THE STORY OF A BOULDER.
THE STORY OF A BOULDER
OR
GLEANINGS FROM THE NOTE-BOOK OF A FIELD GEOLOGIST
BY
ARCHIBALD GEIKIE
OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN.
Illustrated with Woodcuts.
EDINBURGH: THOMAS CONSTABLE AND CO. HAMILTON, ADAMS, AND CO., LONDON.
MDCCCLVIII.
EDINBURGH: T. CONSTABLE, PRINTER TO HER MAJESTY.
TO
GEORGE WILSON, M.D., F.R.S.E.
REGIUS PROFESSOR OF TECHNOLOGY IN THE UNIVERSITY OF EDINBURGH,
THESE PAGES
ARE
AFFECTIONATELY INSCRIBED.
PREFACE
The present Volume has been written among the rocks which it seeks
to describe, during the intervals of leisure of a field-geologist.
Its composition has been carried on by snatches, often short and far
apart, some of the descriptions having been jotted down on the spot
by streamlet and hill-side, or in the quiet of old quarries; others,
again, in railway-carriage or stage-coach. By much the larger portion,
however, has been written by the village fireside, after the field-work
of the day was over--a season not the most favourable to any mental
exercise, for weariness of body is apt to beget lassitude of mind. In
short, were I to say that these Chapters have been as often thrown
aside and resumed again as they contain paragraphs, the statement would
probably not exceed the truth. But the erratic life of an itinerant
student of science is attended with yet greater disadvantages. It
entails an absence from all libraries, more especially scientific
ones, and the number of works of reference admissible into his _parva
supellex_ must ever be few indeed. With these hindrances, can the
writer venture to hope that what has thus been so disjointed and
unconnected to him, will not seem equally so to his readers? Yet if his
descriptions, written, as it were, face to face with Nature, are found
to have caught some tinge of Nature's freshness, and please the reader
well enough to set him in the way of becoming a geologist, he shall
have accomplished all his design.
It cannot be too widely known, or too often pressed on the attention,
especially of the young, that a true acquaintance with science, so
delightful to its possessors, is not to be acquired at second-hand.
Text-books and manuals are valuable only so far as they supplement
and direct our own observations. A man whose knowledge of Nature is
derived solely from these sources, differs about as much from one who
betakes himself to Nature herself, as a dusty, desiccated mummy does
from a living man. You have the same bones and sinews in both; but
in the one they are hard and dry, wholly incapable of action; in the
other they are instinct with freshness and life. He who would know what
physical science really is, must go out into the fields and learn it
for himself: and whatever branch he may choose, he will not be long in
discovering that a forenoon intelligently spent there must be deemed
of far more worth than days and weeks passed among books. He sees the
objects of his study with his own eyes, and not through "the spectacles
of books;" facts come home to him with a vividness and reality they
never can possess in the closet; the free buoyant air brightens his
spirits and invigorates his mind, and he returns again to his desk or
his workshop with a store of new health and pleasure and knowledge.
Geology is peculiarly rich in these advantages, and lies in a manner
open to all. No matter what may be the season of the year, it offers
always some material for observation. In the depth of winter we have
the effects of ice and frost to fall back upon, though the country
should lie buried in snow; and then when the longer and brighter
days of spring and summer come round, how easily may the hammer be
buckled round the waist, and the student emerge from the dust of town
into the joyous air of the country, for a few delightful hours among
the rocks; or when autumn returns with its long anticipated holidays,
and preparations are made for a scamper in some distant locality,
hammer and note-book will not occupy much room in the portmanteau,
and will certainly be found most entertaining company. The following
pages--forming a digest of the Carboniferous rocks--may, perhaps, in
some measure, guide the explorations of the observer, by indicating
to him the scope of geological research, the principles on which the
science rests, and the mode in which it is pursued. But I repeat, no
book, no lecture-room, no museum, will make a geologist of him. He must
away to the fields and study for himself, and the more he can learn
there he will become the better geologist.
He need not burden himself with accoutrements. A hammer, pretty stout
in its dimensions, with a round blunt face and a flat sharp tail; a
note-book and a good pocket-lens, are all he needs to begin with.
