to a few feet, while they thicken southward towards Pennsyl-
vania. In Pennsylvania the rocks include the Carboniferous,
and the whole thickness is at least 40,000 feet. This is exclusive
UNSTRATIFIED CONDITION. 117
of the Triassic, which may add a few thousands to the amount.
In Virginia the thickness is still greater ; but no exact estimate
has been made. In Indiana and the other States west it is only
4000, although extending, a.s in Pennsylvania, to the top of the
Carboniferous. The greater part of the continent of North Ame-
rica east of the Mississippi is destitute of rocks above the Carboni-
In Europe the rocks of the later periods are far more complete
than in North America, while the older also, according to the
estimates stated, exceed the American. In Great Britain the
thickness to the top of the Carboniferous is over 00,000 feet, and
from the Carboniferous to the top of the series little less than
10,000 feet more. This amount is the sum of the thickest deposits
of the several formations, and not the thickness observed in any
' 2. UNSTRATIFIED CONDITION.
128. The larger part of the crystallized rocks are sedimentary
rocks altered or crystallized by heat or other means ; and they are,
therefore, not true examj^les of unstratified rocks. In general they
still retain the lines of deposition distinct. When gneiss and mica
schist are found in alternations, with one another, it is plain that
each layer corresponds to a separate layer in the original deposit,
and the beds, although crystalline, are still as really stratified as
they ever were.
In some metamorphic rocks, however, the appearance of stratifi-
cation is lost; and such may be properly said to be unstratified.
Yet it should be understood that the name does not imply that
they never were stratified, but that this is now their apparent con-
dition. Granite and syenite are unstratified rocks of this kind. In
much granite there is no lamination, no arrangement of the con-
stituent minerals in parallel planes, no evidence of subdivision
into layers. But even this true granite, a few miles off", may become
a schistose or gneissoid rock, and, a short distance farther on,
by gradual transition, a gneiss in which a schistose structure is
Examples of the unstratified condition are common among true
igneous rocks. The ridges of trap or dolerite which range in lofty
masses over many districts ā as the Palisades on the Hudson, Mounts
Tom and Holyoke and other trap ridges of the Connecticut valley,
the trap of the Giants' Causeway and of Fingal's Cave ā are some
of these examples. The rocks were melted when they came up
to the light through fissures, and they now stand without any
marks of stratification. The sketch below represents a scene
among rocks of this kind in Australia. The dome-shaped masses of
ā ā 5" si-' /f^'tJ tt "< 1 r ā -' 'ā¢ -J , '
.'CI! IT V- f t Vf^Jk r I i H- Y.\- i-f i- f f 1 1 i I [[
Basaltic columns, coast of Illawana, New South Wales.
trachyte in some regions of ancient volcanoes, and the interior mass
of many great volcanoes, ā sometimes exposed to view through rend-
ings of the mountain or denudation by water, ā are also examples.
Ikit the ordinary outflows of liquid rock from volcanoes usually
produce layers, which are covered afterwards by others in succession ;
and volcanic mountains, therefore, have to a great extent a strati-
fied arrangement of the rock-material, and not less perfectly so
than bluffs of stratified limestone. Moreover, the same rock which
forms the Giants' Causeway may in other places be interstratified
among sandstones and shales ; for the layer of igneous outflow,
wherever it takes place, may be followed afterwards by deposits
of sand or other sediment.
129. Another example of unstratified material is found in the
loose pebbles and stones which cover a large part of the northern
half of both the American and European continents. Any ordi-
nary mode of action by water lays down sediments in layers. But
these accumulations ā often called chift ā are of vast extent and
without layers. Wherever the same kind of material is in layers,
it is then said to be stratified; and thus it is distinguished from the
There may, therefore, be both stratified and unstratified sedi-
ments, and stratified and unstratified igneous rocks ; and by the
obliteration of the planes of deposition by metamorphism there
may be unstratified metamoriDhic rocks like granite, as well as
130. On the subject of the structure of these rocks, it is only
necessary to refer to the ordinary massive structure of granite
and trachyte, etc., and to the columnar structure mot with among
igneous rocks. The hist is represented in the figure given above.
