OF THE PHENOMENA PECULIAR TO UNSTRATIFIED BODIES.
1. _Metallic Veins._
49. THE unstratified minerals exist either in veins, intersecting the
stratified, or in masses surrounded by them. Veins are of various
kinds, and may in general be defined, separations in the continuity of
a rock, of a determinate width, but extending indefinitely in length
and depth, and filled with mineral substances, different from the rock
itself. The mineral veins, strictly so called, are those filled with
crystallized substances, and containing the metallic ores.
That these veins are of a formation subsequent to the hardening and
consolidation of the strata which they traverse, is too obvious to
require any proof; and it is no less clear, from the crystallized
and sparry structure of the substances contained in them, that these
substances must have concreted from a fluid state. Now, that this
fluidity was simple, like that of fusion by heat, and not compound,
like that of solution in a menstruum, is inferred from many phenomena.
It is inferred from the acknowledged insolubility of the substances
that fill the veins, in any one menstruum whatsoever; from the total
disappearance of the solvent, if there was any; from the complete
filling up of the vein by the substances which that solvent had
deposited; from the entire absence of all the appearances of horizontal
or gradual deposition; and, lastly, from the existence of close
cavities, lined with crystals, and admitting no egress to any thing but
heat.
50. To the same effect may be mentioned those groups of crystals
composed of substances the most different, that are united in the same
specimen, all intersecting and mutually impressing one another. These
admit of being explained, on the supposition that they were originally
in fusion, and became solid by the loss of heat; a cause that acted
on them all alike, and alike impelled them to crystallize: But the
appearances of simultaneous crystallization seem incompatible with the
nature of deposition from a solvent, where, with respect 16 different
substances, the effects must take place slowly, and in succession.
51. The metals contained in the veins which we are now treating of,
appear very commonly in the form of an ore, mineralized by sulphur.
Their union with this latter substance can be produced, as we know, by
heat, but hardly by the way of solution in a menstruum, and certainly
not at all, if that menstruum is nothing else than water. The metals,
therefore, when mineralized by sulphur, give no countenance to the
hypothesis of aqueous solution; and still less do they give any when
they are found native, as it is called, that is, malleable, pure
and uncombined with any other substance. The great masses of native
iron found in Siberia and South America are well known; and nothing
certainly can less resemble the products of a chemical precipitation.
Gold, however, the most perfect of the metals, is found native most
frequently; the others more rarely, in proportion nearly to the
facility of their combination with sulphur. Of all such specimens it
may be safely affirmed, that if they have ever been fluid, or even
soft, they must have been so by the action of heat; for, to suppose
that a metal has been precipitated, pure and uncombined from any
menstruum, is to trespass against all analogy, and to maintain a
physical impossibility. But it is certain, that many of the native
metals have once been in a state of softness, because they bear on them
impressions which they could not have received but when they were soft.
Thus, gold is often impressed by quartz and other stones, which still
adhere to it, or are involved in it. Specimens of quartz, containing
gold and silver shooting through them, with the most beautiful and
varied ramifications, are every where to be met with in the cabinets of
the curious; and contain, in their structure, the clearest proof, that
the metal and the quartz have been both soft, and have crystallized
together. By the compactness, also, of the body which they form, they
show, that when they acquired solidity, it was by the concretion of the
whole mass, and not by such partial concretion as takes place when a
solvent is separated from substances which it held in solution.
52. Native copper is very abundant; and some specimens of it have been
found crystallized. Here the crystallization of the metal is a proof
that it has passed from a fluid to a solid state; and its purity is a
proof that it did not make that transition by being precipitated from a
menstruum.
53. Again, pieces of native manganese have been found possessing so
exactly the characters peculiar to that metal when reduced in our
furnaces, that it is impossible to consider them as deriving their
figure and solidity from any cause but fusion. The ingenious author
who describes these specimens, La Peyrouse, was so forcibly struck
with this resemblance, that he immediately drew the same conclusion
from it which is drawn here, attributing the only difference, which
he remarked between the native and the artificial _regulus_, to the
different energy with which the same agent works when employed by
nature and by art.[17]
[Footnote 17: Theory of the Earth, vol. i. p. 68. Journal de Phys.