Having these, let him seek to learn the general characters' of the
more common rocks, aiding himself, where he can, by a comparison with
the specimens of a museum, or, failing that, with the descriptions of
a text-book. Let him then endeavour to become acquainted with some of
the more characteristic fossils of the district in which he resides,
so as to be able to recognise them wherever they occur. Private
collections and local museums are now becoming comparatively common,
and these, where accessible, will aid him vastly in his studies. Having
at length mastered the more abundant rocks and organic remains of his
neighbourhood, let him try to trace out the connexion of the different
strata across the country, so as to understand its structure. For
this purpose it will be necessary to examine every ravine and natural
exposure of the rocks, along with quarries, ditches, railway-cuttings,
and, in short, the whole surface of the district. A general notion
of the geology of the place will, not perhaps be of very difficult
attainment; and this done, the observer should attempt to put down
the connexion of the rocks on paper, for till this is accomplished he
will have at the best but an imperfect, and perhaps incorrect notion
of the subject. The best map of the district should be obtained, also
a clinometer, or instrument for ascertaining the angle at which rocks
_dip_ with the horizon, and a pocket-compass with which to mark the
direction of the _dip_ and _strike_ of strata, that is, the _outcrop_,
or line which they form when they come to the surface. Thus armed, he
may commence a geological survey of his neighbourhood. Wherever he
sees a bed of rock exposed, it should be marked down on his map with
an arrow pointing to the direction in which the stratum is dipping,
the angle of dip, ascertained by the clinometer, being put alongside.
The nature of the rock, whether sandstone, shale, limestone, or
greenstone, must be set down at the same place, and, to save room,
a system of marks for the different rocks may be conveniently used.
When a sufficient area of ground has been thus traversed, the student
may find, say a row of arrows on his map all pointing due west, and
indicating a set of quarries about a quarter of a mile distant from one
another, the rock in each of them dipping to the west. If there be at
the one end a limestone containing certain fossils, and at the other
end a stratum exactly similar, containing the same fossils, while the
quarries between display the same rock, he will infer, of course, that
the whole is one limestone, and will accordingly draw a line from the
last quarry on the north to the last on the south, connecting them all
together. If the bed dips steeply down, the line will be narrower,--if
but slightly inclined, it will be broader; the breadth of such a line
(which may be coloured to taste) always varying with the thickness
of the stratum and the angle which it makes with the horizon. In a
district where faults and curvatures along with trap-rocks abound, the
mapping becomes more complex, but the principle remains the same--a
curved stratum on the ground making a similarly curved band on the map,
and a fault or dislocation of a set of beds producing, in the same way,
a corresponding break in the lines traced. In short, a geological map
should be as far as possible a transcript of the surface rocks of a
country. The beginner should avoid, however, attempting too much; it
will be enough for him at first to have mastered the leading features
of the geology of his district; the details cannot be shown save on
a map of a large scale, and are better transferred to his note-book.
The use of such mapping is to enable us to gain a correct knowledge of
the geological structure of a country, and of the relation of rocks to
each other as regards age, origin, &c. Bacon tells us that "writing
makes an exact man;" we may say with equal truth that mapping makes
an exact geologist. It is sometimes easy enough to obtain a notion of
the general character of a district by taking a few rambles across it;
but we can never know it thoroughly until we have mapped it. And this
is done not as mere dry routine, or by a series of hard uninteresting
rules. In reading off the geological structure of a country, we
ascertain its history during many thousand ages long prior to that
of man. We become, as it were, interpreters of hieroglyphics, and
historians of long-perished dynasties.
Those who have had experience of field-geology, know how vain it is
to attempt to compress into a page or two the results of years, and
that a few vague general directions are about the utmost that can be
attempted. The practice of the science cannot be taught in books, far
less in prefaces, neither can it be learned from them. And so I once
more repeat the advice: Get away to the fields. Seek to decipher the
geological records for yourself, and look with your own eyes into the
long series of ages whose annals lie inscribed among the rocks. If you
can secure the co-operation of a few companions, so much the better.
Half-a-dozen hammers zealously at work in a richly fossiliferous
stratum will soon pile up a tolerable collection of its treasures. But
whether singly or in company, use your eyes and your hammer, and even
though in the end you should never become a geologist, you will in the
meantime gain health and vigour, and a clearness of observation, that
will stand you in good stead through life.
