TRANSCRIBER'S NOTE
Italic text is denoted by _underscores_.
Superscripts are denoted by ^ eg Lith^{rs} Edin^r.
Basic fractions are displayed as ½ ⅓ ¼ etc; other fractions are shown
in the form a/b, eg 3/11 or 13/(34×2).
The caption for an illustration is displayed as a sidenote in the
etext. It was shown as a page footnote in the original text.
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within the
text and consultation of external sources.
More detail can be found at the end of the book.
INTRODUCTION
TO THE STUDY OF
PALÆONTOLOGICAL BOTANY
INTRODUCTION
TO THE STUDY OF
PALÆONTOLOGICAL BOTANY
BY
JOHN HUTTON BALFOUR, A.M. M.D. EDIN.
F.R.S., SEC. R.S.E., F.L.S.
PROFESSOR OF MEDICINE AND BOTANY IN THE UNIVERSITY OF EDINBURGH,
REGIUS KEEPER OF THE ROYAL BOTANIC GARDEN,
AND QUEEN'S BOTANIST FOR SCOTLAND
WITH FOUR LITHOGRAPHIC PLATES, AND UPWARDS OF
ONE HUNDRED WOODCUTS
EDINBURGH
ADAM AND CHARLES BLACK
1872
_Printed by_ R. & R. CLARK, _Edinburgh_.
TO
PROF. HEINRICH ROBERT GOEPPERT, M.D.,
DIRECTOR OF THE BOTANIC GARDEN, BRESLAU;
ONE OF THE MOST EMINENT PALÆONTOLOGICAL
BOTANISTS OF EUROPE,
The following Treatise
IS DEDICATED, WITH BEST RESPECTS, BY HIS
OBLIGED FRIEND
THE AUTHOR.
PREFACE.
The subject of Fossil Botany or Palæophytology has formed a part of
the Course of Botany in the University of Edinburgh for the last
twenty-five years, and the amount of time devoted to the exposition
of it has increased. The recent foundation of a Chair of Geology and
of a Falconer Palæontological Fellowship in the University seems to
require from the Professors of Zoology and Botany special attention
to the bearings of their departments of science on the structure
of the animals and plants of former epochs of the Earth's history.
No one can be competent to give a correct decision in regard to
Fossils, unless he has studied thoroughly the present Fauna and Flora
of the globe. To give a well-founded opinion in regard to extinct
beings, it is essential that the observer should be conversant with
the conformation and development of the living ones now on the
earth; with their habits, modes of existence and reproduction, the
microscopic structure of their tissues, their distribution, and their
relation to soil, the atmosphere, temperature, and climate.
There can be no doubt that to become a good Fossil Geologist a
student must begin with living animals and plants. The study of
Geology must be shared by the Petralogist, who looks at the
condition of the rocks of the globe, the minerals forming them, and
their mode of formation; the Chemist, who determines the materials
which enter into the composition of minerals and rocks; the
Naturalist, who examines the plants and animals found in the various
strata; and perhaps also the Natural Philosopher, who calculates from
independent sources the phases of the Earth's history. It may be said
thus to combine all these students of Science in one brotherhood.
Much has been done by the efforts of such men as Hutton and Werner,
who were engaged chiefly in considering the mineral department of
Geology; but it is clear that the Science could not have attained its
present position without the continued labours of those who have been
examining fossils in their relations to time and space. Had it not
been for the researches of Palæontologists, Geology could not have
made its present advance.
In my Class Book of Botany I have given an introduction to
Palæophytology, and it occurred to me that it might be useful to
students to publish this in a separate form, with additions in
both the letterpress and the illustrations. The institution of
the Palæontological Fellowship, in memory of my former friend Dr.
Falconer, has brought the subject specially under my notice. The
Fellowship has been promoted chiefly by my friend and former pupil
Dr. Charles Murchison, a gentleman fond of science and of his Alma
Mater, the University of Edinburgh, where he and Falconer studied and
took their degrees.
The first award of the Fellowship has been made to a distinguished
student, who acquitted himself with the greatest credit during the
three days of examination on Geology, Zoology, and Botany. I trust
that the Fellowship will continue to stimulate our eminent students
in future years.
Having been a student of Natural Science along with Dr. Falconer, I
feel a peculiar interest in doing what I can to promote the study
of a subject to which he so successfully devoted his energies.
In my endeavour to do so I have been encouraged by my friend and
former pupil, Mr. William Carruthers, at the head of the Botanical
Department of the British Museum, and a former student in Edinburgh
under the late Professor Fleming. He has done much to advance our
knowledge of Fossil Botany, and to him I am indebted for two of the
plates and some of the woodcuts which illustrate this publication.
He has given me most efficient assistance, and I have to return my
best thanks for his kind aid. I am also indebted to my colleague,
Professor Geikie, for his valued assistance.
The neighbourhood of Edinburgh is rich in Fossils of the
Carboniferous epoch, and much yet remains to be done to illustrate
its Palæontology. Such labourers as Geikie and Peach may be expected
to give great assistance in the furtherance of our knowledge of
Scottish Geology, so as to form a school which shall revive the
reputation enjoyed by Edinburgh in the days of Hutton and Jameson.
If I can be useful in encouraging students to take up the study of
Palæontological Botany, and to prosecute it with vigour, I shall feel
that this introductory treatise has not been issued in vain. As one
of the few surviving relations of Dr. James Hutton, I am glad to be
able to show an interest in a science which may aid in elucidating
the "Theory of the Earth."
