontal lining ; and the
dotted line over the Silurian area divides the Upper Silurian (m)
from the Lower Silurian [l).
SUBDIVISIONS IN THE HISTORY.
1:53
^â– z.
134 AZOIC AGE.
The Devonian, by heavy vertical lines.
The Carboniferous, by light cross-lines on a black ground, or by a black
surface, or by dots on a black ground (the^r^^ the Sub-carboniferous,
the second the Coal formation, the third the Permian).
The Reptilian, including the Triassic, Jurassic, and Cretaceous, by
lines sloping from the right to the left {/), the Cretaceous being
distinguished by having the lines broken.
The Tertiary, by lines sloping from the left to the right f \ ).
The Azoic, by irregular line-dottings.
The surface without markings is occupied by rocks of undeter-
mined age, that on the east mostly crystalline.
I. AZOIC TIME OR AGE.
Reality of the Age. — The Azoic age is the age in the earth's his-
tory preceding the appearance of animal life. The fact of the
existence of the globe at one time in a state of universal fusion
is placed beyond reasonable doubt. And whatever events occurred
upon the globe from the era of the elevated temperature necessary
to fusion, down to the time when the climate and waters had become
fitted for animal life, are events in the Azoic age. The age must,
therefore, stand as i\\e first in geological history, wliether science can
point out unquestionably the rocks of that age or not.
The fossils of true PalEeozoic and Mesozoic rocks have often been obliterated
by the crj'stallization of these rocks; and, as the oldest rocks of the globe are
nearly all crystallized, the question may always arise with respect to ony par-
ticular one of them, whether it may not once have been filled with fossils. After
having reached what was thought to be the lowest fossiliferous beds in Great
Britain, fossils have been found in others inferior, carrying the Silurian down
to a still lower level by transferring to it what had been regarded as Azoic.
Such changes are part of the progress of the science, and cause little inconve-
nience to the system, provided the order of succession be rightly given.
Stratigraphical limits in North America. — The Azoic rocks in
North America^' at present include all that are older than the Pots-
dam sandstone of New York, — the first of the Silurian. This sand-
stone is spread out in nearly horizontal layers, conformable with the
overlying Silurian beds, but rests on crystalline rocks, which are
ui^turned at all angles and folded or crumpled on a scale of great
* The Azoic system of North America was first distinctly recognized in its
true importance in the Report of Foster & "Whitney on the Lake Superior
region. The rocks inferior to the Silurian have been called by Murchison the
Bottom rocks. They are part of ihe prinutri/ of the old geologists.
GEOailAPIIICAL DISTRIBUTION.
135
extent. These beds, thus crystallized and flexed or disturbed before
the Potsdam sands were deposited, are the Azoic. Tlic following
sections illustrate this ])oint. In each, the Azoic, numbered 1, in its
usual disturbed condition, is overlaid nearly horizontally by the
Silurian beds, 2 a being the Potsdam sandstone, 2 h the Calcilerous
sandrock, 3 the Trenton limestones, 4 a the Utica shale.
Fig. 136.
Fi-. i.-sr.
Fiir. 1.38.
Fiy. 136, by Emmons, from Essex co., N.Y.; 1 is liyperstlieiio rock, or hyinuitc. — Fig. 1.07,
by Owen, from Black River, south of Lake Superior; 1 is a granitic rock, la,cliloritic ami
ferrugiuous slates. — Fig. 138, by Logan, from the south side of the St. Lawrence in Canada,
between Cascade Point and St. Louis Ilapids; 1, gneiss.
Geographical distribution. — The Azoic rocks constitute the only
universal formation. They cover the whole globe, and were the
floor of the oceans and the rocks of all emerged land when animal
life was first created. But subsequent operations over the sphere
have buried the larger part of the ancient surface, and to a great
extent worn away and worked up anew its material; so that the
area of the old floor now exposed to view is small.
The areas of the earth's crust over which the Azoic rocks are now
exposed are either, —
1. Those which have always remained uncovered.
2. Those which have been covered by later strata, but from
Avhich these superimposed beds have been simply washed away,
without much disturbance.
3. Those once covered, like the last, but Avhich, in the course
of the upturnings of mountain-making, have been thrust upward
among the displaced strata, and in this way have been brought out
to the liirht.