CONTENTS.
CHAPTER I.
PAGE
Scene near Colinton in midsummer--A grey travelled Boulder--Its
aspect and contents--Its story of the past,
1
CHAPTER II.
Exterior of the boulder--Travelled stones a difficult problem--Once
referred to the Deluge--Other theories--Novelty of the true
solution--Icebergs formed in three ways--Progress and scenery
of an iceberg--Its effects--Size of icebergs--Boulder clay
had a glacial origin--This explanation confirmed by fossil
shells--Laws of the distribution of life--Deductions,
6
CHAPTER III.
How the boulder came to be one--"Crag and tail"--Scenery of
central Scotland: Edinburgh--"Crag and tail" formerly
associated in its origin with the boulder-clay--This
explanation erroneous--Denudation an old process--Its
results--Illustration from the Mid-Lothian coal-field--The
three Ross-shire hills--The Hebrides relics of an
ancient land--Scenery of the western coast--Effects of
the breakers--Denudation of the Secondary strata of the
Hebrides--Preservative influence of trap-rocks--Lost
species of the Hebrides--Illustration--Origin of the
general denudation of the country--Illustrative action
of streams--Denudation a very slow process--Many old
land-surfaces may have been effaced--Varied aspect of the
British Islands during a period of submergence--Illustration,
18
CHAPTER IV.
Interior of the boulder--Wide intervals of
Geology--Illustration--Long interval between the
formation of the boulder as part of a sand-bed,
and its striation by glacial action--Sketch of the
intervening ages--The boulder a Lower Carboniferous
rock--Cycles of the astronomer and the geologist
contrasted--Illustration--Plants shown by the boulder once
grew green on land--Traces of that ancient land Its seas,
shores, forests, and lakes, all productive of material
aids to our comfort and power--Plants of the Carboniferous
era--Ferns--Tree-ferns--Calamites--Asterophyllites--
Lepidodendron--Lepidostrobus--Stigmaria--Scene in a ruined
palace--Sigillaria--Coniferæ, Cycadeæ--Antholites, the oldest
known flower--Grade of the Carboniferous flora--Its resemblance
to that of New Zealand,
30
CHAPTER V.
Scenery of the carboniferous forests--Contrast in the
appearance of coal districts at the present day--Abundance
of animal life in the Carboniferous era--Advantages
of palæontology over fossil-botany--Carboniferous
fauna--Actiniæ--Cup-corals--Architecture of the present
day might be improved by study of the architecture of the
Carboniferous period--Mode of propagation of corals--A
forenoon on the beach--Various stages in the decomposition of
shells--Sea-mat--Bryozoa--Fenestella--Retepora--Stone-lilies--
Popular superstitions--Structure of the stone-lilies--Aspect
of the sea-bottom on which the stone-lilies
flourished--Sea-urchins--Crustacea, their high
antiquity--Cyprides--Architecture of the Crustacea and
mollusca contrasted--King-crabs,
59
CHAPTER VI.
Carboniferous fauna continued--George Herbert's ode on
"Man"--His idea of creation--What nature teaches on
this subject--Molluscous animals--Range of species in
time proportionate to their distribution in space--Two
principles of renovation and decay exhibited alike in the
physical world and the world of life--Their effects--The
mollusca--Abundantly represented in the carboniferous
rocks--Pteropods--Brachiopods--Productus--Its alliance with
Spirifer--Spirifer--Terebratula--Lamellibranchs--Gastropods--
Land-snail of Nova Scotia--Cephalopods--Structure of
orthoceras--Habits of living nautilus,
86
CHAPTER VII.
Classification of the naturalist not always correspondent with
the order of nature--Incongruous grouping of animals
in the invertebrate division--Rudimentary skeleton
of the cephalopods--Introduction of the vertebrate
type into creation--Ichthyolites of the carboniferous
rocks--Their state of keeping--Classification of fossil
fishes--Placoids--Ichthyodorulites--Ganoids--Their structure
exemplified in the megalichthys and holoptychius--Cranium of
megalichthys--Its armature of scales--Microscopic structure of
a scale--Skeleton of megalichthys--History of the discovery
of the holoptychius--Confounded with megalichthys--External
ornament of holoptychius--Its jaws and teeth--Microscopic
structure of the teeth--Paucity of terrestrial fauna in coal
measures--Insect remains--Relics of reptiles--Concluding
summary of the characters of the Carboniferous fauna--Results,
110
CHAPTER VIII.