In writing this work I have taken for granted that the reader is
acquainted with the Elements of Botany, and knows the general
structure of plants of the present day. I have not, therefore,
hesitated to use the ordinary Botanical terms without explanation. I
am satisfied that no one can study Fossil Botany properly unless he
has studied Modern Botany.
Those readers who may find any difficulty as to technical terms I
would refer to my Botanist's Companion, where a full Glossary is
given.
27 INVERLEITH ROW,
_May 1872_.
TABLE OF CONTENTS.
PAGE
INTRODUCTORY REMARKS 1
DETERMINATION OF FOSSIL PLANTS 3
MODE OF PRESERVATION OF FOSSIL PLANTS 8
EXAMINATION OF THE STRUCTURE OF FOSSIL PLANTS 12
FOSSILIFEROUS ROCKS 20
NATURAL ORDERS TO WHICH FOSSIL PLANTS BELONG 22
PERIODS OF VEGETATION AMONG FOSSIL PLANTS 25
FOSSIL FLORA OF THE PRIMARY OR PALÆZOIC PERIOD 26
REIGN OF ACROGENS 26
FLORA OF THE CARBONIFEROUS EPOCH 36
FLORA OF THE PERMIAN EPOCH 71
FOSSIL FLORA OF THE SECONDARY OR MESOZOIC PERIOD 72
REIGN OF GYMNOSPERMS 72
FLORA OF THE TRIAS AND LIAS EPOCHS 77
FLORA OF THE OOLITIC EPOCH 80
FLORA OF THE WEALDEN EPOCH 84
FOSSIL FLORA OF THE TERTIARY OR CAINOZOIC PERIOD (INCLUDING
THE CRETACEOUS EPOCH) 87
REIGN OF ANGIOSPERMS 87
FLORA OF THE CHALK 87
FLORA OF THE EOCENE EPOCH 90
FLORA OF THE MIOCENE EPOCH 92
FLORA OF THE PLIOCENE EPOCH 98
GENERAL CONCLUSIONS 101
RECAPITULATION 103
WORKS ON FOSSIL BOTANY 105
EXPLANATION OF PLATES 111
INDEX 113
LIST OF WOODCUTS.
FIG. PAGE
1. Section of Peuce Withami, Lindley and Hutton 3
2. Bark of Araucaria 5
3. Markings on Araucaria bark 6
4. " " 7
5. " " 7
6. Leaf of Araucaria 7
7. Nicolia Owenii (Carr.) 11
8. Bryson's instrument for slitting Fossils 14
9. Tree-fern 27
10. Asplenium 28
11 _a._ Bifurcating Trunk of a Tree-fern (Alsophila Perrottetiana) 29
11 _b._ Rhizome of Lastrea Filix-mas 29
12. Transverse section of stem of a Tree-fern (Cyathea) 29
13. Scalariform vessels from Tree-fern 30
14. Sporangia of a Fern 30
15. Lycopodium clavatum 30
16. Spore-case, containing Microspores of Lycopodium 30
17. " " Macrospores of Selaginella 30
18. Fructification of Equisetum maximum 31
19. Polygonal scale of Equisetum 32
20. Spore of Equisetum--filaments contracted 32
21. " " " expanded 32
22. Marsilea Fabri 33
22 _bis._ Adiantites Lindseæformis 41
23. Pecopteris (Alethopteris) aquiline 43
24. " (Alethopteris) heterophylla 43
25. Neuropteris Loshii 43
26. " gigantean 43
27. " acuminate 43
28. Sphenopteris affinis 43
29. Cyclopteris dilatata 43
30. Stem of Caulopteris macrodiscus 44
31. " " Balfouri (Carr.) 44
32. " " Morrisi (Carr.) 44
33. " Sigillaria pachyderma 45
34. Sigillaria reniformis 45
35. " pachyderma 46
36. " (Favularia) tessellate 46
37. " pachyderma 46
38. Stigmaria ficoides 47
39. " " (S. anabathra of Corda) 47
40. Bifurcating stem of Lepidodendron obovatum (elegans) 49
41. Stem of Lepidodendron crenatum 49
42. Fructification of Lepidodendron 50
43. Longitudinal section of Fructification of Triplosporites 50
44. (1). Fruit of Selaginella spinulosa, A. Braun (Lycopodium
selaginoides, Linn.) 51
(2). Scale and sporangium from the upper part of cone 51
(3). Antheridian microspores from ditto 51
(4). Macrospore 51
(5). Scale and sporangium from lower part of cone, containing
macrospores 51
(6). Fruit of Lepidostrobus ornatus (Hooker) 51
(7). Three scales and sporangia of ditto 51
(8). Microspores from sporangia of the upper part of the
cone of Triplosporites Brownii, Brongn. 51
(9). Macrospore from the sporangia of the lower part 51
(10). Scales and sporangia of a cone of Flemingites 51
45 _a._ Calamites Suckovii 57
45 _b._ Septum or Phragma of a Calamite 57
46. Vertical stems of Calamites--in coal-measures of Treuil,
near St. Etienne 58
47. Fruits of Equisetum and Calamites 60
(1). Equisetum arvense, L. 60
(2). Portion of sporangium wall 60
(3, 4). Spores--elaters free 60
(5). Longitudinal section of part of one side of cone 60
(6). Transverse section of cone 60
(7). Calamites (Volkmannia) Binneyi (Carr.) 60
(8). Portion of sporangium wall 60
(9). Two spores 60
(10). Longitudinal section of part of one side of cone 60
(11). Transverse section of cone 60
48. Foliage and fruits of Calamites 62
(1, 2). Asterophyllites 62
(3, 4). Annularia 62
(5, 6). Sphenophyllum 62
49. Araucarioxylon Withami, Krauss (Pinites Withami) 63
50. Trigonocarpum olivæforme 63
51. Cardiocarpum Lindleyi (Carr.) 65
52. " " 65
53. Cardiocarpum anomalum (Carr.) 66
54. Pothocites Grantoni (Paterson) 67
55, 56. Walchia piniformis (Sternb.) 72
57. Pinus sylvestris 73
58. Abies excelsa 73
59. Larix Europæa 73
60. Cedrus Libani 73
61. Araucaria excelsa 74
62. Woody tubes of fir--single rows of discs 74
63. " " --double and opposite rows of discs 74
64. Woody tubes of Araucaria excelsa--double and triple
and alternate rows of discs 74
65. Longitudinal section of stem of a Gymnosperm 74
66. Linear leaves of Pinus Strobus 75