There are all shades of perfection in this columnar structure, from
prisms of great height with perfectly plane sides, to a mere ten-
dency to split in prismatic forms ; and also from this less perfect
prismatic character, to the massive structure with no trace of
For a continuation of tliis subject, see the chapter on igneous
operations, under Dynamical Geology.
131. (1.) General nature of veins. ā The vein condition. ā Veins are
narrow plates of rock intersecting other rocks. They are the fill-
ings of cracks or fissures ; and, as these cracks or fissures may either
extend through the earth's crust to the interior and divide it
for long distances, or reach down only for a limited depth, or be
confined to single strata, so veins are exceedingly various in extent.
They may be no thicker than paper, or they may be scores of
rods in width, like the great fissures opened at times to the earth's
inner regions by subterranean agency. They may be clustered
so as to make a perfect net-work through a rock, or may be few
and distant. And, as strata have been faulted, so veins also may
have their faults or displacements. All those subterranean move-
ments that produce joints and fractures in rocks may give origin
(2.) Subdivisions.ā Veins are divided into dihes and proper veins.
Dikes are filled by volcanic rocks, basalt, trap, or some other ig-
neous rocks, and have regular and well-defined walls.
Veins are occupied by quartz, granitic rocks, metallic ores,
calcite, fluor spar, heavy spar, etc., ā ingredients which are less ob-
viously a liquid injection from beiow, and probably never of this
nature. They are generally irregular in form, often indistinct in
their walls, and very varying in their ingredients. They abound
in regions of metamorphic rocks. Veins have been subdivided
into kinds : but the divisions need not here be considered.
(3.) Forms and faults of veins and dikes. ā Fig. 116 represents
two simple veins or dikes (a a and h h) intersecting stratified rocks.
Fig. 117, a net-work of small veins.
Fig. lis, small veins of quartz intersecting gneiss, ā the mass five feet square.
The veins do not all cross one another, and correspond to the cracks which
result from contraction, as by sun-drying or cooling, rather than to those of
any other mode of Assuring.
Fig. 119. Two veins a a', presenting some of the common irregularities of
mineral veins in size, the enlarged parts containing mostly the ore : a is faulted
by another vein h, which is of subsequent formation.
120, 121, 122. Examples of granitic veins of very large size in a
gneissoid granite, showing their subdivisions and various irregularities (taken
by the author from granitic rocks near Valparaiso). The veins undergo con-
stant changes of size, and in some places encircle masses of rock resembling
::ā 'ā¢':: '
:-:ā ā o
the rock outside. The rock adjoining the vein is more micaceous than that at
a distance, and the direction of the lamination (as indicated in the figures) varies
with some reference to the intersecting veins, curving approximately parallel
to the veins on two opposite sides >n and Ā», and not at all so on the other two
o and p. The subdivisions of the veins in fig. 121 cross one another in an alter-
nate manner, a cutting d and e but cut by c, and b cut by c, d, and e ; and in
122, although the veins are similar in constitution, one cuts the other; and in
120 the two crossing veins are broken and subdivided at the intersection so as
to appear like one vein stretching off" in two directions like a letter X.
Fig. 123. A vein a faulted by h, ā whence it is inferred thiit h is subse-
quent to a in age. Also a vein 1 faulted by 2 and again by 3, and 3 faulted
by 4 : 2 and 3, therefore, were subsequent in age to 1, and 4 was subsequent to 3.
The faulting is exhibited also in the layers of the stratified rocks which the
Figs. 124, 125, 126. Veins much broken or faulted : in 124, four faults within
a length of eighteen inches ; in 125, six faults in six feet ; in 126, the broken
parts of the vein of unequal breadth.