Janvier, 1786.]
54. All these appearances conspire to prove, that the materials which
fill the mineral veins were melted by heat, and forcibly injected, in
that state, into the clefts and fissures of the strata. These fissures
we must conceive to have arisen, not merely from the shrinking of the
strata while they acquired hardness and solidity, but from the violence
done to them, when they were heaved up and elevated in the manner which
has already been explained.[18]
[Footnote 18: NOTE XIII.]
55. When these suppositions are once admitted, the other leading facts
in the history of metallic veins will be readily accounted for. Thus,
for instance, it is evident to what we must ascribe the fragments of
the surrounding rock that are often found immersed in the veins, and
encompassed on all sides by crystallized substances. These fragments
being no doubt detached by the concussion, which at once tore asunder
and elevated the strata, were sustained by the melted matter that
flowed at the same time upward through the vein. Large masses of rock
are often found in this manner completely insulated; one of these,
which M. De Luc has described with great accuracy, is no less than a
vast segment of a mountain.[19]
[Footnote 19: Lettres Physiques, &c. tom. iii. P. 361.]
56. The immense violence which has accompanied the formation of mineral
veins, is particularly marked by the slips and shifts of the strata on
each side of them all tending to show what mighty changes have taken
place in those regions, which our imagination erroneously paints as
the abode of everlasting silence and rest. This shifting of the strata
is best observed, where the veins make a transverse section of beds of
rock, considerably inclined to the horizon. There it is common to see
the beds on one side of the vein slipped along from the corresponding
beds on the other side, and removed sometimes in a horizontal,
sometimes in an oblique direction. In this way, not only the strata
are shifted, but veins, which intersect one another, are also shifted
themselves. They are _heaved_, as it is called in the significant
language of the miners, and forced out of their direction. It is
impossible, in such a case, but to connect in the mind the formation
of the vein, and the production of the slips which accompany it, and to
regard them as parts of the same phenomenon.
57. Where these slips are horizontal, and exhibit great bodies of
strata carried from their place, while the parts of the transferred
mass remain undisturbed relatively to one another, they furnish a dear
proof, that this change of plaice has not arisen from the falling in
of the roofs of caverns, as some geologists suppose. The horizontal
direction, and the regularity of the movement, are incompatible with
the action of such a cause as this; and indeed it is highly interesting
to remark, in the midst of the signs of disturbance which prevail in
the bowels of the earth, that there reigns a certain symmetry and
order, which indicate the action of a force of incredible magnitude,
but slow and gradual in its effects. The parts of the mass moved are
undisturbed relatively to one another; what has been broken has been
cemented; the breaches of continuity have been filled up and healed;
and every where we see the operation of a cause that could unite as
well as separate. The twofold action of heat to expand and to melt,
could scarce be pointed out more clearly by any system of appearances.
58. As a long period was no doubt required for the elevation of the
strata, the rents made in them are not all of the same date, nor the
veins all of the same formation. This is clear in the case of one vein
producing a shift or slip in another; for the vein which forces the
other out of its place, and preserves its own direction, is evidently
the more recent of the two, and must have had its materials in a
state of activity, when those of the other were inert. Sometimes,
also, at the intersection of two veins, we may trace the current of
the materials of the one, across those of the other; and here, of
consequence, the relative antiquity is determined just as in the former
instance.