67. Cone of Pinus sylvestris 75
68. " Cupressus sempervirens 75
69. Scale of mature cone of Pinus sylvestris 75
70. Fruiting branch of Juniperus communis 76
71. Branch of Taxus baccata 76
72. Male flower of Yew 76
73. Fruit of Yew 76
74. Cycas revoluta 77
75. Encephalartos (Zamia) pungens 77
76. Schizoneura heterophylla 78
77. Zamites 79
78. Pterophyllum Pleiningerii 80
79. Nilssonia compta (Pterophyllum comptum of Lindley
and Hutton) 80
80. Palæozamia pectinata (Zamia pectinata of Brongniart, and
Lindley and Hutton) 80
81. Brachyphyllum mammillare 81
82. Equisetum columnare 81
83. Araucarites sphærocarpus (Carr.) 82
84. Termination of a scale of ditto 82
85. Section of a scale of ditto 82
86. The Dirt-bed of the island of Portland 83
87. Cycadoidea megalophylla (Mantellia nidiformis of Brongniart) 83
88. Kaidacarpum ooliticum (Carr.) 84
89. Pandanus odoratissimus 84
90. Fossil wood, Abietites Linkii 85
91. Sequoiites ovalis 88
92. Pinites ovatus (Zamia ovata of Lindley and Hutton) 89
93. Palmacites Lamanonis 90
95. Comptonia acutiloba 92
96. Acer trilobatum 93
97. Ulmus Bronnii 93
98. Rhamnus Aizoon 94
99. Alnus gracilis 95
100. Taxites or Taxodites Campbellii 95
101. Rhamnites multinervatus 95
102. Equisetum Campbellii 96
PALÆONTOLOGICAL BOTANY.
The study of the changes which have taken place in the nature of
living beings since their first appearance on the globe till the
period when the surface of the earth, having assumed its present
form, has been covered by the creation which now occupies it,
constitutes one of the most important departments in Geology. It is,
as Brongniart remarks, the history of life and its metamorphoses.
The researches of geologists show clearly that the globe has
undergone various alterations since that "beginning" when "God
created the heavens and the earth." These alterations are exhibited
in the different stratified rocks which form the outer crust of the
earth, and which were chiefly sedimentary deposits produced by the
weathering of the exposed rocks. Remains of the plants and animals
living on the globe at the time of the formation of the different
beds are preserved in them. Elevations and depressions of the surface
of the earth affected the organisms on its surface, and gave to
successive deposits new faunas and floras. Some of these epochs have
been marked by great changes in the physical state of our planet,
and they have been accompanied with equally great modifications
in the nature of the living beings which inhabited it. The study
of the fossil remains of animals is called Palæozoology (παλαιός,
ancient, and ζῷον, animal), while the consideration of those of
vegetables is denominated Palæophytology (παλαιός and φυτόν, a plant).
Both are departments of the science of Palæontology, which has been
the means of bringing geology to its present state of advancement.
The study of these extinct forms has afforded valuable indications
as to the physical state of the earth, and as to its climate at
different epochs. This study requires the conjunct labours of the
Zoologist, the Botanist, and the Petralogist.
The vegetation of the globe, during the different stages of its
formation, has undergone very evident changes. At the same time there
is no reason to doubt that the plants may all be referred to the
great classes distinguished at the present day--namely, Thallogens,
including such plants as Lichens, Algæ, and Fungi; Acrogens, such
as Ferns and Lycopods; Gymnosperms, such as Cone-bearing plants and
Cycads; Endogens, such as Palms, Lilies, and Grasses; and Exogens,
such as the common trees of Britain (excluding the Fir), and the
great mass of ordinary flowering plants. The relative proportion of
these classes, however, has been different, and the predominance of
certain forms has given a character to the vegetation of different
epochs. The farther we recede in geological history from the present
day, the greater is the difference between the fossil plants and
those which now occupy the surface. At the time when the coal-beds
were formed, the plants covering the earth belonged to genera and
species not existing at the present day. As we ascend higher, the
similarity between the ancient and the modern flora increases, and in
the latest stratified rocks we have in certain instances an identity
in species and a considerable number of existing genera. At early
epochs the flora appears to have been uniform, to have presented less
diversity of forms than at present, and to have been similar in the
different quarters of the globe. The vegetation also indicates that
the nature of the climate was different from that which characterises
the countries in which these early fossil plants are now found.
DETERMINATION OF FOSSIL PLANTS.
[Illustration: Fig. 1.]
[Sidenote: Fig. 1. Section of _Peuce Withami_, after Lindley and
Hutton, a fossil Conifer of the coal epoch. Punctated woody tissue
seen.]