Sand and gravel of the boulder--What they suggested--Their
consideration leads us among the more mechanical operations
of Nature--An endless succession of mutations in the
economy of the universe--Exhibited in plants In animals--In
the action of winds and oceanic currents--Beautifully
shown by the ceaseless passage of water from land to
sea, and sea to land--This interchange not an isolated
phenomenon--How aided in its effects by a universal process
of decay going on wherever a land surface is exposed to
the air--Complex mode of Nature's operations--Interlacing
of different causes in the production of an apparently
single and simple effect--Decay of rocks--Chemical
changes--Underground and surface decomposition--Carbonated
springs--The Spar Cave--Action of rain-water--Decay of
granite--Scene in Skye--Trap-dykes--Weathered cliffs of
sandstone--Of conglomerate--Of shale--Of limestone--Caverns
of Raasay--Incident--Causes of this waste of calcareous
rocks--Tombstones,
138
CHAPTER IX.
Mechanical forces at work in the disintegration of
rocks--Rains--Landslips--Effects of frosts--Glaciers and
icebergs--Abrading power of rivers--Suggested volume on the
geology of rivers--Some of its probable contents--Scene
in a woody ravine--First idea of the origin of the ravine
one of primeval cataclysms--Proved to be incorrect--Love
of the marvellous long the bane of geology--More careful
examination shows the operations of Nature to be singularly
uniform and gradual--The doctrine of slow and gradual change
not less poetic than that of sudden paroxysms--The origin
of the ravine may be sought among some of the quieter
processes of Nature--Features of the ravine Lessons of the
waterfall--Course of the stream through level ground--True
history of the ravine--Waves and currents--What becomes of the
waste of the land--The Rhone and the Leman Lake--Deltas on the
sea-margin--Reproductive effects of currents and waves--Usual
belief in the stability of the land and the mutability of the
ocean--The reverse true--Continual interchange of land and
sea part of the economy of Nature--The continuance of such a
condition of things in future ages rendered probable by its
continuance during the past,
157
CHAPTER X.
The structure of the stratified part of the earth's
crust conveniently studied by the examination of a
single formation--A coal-field selected for this
purpose--Illustration of the principles necessary to such
an investigation--The antiquities of a country of value in
compiling its pre-historic annals--Geological antiquities
equally valuable and more satisfactorily arranged--Order
of superposition of stratified formations--Each formation
contains its own suite of organic remains--The age of
the boulder defined by this test from fossils--Each
formation as a rule shades into the adjacent ones--Mineral
substances chiefly composing the stratified rocks few
in number--Not of much value in themselves as a test of
age--The Mid-Lothian coal-basin--Its subdivisions--The
limestone of Burdiehouse--Its fossil remains--Its probable
origin--Carboniferous limestone series of Mid-Lothian--Its
relation to that of England--Its organic remains totally
different from those of Burdiehouse--Structure and
scenery of Roman Camp Hill--Its quarries of the mountain
limestone--Fossils of these quarries indicative of an ancient
ocean-bed--Origin of the limestones--Similar formations still
in progress--Coral-reefs and their calcareous silt--Sunset
among the old quarries of Roman Camp Hill,
178
CHAPTER XI.
Intercalation of coal seams among mountain limestone beds of
Mid-Lothian--North Greens seam--Most of our coal seams
indicate former land-surfaces--Origin of coal a debated
question--Erect fossil trees in coal-measures--Deductions to
be drawn therefrom--Difference between the mountain limestone
of Scotland and that of England--Coal-bearing character of the
northern series--Divisions of the Mid-Lothian coal-field--The
Edge coals--Their origin illustrated by the growth of modern
deltas--Delta of the Nile--Of the Mississippi--Of the
Ganges--Progress of formation of the Edge coals--Scenery
of the period like that of modern deltas--Calculations of
the time required for the growth of a coal-field--Why of
doubtful value--Roslyn Sandstone group--Affords proofs of
a general and more rapid subsidence beneath the sea--Its
great continuity--Probable origin--Flat coals--Similar
in origin to the Edge coals below--Their series not now
complete--Recapitulation of the general changes indicated by
the Mid-Lothian coal-field,
204
CHAPTER XII.