59. The want of any appearance of stratification in mineral veins has
already been taken notice of. There is, however, to be observed, in
many instances, a tendency to a regular arrangement of the substances
contained in them; those of the same kind forming coats parallel to the
sides of the vein, and nearly of an equal thickness. This phenomenon
is considered as one of the strongest arguments in favour of the
Neptunian system, but has nothing in it, in the least incompatible
with that theory which ascribes the formation of veins to the action
of subterraneous heat. When melted matter from the mineral regions was
thrown up into the veins, that which was nearest to the sides would
soonest lose its heat. The similar substances, also, would unite while
this process was going forward, and would crystallize, as in other
cases of congelation, from the sides toward the interior. There is the
more reason for supposing this to have been the case, that the same
sort of coating is often observed on the inside of close cavities,
which are, nevertheless, so constructed, as to afford a demonstration
that no chemical solvent was ever included in them, (§ 74.) Some
veins, it must also be considered, may have been filled by successive
injections of melted matter, and this would naturally give rise to a
variety of separate incrustations.[20]
[Footnote 20: See some farther remarks on this subject at NOTE XIII.]
60. In the view now given of metallic veins, they have been considered
as traversing only the stratified parts of the globe. They do,
however, occasionally intersect the unstratified parts, particularly
the granite, the same vein often continuing its course across rocks
of both kinds, without suffering any material change; and, if we have
hitherto paid no attention to this circumstance, it is because the
order pursued in this essay required, that the relation of the veins
to stratified bodies should be first treated of. Besides, the facts
in the natural history of veins, whether contained in stratified or
unstratified rocks, are so nearly alike, that in a general view of
geology, they do not require to be distinguished. It is material to
remark, that, though metallic veins are found indiscriminately in all
the different kinds of rock, whether stratified or otherwise, they are
most abundant in the class of primary schisti. All the countries most
remarkable for their mines, and the mountains distinguished by the
name of metalliferous, are primary, and the instance of Derbyshire is
perhaps the most considerable exception to this rule that is known.
This preference, which the metals appear to give to the primary strata,
is very consistent with Dr Hutton's theory, which represents the rocks
of that order as being most changed from their original position, and
those on which the disturbing forces of the subterraneous regions have
acted most frequently, and with greatest energy. The primary strata are
the lowest, also, and have the most direct communication with those
regions from which the mineral veins derive all their riches.
2. _Of Whinstone._
61. Beside the veins filled with spar, and containing the metallic
ores, the strata are intersected by veins of whinstone, porphyry, and
granite, the characters of which are next to be examined.
The term _whin_, or _whinstone_, with Dr. Hutton, like the word _trap_,
with the German mineralogists, denotes a class of stones, comprehending
several distinct species, or at least varieties. The common _basalt_,
the _wacken_, _mullen_, and _crag_ of Kirwan, the _grûnstein_ of
Werner, and the _amygdaloid_, are comprehended under the name of whin.
All these stones have a tendency to a spathose structure, and discover
at least the rudiments of crystallization. They are, at the same time,
without any mark of stratification in their internal texture, as they
are also, for the most part, in their outward configuration; and, as
the different species here enumerated compose, not unfrequently, parts
of the same continuous rock, the change from one to another being made
through a series of insensible gradations, they may safely be regarded
by the geologist as belonging to the same _genus_.
62. Whin, though not stratified, exists in two different ways, that is,
either in veins, (called in Scotland _dykes_,) traversing the strata
like the veins already described, or in irregular masses, incumbent
on the strata, and sometimes interposed between them. In both these
forms, whinstone has nearly the same characters, and bears, in all its
varieties, a most striking resemblance to the lavas which have actually
flowed from volcanoes on the surface of the earth. This resemblance
is so great, that the two substances have been often mistaken for one
another; and many rocks, which have been pronounced to be the remains
of extinguished volcanoes, by mineralogists of no inconsiderable
name, have been found, on closer examination, to be nothing else than
masses or veins of whinstone. This latter stone is indeed only to be
distinguished from the former, by a careful examination of the internal
characters of both; and chiefly from this circumstance, that whinstone
often contains calcareous spar and zeolite, whereas neither of these
substances is found in such lavas, as are certainly known to have been
thrown out by volcanic explosions.