Fossil plants are by no means so easily examined as recent species.
They are seldom found in a complete state. Fragments of stems,
leaves, and fruits, are the data by which the plant is to be
determined. It is very rare to find any traces of the flowers. The
parts of fossil plants are usually separated from each other, and
it is difficult to ascertain what are the portions which should be
associated together so as to complete an individual plant. Specimens
are sometimes preserved, so that the anatomical structure of the
organs, especially of the stem, can be detected by very thin slices
placed under the microscope. In the case of some stems the presence
of punctated woody tissue (Fig. 1) has proved of great service as
regards fossil Botany; this structure, along with the absence of
large pitted ducts, serving to distinguish Conifers. The presence
of scalariform vessels indicates a plant belonging to the vascular
Cryptogams, of which the fern is the best known example. The cautions
to be observed in determining fossil plants are noticed by Dr. Hooker
in the Memoirs of the Geological Survey of Great Britain (vol. ii.
p. 387). At the present day, the same fern may have different forms
of fronds, which, unless they were found united, might be reckoned
distinct genera; and remarkable examples are seen in Niphobolus
rupestris and Lindsæa cordata. Moreover, we find the same form of
frond belonging to several different genera, which can only be
distinguished by the fructification; and as this is rarely seen in
fossil ferns, it is often impossible to come to a decided conclusion
in regard to them. A leaf of Stangeria paradoxa was considered
by an eminent botanist as a barren fern frond, but it ultimately
proved to be the leaf of a Cycad. The leaf of Cupania filicifolia,
a Dicotyledon, might easily be mistaken for that of a fern; it
resembles much the frond of a fossil fern called Coniopteris. The
diverse leaves of Sterculia diversifolia, if seen separately, might
easily be referred to different plants. In the same fern we meet
also with different kinds of venation in the fronds. Similar remarks
may be made in regard to other plants. Harvey has pointed out many
difficulties in regard to sea-weeds.
As regards the materials for a fossil flora, the following remarks of
Hugh Miller deserve attention:--
"The authors of Fossil Floras, however able or accomplished they may
be, have often to found their genera and species, and to frame their
restorations, when they attempt these, on very inadequate specimens.
For, were they to pause in their labours until better ones turned up,
they would find the longest life greatly too short for the completion
of even a small portion of their task. Much of their work must be
of necessity of a provisional character--so much so, that there are
few possessors of good collections who do not find themselves in
circumstances to furnish both addenda and errata to our most valuable
works on Palæontology. And it is only by the free communication of
these addenda and errata that geologists will be at length enabled
adequately to conceive of the by-past creations, and of that gorgeous
Flora of the Carboniferous age, which seems to have been by far the
most luxuriant and wonderful which our emphatically ancient earth
ever saw."
[Illustration: Fig. 2.]
[Sidenote: Fig. 2. Bark of _Araucaria imbricata_.]
The bark of trees at the present day often exhibits different kinds
of markings in its layers. This may be illustrated by a specimen of
Araucaria imbricata, which was destroyed by frost in the Edinburgh
Botanic Garden on 24th December 1861. The tree was 24½ feet high,
with a circumference of four feet at the base of the stem, and
had twenty whorls of branches. The external surface of the bark
is represented in Fig. 2. There are seen scars formed in part by
prolongations from the lower part of the leaves, which have been
cut off close to their union with the stem. The base of each leaf
remaining in the bark has the form of a narrow elongated ellipse,
surrounded by cortical foliar prolongations. The markings on the
bark, when viewed externally, have a somewhat oblique quadrilateral
form. On removing the epiphlœum or outer bark, and examining its
inner surface, we remark a difference in the appearance presented
at the lower and upper part of the stem. In the lower portion the
markings have an irregular elliptical form, with a deep depression,
and fissures where the leaves are attached (Fig. 3). Higher up the
epiphlœal markings assume rather more of a quadrilateral form, with
the depressions less deep, and the fissures for the leaves giving off
prolongations on either side. Farther up the markings are smaller in
size, obliquely quadrilateral, and present circular clots along the
boundary lines chiefly (Fig. 4). Higher still the quadrilateral form
becomes more apparent, and the dots disappear (Fig. 5). The epiphlœum
thus presents differences in its markings at different heights on the
stem.
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
[Illustration: Fig. 5.]
[Sidenote: Figs. 3, 4, and 5. Markings on Araucaria bark.]
The part of the bark immediately below the epiphlœum is well
developed, and is of a spongy consistence. When examined
microscopically it is seen to be composed of cells of various
shapes--some elongated fusiform, others rhomboidal, others with
pointed appendages. The variety of forms is very great, but it is
possible that this may be partly owing to the effects of frost on the
cells. On the spontaneous separation of the bark, the portion below
the epiphlœum was seen to consist of distinct plates of a more or
less quadrilateral form, with some of the edges concave and others
convex, a part in the centre indicating the connection with the leaf,
along with which it is detached. In Fig. 6 a leaf is shown with one
of these plates attached.
[Illustration: Fig. 6.]
[Sidenote: Fig. 6. Leaf of Araucaria with a portion of bark.]