In cases like those of figures 136, 137, in which the Silurian
rocks are spread in nearly horizontal layers over the borders of
an area made up of tilted Azoic rocks, the Azoic area either has
been always uncovered, or has become so from denudation ; but
in mountain-regions where the Silurian locks have been folded
up in the mountain-making, the Azoic below may have been brought
136
AZOIC AGE.
up to view in the same process. Moreover, the Azoic, if it had not
undergone flexures before the Sihirian beds were laid down, would
partake of the Silurian flexures, or, in other words, be conformable
to the Silurian strata. But if it had been flexed or tilted in some
previous period of disturbance, then the Azoic would be unconform-
able to the Silurian, although both were finally upthrown together
in the making of the mountains.
In the study of Azoic regions these points require sj)ecial investi-
gation.
The Azoic areas of North America of the first kind (and partially,
it may be, of the second) are shown, as far as now ascertained, on the
accompanying map of the Azoic continent. The Azoic lands on this
Fig. 1.39.
Azuic Map of North America.
chart are represented as the dry land of the era, while the rest of
the continent is submerged. They are concluded to have been thus
dry, because no marine beds cover them, Avhile, on either border,
marine beds (Silurian and later) commence and spread widely over
the most of the continent. The outline of the continent and of the
GEOGRAnilCAL DISTRIIJUTION. lo7
great lakes enables the reader to perceive the relative positions ol'
these Azoic lands. Tlierc may have been other areas along the
Appalachians, and over the Rocky Mountain region, which future
study will bring to liglit.
The Azoic regions laid down are —
1. Canada north of the 8t. Lawrence, reaching northeast from
Lakes Huron and Superior to Labrador (C C), and the continuation
northwest (B B) to the Arctic Ocean.
2. An isolated area in northern New York, — a peninsular pro-
longation, it may be considered, of the Canada region, — covering for
the most part Essex, Clinton, Franklin, St. Lawrence, Hamilton,
and Warren cos., and part of Saratoga, Fulton, Herkimer, Lewis,
and Jeflerson cos.
3. A similar area south of Lake Superior (S).
4. West of the Mississippi, a small area in Missouri, in which the
famous Iron Mountains are situated ; the Black Hills in Dakota,
and the Laramie Range in Nebraska, as recently observed by Dr.
Hayden ; part of the Ozark Mountains in Arkansas.
In northern New Jersey there are Azoic gneiss, limestone, and other crystalline
rocks containing great beds of the ore called Franklinite, analogous to the iron-
ore beds of northern Xew York : the lowest Silurian beds cover them un-
conformably. Professors Rogers have described the occurrence of Azoic rock.<
in the Appalachians j but, although probably occurring in the range, the evi-
dence is not yet conclusive that the rocks so designated antedate the Silurian.
Professor Safford mentions rocks of the Azoic age in eastern Tennessee, — a
part of the same mountains; but they are stated to \)Q conformable, as far as yet
investigated, to the Silurian.
The map of New York and Canada in the chapter on the Silu-
rian shows more precisely the form of the New York Azoic and
that north of the St. Lawrence. It represents also the Silurian
and Devonian strata of the State as they become successively the
surface-rocks on going from the Azoic southward. Adjoining the
Azoic (numbered 1) is the earliest Silurian, No. 2, which outcrops
where it is represented, but is supposed to underlie the strata num-
bered 3, 4, 5, etc. So No. 3 is the next formation which outcrops,
while it probably underlies all the beds 4, 5, etc. The Azoic is thus
the basement, and each successive stratum was a new deposit over
it in the seas that bordered at the time the Azoic dry land.
In Europe the Azoic system has been distinctly recognized in
Norway and Sweden and in Bohemia underlying the Silurian
unconformably. The great iron-regions of Sweden are probably
of this age. In geological maps of other parts of the world
(and those of Europe and America are not always excepted) it is
138 AZOIC AGE.
»
common to color the regions covered by crystalline rocks all alike,
without reference to their differences of age. Thus the metamor-
phic rocks of various ages are confounded, as they are also in the
unfortunate name they sometimes bear, of hjpogene rocks.
Kinds of rocks. — The rocks are mostly of the metamorphic
series, related to granite, gneiss, syenite, and the like. But they
embrace only the most ancient of these rocks ; for the granites
and schists of New England, of Cornwall, the Alps, and many
other regions, belong to later ages.