Figs. 127, 128, 129. Other faulted veins, 127 a and b, six feet apart, and still
different in their faults ; 128, 129, other interrupted veins. These dissimilarities
between the parts of one faulted vein, as in 126, and between the parts of two
parallel veins, as in 127, arise from an oblique shove of the parts either at the
time of the fracturing in which the veins themselves originated, or at some
The points here illustrated are, ā
The great irregularities of size in veins along their courses,
swelling out and contracting ; their occasional reticulations ; their
frequently embracing portions of the enclosed rock ; their nume-
rous faultings or breaks and displacements.
132. (4.) Structure. ā Dikes. ā Dikes consist essentially of the
same kind of material from side to side and at all heights, where not
altered by exposure to the air. The structure may be simply
massive, or cracked irregularly, as in many volcanic dikes. But
frequently there are transverse fractures, producing a columnar
structure, so that a dike is like a pile of
columns. For a short distance from the
walls the structure is generally imperfect
(fig. 130) ; and in many cases there is an
earthy layer along the sides, or even a lami-
nated structure parallel with the walls
(fig. 131), produced by the friction of the
rising liquid mass against the walls of the
133. Veins never have the transverse columnar structure of
dikes. The simplest consist of one kind of material, ā as quartz,
granite, heavy spar, ā and are alike from side to side. But others
have a banded structure not found in dikes, consisting in an
arrangement of the material parallel to the walls. Fig. 132
represents such a vein, consisting of eleven bands : 1, 3, and 6 are
65 43212 4 50
1 1 1 1
bancU of quartz ; 2, 4, of a gneissoid granite ; and 5, of gneiss.
Of banded veins, the simplest is a vein with three bands, one
central; but the number may be a score or
Instead of being simply rock-material, as in
fig. 132, the bands may be partly metallic ores
of difterent kinds, and calcite, heavy spar, fluor
spar, may make the alternating bands instead
of granite or gneiss. A great vein at Frei-
berg consists of layers of blende, quartz, fluor
spar, pyrites, heavy spar, calcite, each two or
three times repeated, the layers nearly corre-
sponding on either side of the middle seam.
Thus this banded structure is as much cha-
racteristic of veins as the columnar structure is of dikes
fails of the peculiarity in their simpler kinds.
The bands of a vein are far from uniform at different heights,
even when the width of the vein is constant ; and they vary exceed-
ingly through the contractions and expansions which take place at
intervals. The expanded portions may alone be banded, or consist
of layers parallel to the sides, or contain ore.
The mineral or rock-material accompanying the ore in a vein is
called the vein-stone, or gangue. The most common kinds of vein-
stone are quartz, calcite, barytes, and fluor.
In studying veins, besides noting their extent, mineral cha-
racter, and structure, it is important to ascertain their strike and
angle of dip. There is generally an approximate uniformity of
strike in a given region ; and frequently the direction is parallel to
the principal line of elevation in the region. The nature of the
walls or adjoining rock, and systems of faults, are other points that
should receive close attention.
134. False veins. ā Besides the veins and dikes described, there
are also false veins. These false veins are fissures filled by sand
or clay from above. They are readily distinguished by the sedi-
mentary nature of the material ; for all true dikes or veins
are occupied by crystalline rocks or minerals. In a similar
manner earth and organic remains may be washed into caverns
or any open spaces in rocks, and so make, in the very body
of an old record, a false entry. Such a conjunction of com-
paratively modern fossils with more ancient may lead to error,
unless the facts are carefully studied and the true explanation
124 LITHOLOGICAL GEOLOGY.
In the language of miners, ā
A lode is a vein containing ore.
The hanging icall of a vein is the upper wall when the vein has an oblique
dip; and the opposite is the foot-wall.
The Jliiccan is the half-decomposed rock adjoining a vein; and a thin, clayey
layer along either side of a vein is called the selvage.
A horse is a body of rock, like the wall-rock, occurring in the course of a
A comb is one of the layers in a banded vein, ā so called especially when its
surface is more or less set with crystals.
GENERAL DIVISIONS IN THE HISTORY.
1. Nature of subdivisions in history. ā The methods of ascer-
taining the true succession or chronological order of the rocks
have been explained in ^Ā§ 122-125, and in connection ( Ā§ 126 ) a
brief mention is made of the grander divisions of the series. Some
further explanations are necessary as introductory to the survey
of geological history.