The appearances presented by the outer and middle bark of Araucaria
imbricata bear a marked resemblance to those exhibited by certain
fossils included in the genera Sigillaria and Lepidodendron. The
sculpturesque markings on the stems of these fossil plants indicate
their alliance to the ferns and lycopods of the present epoch. But
it is evident, from these markings, that much caution is required
in making this determination. Other points of structure must be
examined before a proper decision can be formed. When, for instance,
the presence of scalariform tissue, or of punctated woody tissue,
has been satisfactorily shown under the microscope, we are entitled
to hazard an opinion as to the affinities of the fossils. In many
instances, however, external appearances are the only data on which
to rely for the determination of fossil genera and species; and rash
conclusions have often been drawn by geologists who have not been
conversant with the structure of plants. The Araucaria markings point
out the need of care in drawing conclusions, and their variations at
different parts of the bark indicate the danger of a rash decision
as to species. There can be no doubt that in vegetable Palæontology
the number of species has been needlessly multiplied--any slight
variation in form having been reckoned sufficient for specific
distinction. We can conceive that the Araucaria bark markings in a
fossil state might easily supply several species of Lepidodendron.
A naturalist, with little knowledge of the present flora of the
globe, ventures sometimes to decide on an isolated fragment. Hence
the crude descriptions of fossil vegetable forms, and the confusion
in which Palæophytology is involved. Every geologist who examines
fossil plants ought to be well acquainted with the minute structure
of living plants, the forms of their roots, stems, leaves, fronds,
and fructification; the markings on the outer and inner surfaces of
their barks, on their stems, and on their rhizomes; the localities in
which they grow, and the climates which genera and species affect in
various parts of the world. (Professor Balfour in the Proceedings of
the Royal Society of Edinburgh, April 1862, vol. iv. p. 577.)
MODE OF PRESERVATION OF FOSSIL PLANTS.
The mode in which plants are preserved in a fossil state may be
referred to four principal classes:--1. Casts of the plants; from
which all the original substance and structure have been removed
subsequently to the burial of the plants, and to the greater or
less induration of the rocks in which they are entombed. Such casts
are occasionally hollow, but more frequently they consist of the
amorphous substance of the rock which has filled up the cavity,
and which exhibits, often with remarkable minuteness, the external
aspects of the original specimen. 2. Carbonisation; in which the
original substance of the plant has been chemically altered and
converted into lignite or coal. All trace of the form of the original
plant is generally lost, as is the case with the extensive beds of
coal; but frequently, when the organism has been buried in a bed of
clay, the external appearance is faithfully preserved, as in the
ferns and other foliage found in the shales of the coal-measures.
3. Infiltration; in which the vegetable tissues, though carbonised,
retain their original form from the infiltration of some mineral in
solution, chiefly lime or silex, which has filled the empty cells
and vessels, and so preserved their original form. This mode of
preservation occurs in the calcareous nodules in coal-beds, in the
remarkable ash-beds discovered by Mr. Wünsch in Arran, and generally
in the secondary rocks. 4. Petrifaction; in which the structure is
preserved, but the whole of the original substance has been replaced,
atom for atom, by an inorganic substance, generally lime, silex, or
some ore of iron. This is the condition of the beautiful fossils
from Antigua, and of many stems and fruits from rocks of all ages in
Britain.
Carbonised vegetables, or those which have passed into the state
of Lignites, often undergo modifications which render it difficult
to understand them rightly. Sometimes a portion of the organs of
vegetables which have passed into the state of lignite is transformed
into pyrites, or else pyrites of a globular shape is found in
the middle of the tissue, and may be taken for a character of
organisation. The section of certain Dicotyledonous fossil woods, in
that case, may resemble Monocotyledons. Petrifaction, as in the case
of silicified woods, often preserves all the tissues equally, at
other times the soft tissues are altered or destroyed; the cellular
tissue being replaced by amorphous chalcedony, while the ligneous
and vascular tissues alone are petrified, so as to preserve their
forms. In some cases the reverse takes place as to these tissues;
the fibrous portions disappear, leaving cavities, while the cells
are silicified. Sometimes we find the parts regularly silicified
at one place, so as to retain the structure, while at another an
amorphous mass of silica is found. In such cases there appear,
as it were, distinct silicified woody bundles in the midst of an
amorphous mass. The appearance depends, however, merely on irregular
silicification or partial petrifaction. Infiltrated fossil woods, by
means of chemical tests, are shown to possess portions of vegetable
tissues cemented into a mass by silica. In some cases we find the
vessels and cells separately silicified, without being crushed into a
compact mass. In these cases, the intercellular substance not being
silicified, the mass breaks down easily; whereas, when complete
silicification takes place, the mass is not friable. Coniferous wood
is often friable, from silicified portions being still separated
from each other by vegetable tissue more or less entire. During
silicification, or subsequent to it, it frequently happens that
the plant has been compressed, broken, and deformed, and that
fissures have been formed which have been subsequently filled with
crystallised or amorphous silica.
[Illustration: Fig. 7.]
[Sidenote: Fig. 7. _Nicolia Owenii_ (Carr.), from the Tertiary Strata
of Egypt.]
Silicified stems of trees have been observed in various parts of
the world, with their structure well preserved, so that their
Endogenous and Exogenous character could be easily determined. The
Rev. W. B. Clarke notices the occurrence of a fossil pine-forest
at Kurrur-Kurrân, in the inlet of Awaaba, on the eastern coast of
Australia. In the inlet there is a formation of conglomerate and
sandstone, with subordinate beds of lignite--the lignite forming
the so-called Australian coal. Throughout the alluvial flat, stumps
and stools of fossilised trees are seen standing out of the ground,
and one can form no better notion of their aspect than by imagining
what the appearance of the existing living forest of Eucalypti and
Casuarinæ would be if the trees were all cut down to a certain level.