Besides true granite and gneiss, there are dioritc, — a rock consisting
of feldspar (albite) and hornblende Avithout quartz (^84); also
extensive ranges of coarse granite-like rocks of grayish and
reddish-brown colors, composed mainly of crystallized labradorite
or a related feldspar (| 55), or of this feldspar with the addition
of the brownish-black and bronzy foliated mineral hypersthene
(I 65), and constituting the rock called hi/perif.e ; also chlorite schist,
while mica schist appears to be absent ; also serpentine, limestone
(or statuary marble), granular quartz (a hard sandstone), and in
some places a hard conglomerate ; also 7nagneiic and specular iron-ore in
immense beds.
There are, in addition, 2^'^'T^'>/ri/ of green, brown, and reddish colors; a
garnet-euphotide (eclogite) and a feldspar-euphotide (^85); soapstone (Rens-
selaerite) (^ 86, [4]); parophite rock and schist (§^ 67, 87); pyroxene rocks ;
ophiolites or verd-antique marble of different A^arieties (^86, [8]).
Part of the feldspar related to labradorite has the composition of andesine
orvosgite; and oligoclase exists in the Swedish Azoic. The Labrador rock
turns gray on weathering. Part of the hyperite contains ordinary hornblende
instead of hypersthene, and some kinds mica or cpidote. The hypersthene is in
foliated pieces or crystals often a little bronze-like in lustre. Good localities for
the opalescent labradorite are the streams of the Adirondack, — especially, says
Professor Emmons, the beaches of East River; also Avalanche Lake, near the
foot of the great slide from Mount McMartin.
The potstone or soapstone called Bensselaerite covers considerable areas in
the towns of Fowler, Canton, Edwards, Hermon, etc., St. Lawrence co., and
at Grenvillc, in Canada, and is cut into slabs for tables, chimney-pieces, fur-
nace-linings, or made into inkstands. The parophite or aluminous potstone
of Diana. Lewis co., N.Y. (^ 87). is also used for inkstands, etc.
Beautiful red and green poi-phyry and a buhrstone are found at Grenville.
Canada.
As crystalline rocks have been formed in various ages, — those
of New England, for example, long after those of the Azoic, — it is
possible that some Azoic rocks have undergone a second or third
alteration subsequent to the original one in the Azoic age. It may
be difficult, in fact, to say which of the rocks retain their original
CHARACTERISTICS OF AZOIC ROCKS. lo'.l
composition. There is, however, no reason to suspect any funda-
mental changes in the granitic or hornblendic rocks or schists.
But the potstones, both magnesian and aluminous, are probably
of later origin. The Rensselaerite has been oVjserved under the
crystalline form of j)yroxene, showing that in part, at least, it has
been made out of pyroxene ; and the aluminous species exists
under the crystalline form of nepheline, giving unequivocal proof
that it has been made out of pre-existing nei^heline crystals, like
the gieseckite of Greenland, which it resembles in aspect and com-
position. The rocks are probably, therefore, the result of th6 altera-
tion of different minerals or rocks after the first Azoic crystalli-
zation. If this be true, they may not be actually Azoic rocks : they
may belong to the same age with the metamorphic rocks of New
England, or to some other period. By one interested in bringing
the events of geological history into their true chronological rela-
tions, — the real end in geological studies, — this will be regarded as
an important question.
Other evidences of alteration since the original crystallization in
northern New York have been observed, — such as the rounded
(juartz crystals of Gouverneur, and the soft spinels of St. Lawrence
CO., called houghite. Even the serpentine of the same region may
come into this category.
Minernh of the Azoic rocks. — Besides the constituent minerals mentioned, — viz.,
quartz, feldspar of different species, hornblende, pyroxene, cpidote, mica, talc,
garnet, — there are also the following common species : tourmaline, scapolitc,
wollastonite, sphene, rutile, graphite, the mica called phlogopite (^ 56), apatite,
chondrodite, spinel, zircon, corundum, — each of which occurs at times in the
crystalline limestone or its vicinity. Mica is found in Grenville, Canada, in
plates between one and two feet square. In addition to these, there are a
number of rare ores of yttrium, cerium, and columbium among the Swedish
Azoic rocks.