Trap-pebbles of the boulder--Thickness of the earth's crust
unknown--Not of much consequence to the practical
geologist--Interior of the earth in a highly heated
condition--Proofs of this--Granite and hypogene
rocks--Trap-rocks: their identity with lavas and
ashes--Scenery of a trappean country--Subdivisions of
the trap-rocks--Intrusive traps--Trap-dykes--Intrusive
sheets--Salisbury Crags--Traps of the neighbourhood of
Edinburgh--Amorphous masses--Contemporaneous trap-rocks of
two kinds--Contemporaneous melted rocks--Tests for their age
and origin--Examples from neighbourhood of Edinburgh--Tufas
or volcanic ashes--Their structure and origin--Example
of contemporaneous trap-rocks--Mode of interpreting
them--Volcanoes of Carboniferous times--Conclusion,
235
THE STORY OF A BOULDER.
CHAPTER I.
Scene near Colinton in midsummer--A grey travelled Boulder--Its
aspect and contents--Its story of the past.
Three miles to the south-west of Edinburgh, and not many hundred yards
from the sequestered village of Colinton, there is a ravine, overshaded
by a thick growth of beech and elm, and traversed beneath by a stream,
which, rising far away among the southern hills, winds through the
rich champaign country of Mid-Lothian. It is, at all seasons of the
year, one of the most picturesque nooks in the county. I have seen it
in the depth of winter--the leafless boughs doddered and dripping, the
rocks dank and bare save where half-hidden by the rotting herbage, and
the stream, red and swollen, roaring angrily down the glen, while the
families, located along its banks, fleeing in terror to the higher
grounds, had left their cottages to the mercy of the torrent. The last
time I visited the place was in the heart of June, and surely never did
woodland scene appear more exquisitely beautiful. The beech trees were
in full leaf, and shot their silvery boughs in slender arches athwart
the dell, intertwining with the broader foliage and deeper green of
the elm, and the still darker spray of the stately fir. The rocks on
either side were tapestried with verdure; festoons of ivy, with here
and there a thread of honey-suckle interwoven, hung gracefully from the
cliffs overhead; each projecting ledge had its tuft of harebells, or
speedwell, or dog-violets, with their blue flowers peeping out of the
moss and lichens; the herb-robert trailed its red blossoms over crag
and stone; the wood-sorrel nestled its bright leaves and pale flowerets
among the gnarled roots of beech and elm; while high over all, alike on
the rocks above and among the ferns below, towered the gently drooping
stalks of the fox-glove. The stream, almost gone, scarcely broke the
stillness with a low drowsy murmur, as it sauntered on among the
_lapides adesos_ of its pebbly channel. Horace's beautiful lines found
again their realization:--
"Qua pinus ingens, albaque populus
Umbram hospitalem consociare amant
Ramis, et obliquo laborat
Lympha fugax trepidare rivo." [1]
[Footnote 1:
Where the tall pine and poplar pale
Delight to cast athwart the vale
A pleasing shade.
While the clear stream low murmuring bells.
And o'er its winding channel toils
Adown the glade.--A. G.
]
It was noon, and the sun shone more brightly and with greater heat
than had been felt for years. The air, heavy and warm, induced a
feeling of listlessness and languor, and the day seemed one for which
the only appropriate employment would have been to read once again
the "Castle of Indolence." But failing that, I found it pleasant to
watch the flickering light shot in fitful gleams through the thick
canopy of leaves, and thus, in the coolness of the shade, to mark
these rays--sole messengers from the sweltering world around--as they
danced from rock to stream, now lighting up the ripples that curled
dreamily on, now chequering some huge boulder that lay smooth and
polished in mid-channel, anon glancing playfully among the thickets
of briar or honeysuckle and vanishing in the shade. Sometimes a
wagtail would alight at hand, or a bee drone lazily past, while even
an occasional butterfly would venture down into this shady covert.
But, with these exceptions, the animal creation seemed to have gone to
sleep, an example which it was somewhat difficult to avoid following.