In a lake in the vicinity there are also some fossilised stumps of
trees, standing vertically. In Derwent Valley, Van Diemen's Land,
fossil silicified trees, in connection with trap rocks, have been
found in an erect position. One was measured with a stem 6 feet high,
a circumference at the base of 7 feet 3 inches, and a diameter at the
top of 15 inches. The stems are Coniferous, resembling Araucaria. The
outer portion of the stem is of a rich brown glossy agate, while the
interior is of a snowy whiteness. One hundred concentric rings have
been counted. The tissue falls into a powdery mass. Silica is found
in the inside of the tubes, and their substance is also silicified.
The erect silicified stems of coniferous trees exist in their natural
positions in the "dirt-bed," an old surface soil in the sandstone
strata of the Purbeck series in the Isle of Portland, Dorsetshire. In
the petrified forests near Cairo silicified stems have been examined
by Brown, Unger, and Carruthers. They belong to dicotyledonous trees
(not coniferous), to which the names of Nicolia Ægyptiaca and Nicolia
Owenii (Fig. 7) have been given. The wood consists of a slender
prosenchyma, abundantly penetrated by large ducts. The walls of the
ducts are marked by small, regularly arranged, oval, and somewhat
compressed hexagonal reticulations. The ducts have transverse
diaphragms. There are numerous medullary rays. The wood in their
stems is converted into chalcedony. (Carruthers on Petrified Forest
near Cairo. Geol. Mag., July 1870.)
EXAMINATION OF THE STRUCTURE OF FOSSIL PLANTS.
When the structure of fossil plants is well preserved, it may be
seen under the microscope by making thin sections after the mode
recommended by Mr. William Nicol, the inventor of the prism which
bears his name, and to whose memory Unger dedicated the genus
Nicolia, which has just been described as constituting the petrified
forest at Cairo. The following is a description of the process of
preparing fossils for the microscope, by Mr. Alexander Bryson. (Edin.
N. Phil. Journal, N. S. iii. 297. Balfour's Botanist's Companion, p.
30.)
"The usual mode of proceeding in making a section of fossil wood is
simple, though tedious. The first process is to flatten the specimen
to be operated on by grinding it on a flat _lap_ made of lead charged
with emery or corundum powder. It must now be rendered perfectly flat
by hand on a plate of metal or glass, using much finer emery than in
the first operation of grinding. The next operation is to cement the
object to the glass plate. Both the plate of glass and the fossil
to be cemented must be heated to a temperature rather inconvenient
for the fingers to bear. By this means moisture and adherent air are
driven off, especially from the object to be operated on. Canada
balsam is now to be equally spread over both plate and object, and
exposed again to heat, until the redundant turpentine in the balsam
has been driven off by evaporation. The two surfaces are now to be
connected while hot, and a slow circular motion, with pressure, given
either to the plate or object, for the purpose of throwing out the
superabundant balsam and globules of included air. The object should
be below and the glass plate above, as we then can see when all the
air is removed, by the pressure and motion indicated. It is proper
to mention that too much balsam is more favourable for the expulsion
of the air-bubbles than too little. When cold, the Canada balsam
will be found hard and adhering, and the specimen fit for slitting.
This process has hitherto been performed by using a disc of thin
sheet-iron, so much employed by the tinsmith, technically called
_sheet-tin_. The tin coating ought to be partially removed by heating
the plate, and when hot rubbing off much of the extraneous tin by a
piece of cloth. The plate has now to be planished on the polished
_stake_ of the tinsmith, until quite flat. If the plate is to be
used in the lathe, and by the usual method, it ought to be planished
so as to possess a slight convexity. This gives a certain amount
of rigidity to the edge, which is useful in slitting by the hand;
while by the method of mechanical slitting, about to be described,
this convexity is inadmissible. The tin plate, when mounted on an
appropriate chuck in the lathe, must be turned quite true, with its
edge slightly rounded and made perfectly smooth by a fine-cut file.
The edge of the disc is now to be charged with diamond powder. This
is done by mingling the diamond powder with oil, and placing it on a
piece of the hardest agate, and then turning the disc slowly round.
Then, by holding the agate with the diamond powder with a moderate
pressure against the edge of the disc, it is thoroughly charged with
a host of diamond points, becoming, as it were, a saw with invisible
teeth. In pounding the diamond, some care is necessary, as also a
fitting mortar. The mortar should be made of an old steel die, if
accessible; if not, a mass of steel, slightly conical, the base of
which ought to be 2 inches in diameter, and the upper part 1½ inch.
A cylindrical hole is now to be turned out in the centre, of ¾ths of
an inch diameter, and about 1 inch deep. This, when hardened, is the
mortar; for safety it may be annealed to a straw colour. The pestle
is merely a cylinder of steel, fitting the hollow mortar but loosely,
and having a ledge or edging of an eighth of an inch projecting round
it, but sufficiently raised above the upper surface of the mortar, so
as not to come in contact while pounding the diamond. The point of
the pestle ought only to be hardened and annealed to a straw colour,
and should be of course convex, fitting the opposing and equal
concavity of the mortar. The purpose of the projecting ledge is to
prevent the smaller particles of diamond spurting out when the pestle
is struck by the hammer."
[Illustration: Fig. 8.]
[Sidenote: Fig. 8. Mr. Bryson's instrument for slitting fossils. A
very simple slicing and polishing machine has been invented by Mr. J.
B. Jordan of the Mining Record Office, and is sold by Messrs. Cotton
and Johnson, Grafton Street, Soho, London. It costs about £10.]