WJiat are subdivisions in history/? ā Many persons, in their study of
geology, expect to find strongly-drawn lines between the ages, or
the corresponding subdivisions of the rocks. But geological his-
tory is like human history in this respect. Time is one in its
course, and all progress one in plan.
vSome grand strokes there may be, ā as in human history there is
a beginning in man's creation, and a new starting-point in the
advent of Christ. But all attempts to divide the course of progress
in man's historical development into ages with bold confines are
fruitless. We may trace out the culminant phases of different
periods in that progress, and call each culmination the centre
of a separate period. But the germ of the period was long work-
ing onward in preceding time, before it finally came to its full
development and stood forth as the characteristic of a new era of
progress. It is the same with the development or history of an
individual being. There are distinct epochs and periods in the
history which all recognize, ā the period of the embryo, of the youth,
of the adult. But no one thinks of marking the hour or day
when one ends and another begins, or of pointing to a visible
physical line that at any given moment was passed. It is all one
progress, while successive phases stand forth in that progress.
in geological history, the earliest events were simply physical.
126 HISTORICAL GEOLOGY.
While the inorganic history was still going on (although finished
in its more fundamental ideas), there was, finally, the intro-
duction of Vtfe, ā a new and great step of progress. That life,
beginning with the lower grades of species, was expanded and ele-
vated through the creations of new types, until the history closed
in the appearance of Man. In this organic history there are suc-
cessive phases of progress, or a series of culminations, with the
creation of Man and Mind as the last and loftiest of these culmi-
nations. As the tribes, in geological order, pass like panoramic
scenes before us, the reality of one age after another becomes
strongly apparent. The age of Mammals, the age of Eejjtiles, and
the age of Coal-Plants come out to view like mountains in the
prospect, ā although if the mind should attempt to define precisely
where the slopes of the mountain end as they pass into the plain
around, it might be greatly embarrassed. It is not in the nature
of history to be divided off by visible embankments ; and it is a
test of the true philosopher to see and appreciate the culmina-
tions of phases in time, or of the successive ideas in the system
of progress, amid the multitude of events and indefinite blendings
that bewilder other minds.
We note here the following important principles : ā
First. The reality of an age in history is marked by the culmina-
tion of some new idea in the system of progress.
Secondly. The beginning of an age will be in the midst of a pre-
ceding age ; and the marks of the future coming out to view are to
be regarded as prophetic of that future.
Thirdly. The end of an age may be as ill defined as its beginning,
although its culminant point may stand out boldly to view.
Thus, the age of Coal-Plants was preceded by the occurrence of
related plants far back in the Devonian. The age of Mammals
was foreshadowed by the appearance of mammals long before, in
the course of the Eeptilian age. And the age of Reptiles was pro-
phesied in types that lived in the earlier Carboniferous age. Such
is the system in all history. Nature has no sympathy with the art
which runs up walls to divide off her open fields.
But the question may arise, whether a geological age is not, after
all, strongly marked off in the rocks. Rocks are but the moving
sands or the accumulations of dead relics of the age they represent,
and are local phenomena, as already explained. Each continent
has its special history as regards rock-making ; and it is only
through the fossils in the rocks that the special histories are com-
bined into a general system. Movements have in all ages disturbed
one hemisphere without afifecting the other, causing breaks in the
SUBDIVISIONS IN THE HISTORY. 127
succession of rocks in one continent or part of a continent that
have no representatives in another.
When an age can be proved, through careful study, to have been
closed by a catastrophe or a transition which was universal in its
effects, the event is accepted as a grand and striking one in geo-
logical history. But the proof should be obtained before the uni-
versality is assumed. Hence the conclusion, ā
Fourthly. The grander subdivisions or ages in geological history,
based on organic progress, should be laid down independently of the
rocks. They are universal ideas for the globe. The rocks are to be
divided off as nearly as practicable in accordance with them.