Now, from these circumstances of affinity between lava and whinstone,
on the one hand, and of diversity on the other, as the formation of
the one is known, it should seem that some probable conclusion may be
drawn concerning the formation of the other. The affinity in question
is constant and essential; the difference variable and accidental;
and this naturally leads to suspect, that the two stones have the
same origin; and that, as lava is certainly a production of fire, so
probably is whinstone.
63. But, in order to see whether this hypothesis will explain the
diversity of the two substances, without which it will not be entitled
to much attention, we must remark, that the presence of carbonate of
lime in a body that has been fused, argues, agreeably to the principles
formerly explained, that the fusion was brought about under a great
compressing force, that is to say, deep in the bowels of the earth, or
in the great laboratory of the mineral regions. We are, therefore, to
suppose that the fusion of the whin was performed in those regions,
where the compression was sufficient to preserve the carbonic gas in
union with the calcareous earth, so that these two substances melted
together, and, on cooling, crystallized into spar. In the lavas, again,
thrown out by volcanic eruption, the fusion, as we know, wherever it
may begin, continues in the open air, where the pressure is only that
of the atmosphere: the calcareous earth, which, therefore, may have
been, in the form of a carbonate, among the materials of this lava,
must be converted into quicklime, and become infusible; hence the want
of calcareous spar in lavas that have flowed at the surface.
Thus, whinstone is to be accounted a subterraneous, or _unerupted_
lava; and our theory has the advantage of explaining both the affinity
and the difference between these stony bodies, without the introduction
of any new hypothesis. In the Neptunian system, the affinity of
whinstone and lava is a paradox which admits of no solution.
64. The columnar structure sometimes found in that species of whinstone
called basaltes, is a fact which has given rise to much discussion;
and it must be confessed, that though one of the most striking and
peculiar characters of this fossil, it is not that which gives the
clearest and most direct information concerning its origin. One
circumstance, however, very much in favour of the opinion that basaltic
rocks owe their formation to fire, is, that the columnar form is
sometimes assumed by the lava actually erupted from volcanoes. Now, it
is certainly of no small importance, to have the synthetic argument
on our side, and to know, that basaltic columns can be produced by
fire; though, no doubt, to give absolute certainty to our conclusion,
it would be necessary to show, that there are in nature no other
means but this by which these columns can be formed. This sort of
evidence is hardly to be looked for; but since the power of fusion,
to produce the phenomena in question, is perfectly established, and
since the production of the same phenomena in the humid way is a mere
hypothesis, if there be the least reason to suspect the action of
subterraneous heat as one of the causes of mineralization, every maxim
of sound philosophy requires that the basaltic structure, in all cases,
should be ascribed to it.
65. The Neptunists will no doubt allege, with BERGMAN, that, in the
drying of starch, clay, and a few other substances, something analogous
to basaltic columns is produced. Here, however, a most important
difference is to be remarked, corresponding very exactly to one of the
characters which we have all along observed to distinguish the products
of aqueous, from those of igneous consolidation. The columns formed
by the substances just mentioned, are distant from one another: they
are separated by fissures which widen from the bottom to the top, and
which arise from the shrinking and drying of the mass. In the basaltic
columns, no such openings, nor vacuity of any kind is found; the
pillars are in contact, and, though perfectly distinct, are so close,
that the sharp edge of a wedge can hardly be introduced between them.
This is a great peculiarity in the basaltic structure, and is strongly
expressive of this fact, that the mass was all fluid together, and
that its parts took their new arrangement, not in consequence of the
separation of a fluid from a solid part, by which great shrinking and
much empty space might be produced; but in consequence of a cause
which, like refrigeration, acted equally on all the parts of the mass,
and preserved their absolute contact after their fluidity had ceased.