Mr. Bryson has contrived an instrument for slitting fossils. The
instrument is placed on the table of a common lathe, which is, of
course, the source of motion (Fig. 8). It consists of a Watt's
parallel motion, with four joints, attached to a basement fixed
to the table of the lathe. This base has a motion (for adjustment
only) in a horizontal plane, by which we may be enabled to place the
upper joint in a parallel plane with the spindle of the lathe. This
may be called the azimuthal adjustment. The adjustment, which in
an astronomical instrument is called the plane of right ascension,
is given by a pivot in the top of the base, and clamped by a screw
below. This motion in right ascension gives us the power of adjusting
the perpendicular planes of motion, so that the object to be slit
passes down from the circumference of the slitting-plate to nearly
its centre, in a perfectly parallel plane. When this adjustment
is made accurately, and the slitting-plate well primed and flat,
a very thin and parallel slice is obtained. This jointed frame is
counterpoised and supported by a lever, the centre of which is
movable in a pillar standing perpendicularly from the lathe table.
Attached to the lever is a screw of three threads, by which the
counterpoise weight is adjusted readily to the varying weight of the
object to be slit and the necessary pressure required on the edge of
the slitting-plate.
The object is fixed to the machine by a pneumatic chuck. It consists
of an iron tube, which passes through an aperture on the upper
joint of the guiding-frame, into which is screwed a round piece of
gun-metal, slightly hollowed in the centre, but flat towards the
edge. This gun-metal disc is perforated by a small hole communicating
with the interior of the iron tube. This aperture permits the air
between the glass plate and the chuck to be exhausted by a small
air-syringe at the other end. The face of this chuck is covered with
a thin film of soft india-rubber not vulcanised, also perforated with
a small central aperture. When the chuck is properly adjusted, and
the india-rubber carefully stretched over the face of the gun-metal,
one or two pulls of the syringe-piston is quite sufficient to
maintain a very large object under the action of the slitting-plate.
By this method no time is lost; the adhesion is made instantaneously,
and as quickly broken by opening a small screw, to admit air between
the glass plate and the chuck, when the object is immediately
released. Care must be taken, in stretching the india-rubber over the
face of the chuck, to make it very equal in its distribution, and as
thin as is consistent with strength. When this material is obtained
from the shops, it presents a series of slight grooves, and is rather
hard for our purpose. It ought, therefore, to be slightly heated,
which renders it soft and pliant, and in this state should now be
stretched over the chuck, and a piece of soft copper wire tied round
it, a slight groove being cut in the periphery of the chuck to detain
the wire in its place. When by use the surface of the india-rubber
becomes flat, smooth, and free from the grooves which at first mar
its usefulness, a specimen may be slit of many square inches, without
resort being had to another exhaustion by the syringe. But when a
large, hard, siliceous object has to be slit, it is well for the
sake of safety to try the syringe piston, and observe if it returns
forcibly to the bottom of the cylinder, which evidences the good
condition of the vacuum of the chuck.
After the operation of slitting, the plate must be removed from
the spindle of the lathe, and the flat lead _lap_ substituted. The
pneumatic chuck is now to be reversed, and the specimen placed in
contact with the grinder. By giving a slightly tortuous motion to
the specimen, that is, using the motion of the various joints, the
object is ground perfectly flat when the length of both arms of the
joints is perfectly equal. Should the leg of the first joint on the
right-hand side be the longer, the specimen will be ground hollow; if
shorter, it will be ground convex. But if, as before stated, they are
of equal length, a perfectly parallel surface will be obtained.
In operating on siliceous objects, I have found soap and water
quite as speedy and efficacious as oil, which is generally used;
while calcareous fossils must be slit by a solution of common soda
in water. This solution of soda, if made too strong, softens the
india-rubber on the face of the pneumatic chuck, and renders a
new piece necessary; but if care is taken to keep the solution of
moderate strength, one piece of india-rubber may last for six months.
The thinner and flatter it becomes, the better hold the glass takes,
until a puncture occurs in the outer portion, and a new piece is
rendered necessary.
The polishing of the section is the last operation. This is performed
in various ways, according to the material of which the organism is
composed. If siliceous, a _lap_ of tin is to be used, about the same
size as the grinding _lap_. Having turned the face smooth and flat, a
series of very fine notches are to be made all over the surface. This
operation is accomplished by holding the edge of an old dinner-knife
almost perpendicular to the surface of the _lap_ while rotating;
this produces a series of _criddles_, or slight asperities, which
detain the polishing substance. The polishing substance used on the
tin lap is technically called lapidaries' rot-stone, and is applied
by slightly moistening the mass, and pressing it firmly against
the polisher, care being taken to scrape off the outer surface,
which often contains grit. The specimen is then to be pressed with
some degree of force against the revolving tin _lap_ or polisher,
carefully changing the plane of action, by moving the specimen in
various directions over the surface.
To polish calcareous objects, another method must be adopted as
follows:--
A _lap_ or disc of willow wood is to be adapted to the spindle of the
lathe, three inches in thickness, and about the diameter of the other
laps (10 inches), the axis of the wood being parallel to the spindle
of the lathe, that is, the acting surface of the wood is the end of
the fibres, the section being transverse.
This polisher must be turned quite flat and smoothed by a plane, as
the willow, from its softness, is peculiarly difficult to turn. It
is also of consequence to remark that both sides should be turned,
so that the _lap_, when dry, is quite parallel. This _lap_ is most
conveniently adapted to the common face chuck of a lathe with a
conical screw, so that either surface may be used. This is made
evident, when we state that this polisher is always used moist, and,
to keep both surfaces parallel, must be entirely plunged in water
before using, as both surfaces must be equally moist, otherwise the
dry surface will be concave and the moist one convex. The polishing
substance used with this _lap_ is putty powder (oxide of tin), which
ought to be well washed, to free it from grit. The calcareous fossils
being finely ground, are speedily polished by this method. To polish
softer substances, a piece of cloth may be spread over the wooden
_lap_, and finely-levigated chalk used as a polishing medium.