Each continent, under these ages, then becomes a special study ;
and its history has its periods and epochs wliich may or may not
correspond in their limits with those of the other continents. Every
transition in the strata, as from limestone to sandstone, clay-beds,
or conglomerate, or from either one to the other, and especially
where there is also a striking change in the organic remains, indi-
cates a transition in the era from one set of circumstances to an-
other, ā it may be a change from one level to another in the conti-
nents, a submergence or emergence, or some other kind of catas-
trophe. All such transitions mark great events in the history of
the continent, and thus divide the era into periods, and periods
into epochs, and epochs, it may be, into sub-epochs. Hence, ā
Fifthly. Through the ages each continent had its special history ;
and the periods and epochs in that history are indicated by changes
or transitions in the rock-formations and their fossils.
It is greatly to the assistance of research that some of the revolu-
tions of the globe have probably been nearly or quite universal.
The one preceding the Mammalian age appears to be an example ;
although, even with regard to this, further investigation is required
before its actual universality can be regarded as established. But
the periods and epochs of America and Europe are not in general
the same in their limits. A near cotemporaneity in rocks may be
proved, but not in the transitions from one rock to another. For
example, the Devonian age has a very different series of periods
and epochs in North America from what it has in Europe, and there
is even considerable diversity between the epochs of New York and
the Atlantic slope, and those of the Mississippi valley. The Car-
boniferous, Reptilian, and Mammalian ages also have their American
epochs and their European, differing from one another ; and the dif-
ferences between the continents increase as we come down to more
modern times. There are Tertiary and Cretaceous rocks in America
as well as Europe, but there is little reason for the assumption that
the transitions fi*om one set of Tertiary or Cretaceous strata to
another were, in the two, cotemporaneous. The point should be
proved, not assumed. We add, therefore, ā
Sixthly. It is an important object' in geology to ascertain as nearly
as possible the parallelism between the periods and epochs marked
off on each continent, and study out the precise equivalents of the
rocks, each for each, that all the special histories may read as parts
of one general history, and thus contribute to the perfection of one
Progress of life as the basis of the subdivision into geo-
logical ages. ā The general principles in the progress of life upon
which the ages are based are shown in the annexed table.*
Age of Man.
Age of Mammals.
Age of Reptiles.
Carboniferous Age. H a
Age of Fishes, or \
Age of MoUusks, )
or Silurian. J
The horizontal bands represent the ages, in succession ; the ver-
tical correspond to different groups of animals and plants.
The Radiates begin with the Lower Silurian, and continue till
now, rather increasing throughout the ages.
The Mollusks have their beginning at the same time, and continue
increasing to the age of Reptiles ; they then pass their maxmium
(as indicated in the figure) and decline.
* The system of ages is essentially the same with that proposed by Professor
Agassiz,-the only difference consisting in calling the Silurian the age of Mol-
lusks, instead of considering both the Silurian and Devonian the age of Fishes.
SUBDIVISIONS IN THE HISTORY. 129
The Articulates, as the table shows, commence in the Silurian
(as Crustaceans and Worms), and continue expanding in numbers
and grade to the present time.
Fishes begin in the Silurian, are very abundant in the Devonian,
and continue on, becoming increasingly diversified to the last, with-
out much rise in grade.
Reptiles begin in the top of the Devonian, and reach their maxi-
mum in the Reptilian age.
Mammals begin in the Reptilian age, and have their maximum in
the Mammalian age.
As to Plants, Sea-weeds (or Algx) are the earliest of the globe, pro-
bably preceding animal life. The Acrogens begin in the Devonian,
or earlier, and have their greatest expansion in the age of Coal-
Plants, where they occur with abundant Conifers. Cycads begin
in the Carboniferous, and have their greatest exjoansion in the Rei3-
tilian age. Dicotyledons begin in the closing period of the Reptilian
age, and expand, along with Palms, through the age of Mammals.
The Silurian is eminently the age of Mollusks ; for this is the
highest branch of the animal kingdom which is represented at that
time in all its grand subdivisions. Brachiopods, Conchifers, Gas-