While thus idly engaged, my eye rested on a large boulder on the
opposite side. It lay partly imbedded in a stiff clay, and partly
protruding from the surface of the bank some way above the stream.
A thick arbour of leafage overhung it, through which not even the
faintest ray of sunshine could force its way. The spot seemed cooler
and more picturesque than that which I occupied, and so, crossing the
well-nigh empty channel, I climbed the bank and was soon seated on the
boulder. A stout hammer is a constant companion in my rambles, and was
soon employed on this occasion in chipping almost unconsciously the
newly-acquired seat. The action was, perhaps, deserving of the satire
of Wordsworth's Solitary:--
"You may trace him oft
By scars, which his activity has left
Beside our roads and pathways, though, thank Heaven!
This covert nook reports not of his hand.
He, who with pocket-hammer smites the edge
Of luckless rock or prominent stone, disguised
In weather-stains, or crusted o'er by Nature
With her first growths, detaching by the stroke
A chip or splinter to resolve his doubts;
And, with that ready answer satisfied,
The substance classes by some barbarous name,
And hurries on; or from the fragments picks
His specimen; if but imply interveined
With sparkling mineral, or should crystal cube
Lurk in its cells and thinks himself enriched.
Wealthier, and doubtless wiser, than before!"
There was nothing in the distant aspect of the boulder to attract
attention. It was just such a mass as dozens of others all round. Nor,
on closer inspection, might anything peculiar have been observed. It
had an irregularly oblong form, about two or three feet long, and half
as high. Ferns and herbage were grouped around it, the wood-sorrel
clustered up its sides, and little patches of moss and lichen nestled
in its crevices. And yet, withal, there was something about it that,
ere long, riveted my attention. I examined it minutely from one end to
the other, and from top to bottom. The more I looked the more did I see
to interest me; and when, after a little labour, some portions of its
upper surface were detached, my curiosity was abundantly gratified.
That grey lichened stone, half hid among foliage, and unheeded by any
human being, afforded me material for a pleasant forenoon's thought.
Will my reader accept an expanded narrative of my reverie?
I can almost anticipate a smile. "What can there be remarkable in such
a grey stone, hidden in a wood, and of which nobody knows anything?
It never formed part of any ancient building; it marks the site of
no event in the olden time; it is linked with nothing in the history
of our country. What of interest, then, can it have for us?" Nay, I
reply, you are therein mistaken. It is, assuredly, linked with the
history of our country--it does mark the passing of many a historical
event long ere human history began; and, though no tool ever came upon
it, it did once form part of a building that rose under the finger of
the Almighty during the long ages of a bygone eternity. To change the
figure, this boulder seemed like a curious volume, regularly paged,
with a few extracts from older works. Bacon tells us that "some books
are to be tasted, others to be swallowed, and some few to be chewed and
digested." Of the last honour I think the boulder fully worthy, and if
the reader will accompany me, I shall endeavour to show him how the
process was attempted by me.
The rock consisted of a hard grey sandstone finely laminated above,
and getting pebbly and conglomeritic below. The included pebbles were
well worn, and belonged to various kinds of rock. The upper part of
the block was all rounded, smoothed, and deeply grooved, and, when
split open, displayed numerous stems and leaflets of plants converted
into a black coaly substance. These plants were easily recognisable
as well-known organisms of the carboniferous strata, and it became
accordingly evident that the boulder was a block of carboniferous
sandstone. The pebbles below, however, must have been derived from
more ancient rocks, and they were thus seen to represent some older
geological formation. In this grey rock, therefore, there could at
once be detected well-marked traces of at least two widely-separated
ages. The evidence for each was indubitable, and the chronology of
the whole mass could not be mistaken. The surface striation bore
undoubted evidence of the glacial period, the embedded plants as
plainly indicated the far more ancient era of the coal-measures, while
the pebbles of the base pointed, though dimly, to some still more
primeval age. I had here, as it were, a quaint, old, black-letter
volume of the middle ages, giving an account of events that were
taking place at the time it was written, and containing on its earlier
pages numerous quotations from authors of antiquity. The scratched
surface, to complete the simile, may be compared to this old work
done up in a modern binding. Let us, then, first of all, look for a
little at the exterior of the volume, and inquire into the origin of
that strangely-striated surface, and of the clay in which the boulder
rested.