No gold has thus far been found in the Azoic. Andalusite, kyanite, and
staurotide are also among the common minerals of crystalline schists not
detected in the Azoic.
Characteristics of the Azoic rocks. — 1. The Azoic rocks are
nearly all crystalline rocks. A few sandstones, slates, and conglo-
merates are the only exceptions ; and these are excessively hard
rocks.
2. The crystalline rocks are remarkable for the small amount
of silica they contain (a fact noticed by T, 8. Hunt). This is
seen in the absence of quartz from many of the rocks (the diorite.
Labrador rock, hyperite), and the abundance of feldspars, like
labradorite, that have a low proportion of silica.
140
AZOIC AGE.
3.
The prevalence of iron is another characteristic (remarked by
J. D. Whitney). This is seen in the abundance of the minerals
(silicates) containing iron, as hornblende, hypersthene, chlorite,
garnet ; also the reddish color of much of the feldspar ; also the
beds of iron-ore, which exceed in extent those of any other age.
4. There are none of the simple silicates of alumina.
Arrangement of the rocks. — Although the Azoic rocks are
mostly crystalline, they follow one another in various alternations,
like the sedimentary beds of later date. In the sections which have
been given, there are alternations of granite, gneiss, schists, lime-
stone, etc. ; and the dip and strike may be studied in the same
manner as in the case of any tilted sandstones or shales. The
following sections represent other examples ; and in them there are
beds of iron-ore, one hundred feet and upwards in thickness, which
are banded with siliceous layers and chloritic schist, showing
thereby a distinctly stratified character. Where most flexed or
folded, there is still a distinction of layers ; and it is owing to this
fact that the rocks may be described as folded ; for folds can be
identified only where the rocks are in sheets. This grand fact is,
then, evident, — that the Azoic rocks are in layers, as much as the
rocks of any later age.
The following section by Logan (real in its general truths, although partly
ideal) exhibits well this fact. It presents to view a stratum of (a) white
Fig. 140.
b a b a b a n i>
granular or crystalline limestone, many times folded, and interstratified with
gneiss and quartz rock (b) ; and over the same region (Grenville and adjacent
country, Canada) the limestone has been traced in linear and curving bands
corresponding to a series of folds.
The followin": sections contain iron-ore beds among the alternations. In
Fig. 141.
Fii!-. 11?.
Fig. 143.
fig. 141 (from the Michigan region, Foster & Whitney) the iron-ore, in extensive
beds (?, i), occurs between chloritic schist (a, a) and diorite {b) : and the iron-
ARRANGEMENT OF AZOIC ROCKS.
141
ore in i' is banded with jasper. In ligs. 142 and 143 (Essex co., N.Y.,
Emmons) the iron-ore, in beds several yards wide, is associajted with gneiss
and quartz rock, and is intcrlaminated with
quartz, the whole dipping together in a
oommon direction, like l)eds of sandstone,
shale, and iron-ore in many regions of sedi-
mentary rocks.
In fig. 144 (Pcnokie Range, south of Lake
Superior, C. Whittlesey) h is hornblende rock
and slaty quai'tz ,• y, quartzite, oO feet thick ;
i, a bed of iron-ore, 25 to 50 feet thick.
In another section, by C. "Whittlesey (described in Foster & Whitney's Report),
taken at the falls of the Menomonee, there are alternations of gneiss, horn-
blende, and quartz rock with talcose and chloritic schists, quartzite, and granu-
lar limestone, and between the limestone layers there is a layer of iron-ore, —
showing again that the iron-ore is in beds conformable with the schists. The
beds, however, may not have great lateral extent ; for the iron may be local in
bands, or imbedded in other kinds of rocks.
In the Missouri region, at Pilot Knob, the ore-strata (says J. D. Whitney)
consist of a series of quartzose beds of great thickness, passing gradually into
specular iron, which frequently forms bands of nearly pure ore, alternating
with bands of quartz more or less mixed with the iron. The ore, moreover,
is often thin-laminated.
At the Adirondack mines, in Essex co., N.Y., one bed, according to Emmons,
is 150 feet thick, and another exceeds 700 feet. In the Michigan region they
are on the same great scale. In Missouri, one of the "iron mountains" — the
Pilot Knob — is 581 feet high, and the other 228 feet ; and huge displaced