In order to study fossil plants well, there must be an acquaintance
with systematic botany, a knowledge of the microscopical structure
of all the organs of plants, such as their roots, stems, barks,
leaves, fronds, and fruit; of the markings which they exhibit on
their different surfaces, and of the scars which some of them
leave when they decay. It is only thus we can expect to determine
accurately the living affinities of the fossil. Brongniart says,
that before comparing a fossil vegetable with living plants, it is
necessary to reconstruct as completely as possible the portion of
the plant under examination, to determine the relations of these
portions to the other organs of the same plant, and to complete the
plant if possible, by seeing whether, in the fossils of the same
locality, there may not be some which belong to the same plant. The
connection of the different parts of the same plant is one of the
most important problems in Palæophytology, and the neglect of it has
led to many mistakes. In some instances the data have been sufficient
to enable botanists to refer a fossil plant to a genus of the present
day, so that we have fossil species of the genera Ulmus, Alnus,
Pinus, etc. Sometimes the plant is shown to be allied to a living
genus, but differing in some essential point, or wanting something to
complete the identity, and it is then marked by the addition of the
term _ites_, as Pinites, Thuites, Zamites, etc.
Before drawing conclusions as to the climate or physical condition
of the globe at different geological epochs, the botanist must be
well informed as to the vegetation of different countries, as to the
soils and localities in which certain plants grow, whether on land
or in the sea, or in lakes, in dry or marshy ground, in valleys or
on mountains, or in estuaries, in hot, temperate, or cold regions.
Great caution must be employed also in predicating from one species
the conditions of another, inasmuch as different species of the
same genus frequently exist in very different habitats, and under
almost opposite conditions of moisture and temperature. It is
only by a careful consideration of all these particulars that any
probable inferences can be drawn as to the condition of the globe.
Considering the physiognomy of vegetation at the present day, we
find remarkable associations of forms. The Palms, although generally
characteristic of very warm countries, are by no means confined to
them; Chamærops humilis extending to Europe as far as lat. 43° to 44°
N., and C. palmetto in North America to lat. 34° to 36° N., while
C. Fortunei, from the north of China, is perfectly hardy in the
south of England. Major Madden mentions the association of Palms and
Bamboos with Conifers at considerable elevations on the Himalayas.
(Edin. Bot. Soc. Trans. iv., p. 185.) Epiphytic Orchids, which
usually characterise warm climates, have representatives at great
elevations, as Oncidium nubigenum at 14,000 feet in the Andes, and
Epidendrum frigidum at from 12,000 to 13,000 feet in the Columbia
mountains. These facts point out the care necessary before drawing
conclusions as to the climate which fossil plants may be supposed to
indicate.
FOSSILIFEROUS ROCKS.
The rocks of which the globe is composed are divided into two
great classes--the Stratified or Aqueous, and the Unstratified or
Igneous. The stratified rocks frequently contain fossil remains,
and are then called fossiliferous; those with no such remains are
designated non-fossiliferous or azoic. The igneous unstratified
rocks, included under the names of Granitic and Trappean, show no
appearance of animal or vegetable remains. Those trap rocks, however,
which have been formed of loose volcanic ashes have often enclosed
and preserved the remains of plants and animals; while even between
the successive beds of old lava-like trap rocks organic remains are
sometimes found. Thus, in Antrim, near the Giant's Causeway, deposits
containing vegetable remains occur inter-stratified with basaltic
rocks. These remains are of Miocene age, and have been referred to
coniferous plants, beeches, oaks, plane trees, etc. Similar plants
have been discovered in a similar position by the Duke of Argyll
in the island of Mull. In trap rocks near Edinburgh, lignite with
distinct structure has also been detected. Silicified wood and coal,
imbedded in trap rocks, have been seen in Kerguelen's Land. The wood
is found enclosed in basalt, whilst the coal crops out in ravines,
in close contact with the overlying porphyritic and amygdaloidal
greenstone. Hooker has also seen silicified wood, in connection with
trap, in Macquarrie's Plains, in Tasmania. Several beds of trap-tuff
or ash, formed into solid compact rock by infiltrated carbonate of
lime, occur in the north-east of Arran, which contain numerous stems,
branches, and fruits of carboniferous plants. These represent the
remains of successive forests which grew on this locality, and were
one after the other destroyed by the ash-showers poured forth from a
neighbouring volcano during its intermittent periods of activity.
Fossil remains are extremely rare in certain rocks, which, from the
changes they have undergone, have been denominated Metamorphic.
These include Gneiss and Mica-slate, which are stratified rocks
subsequently altered by heat and other causes, and so completely
metamorphosed that the traces of organisms have been nearly
obliterated. Nevertheless, recognisable traces of plant and animal
remains have been found in what were recently thought to be azoic
rocks. The absence of organic remains in rocks is therefore not
sufficient to enable us to state that these rocks were formed before
animals or vegetables existed.
The stratified rocks which contain fossils have been divided into
three great groups--the Palæozoic, the Secondary, and the Tertiary,
or into Palæozoic and Neozoic groups. The formations included under
these are exhibited in the following table, taken from Lyell's Manual
of Geology:--
1. Recent. } Post Tertiary. } Recent.
2. Post Pliocene. } }