66. A mark of fusion, or at least of the operation of heat, which
whinstone possesses in common with many other minerals, is its being
penetrated by pyrites, a substance, as has been already remarked, that
is of all others most exclusively the production of fire. Another
mark of fusion, more distinctive of whin, is, that both in veins and
in masses it sometimes includes pieces of sandstone, or of the other
contiguous strata, completely insulated, and having the appearance
of fragments of rock, floating in a fluid sufficiently dense and
ponderous to sustain their weight. Though these fragments have been
too refractory to be reduced into fusion themselves, they have not
remained entirely unchanged, but are, in general, extremely indurated,
in comparison of the rock from which they appear to have been detached.
67. Similar instances of extraordinary induration are observed in the
parts of the strata in contact with whinstone, whether they form the
sides of the veins, or the floors, and roofs of the masses into which
the whinstone is distributed. The strata whether sandy or argillaceous,
in such situations, are usually extremely hard and consolidated; the
former in particular lose their granulated texture, and are sometimes
converted into perfect jasper. This interesting remark was first made
by Dr Hutton, and the truth of it has been verified by a great number
of subsequent observations.
68. To the same excellent geologist we are indebted for the knowledge
of an analogous fact, attendant on the passage of whinstone veins
through coal strata. As the beds of stone where they are in contact
with veins of whin, seem to acquire additional induration, so those
of coal, in like circumstances, are frequently found to have lost
their fusibility, and to be reduced nearly to the condition of coke,
or of charcoal. The existence of coal of this kind has been already
mentioned, and considered as a proof of the operation of subterraneous
heat. In the instances here referred to, that is, where the charring
of the coal is limited to those parts of the strata which are in
contact with the whin, or in its immediate vicinity, the heat is
pointed out as residing in the vein; and this is to be accounted for
only on the supposition of the melted whin, at a period subsequent to
the consolidation of the coal, having flowed through the openings of
the strata. The heat has been powerful enough, in many cases, to drive
off the bituminous matter of the coal, and to force it into colder and
more distant parts. Few facts, in the history of fossils, are more
remarkable than this, and none more directly assimilates the operations
of the mineral regions, with those that take place at the surface of
the earth.
69. Again, the disturbance of the strata, wherever veins of whinstone
abound, if not a direct proof of the original fluidity of the
whinstone, is a clear indication of the violence with which it was
introduced into its place. This disturbance of the position of the
strata, by shifting, unusual elevation, and other irregularities, where
they are intersected by whinstone veins, is a fact so well known to
miners, that when they meet with any sudden change in the lying of the
_metals_, they are wont to foretell their approach to masses, or veins
of unstratified matter; and, in their figurative language, point them
out as the causes of the confusion with which they are so generally
accompanied.[21] The mineral veins likewise, as well as the strata, are
often heaved and shifted by the veins of whinstone.
[Footnote 21: A _Trouble_ is the name which the colliers in this
country give to a vein of whinstone.]
70. Whinstone of every species is found frequently interposed in
tabular masses, between beds of stratified rocks; and it then adds to
the indications of its igneous origin, already enumerated, some others
that are peculiar to it when in this situation. In such instances, it
is not uncommon to find the strata in some places, contiguous to the
whin, elevated, and bent with their concavity upward, so that they
appear clearly to have been acted on by a force that proceeded from
below, at the same time that they were softened, and rendered in some
degree flexible: it is needless to remark, that these effects can be
explained by nothing but the fusion of the whin; and that the great
force with which it was impelled against the strata, could be produced
by no cause but heat, acting in the manner that is here supposed.
71. Again, if it be true that the masses of whin, thus interposed
among the strata, were introduced there, after the formation of the
latter, we might expect to find, at least in many instances, that the
beds on which the whinstone rests, and those by which it is covered,
are exactly alike. If these beds were once contiguous, and have been
only heaved up and separated by the irruption of a fluid mass of
subterraneous lava, their identity should still be recognised. Now,
this is precisely what is observed; it is known to hold in a vast
number of instances, and is strikingly exemplified in the rock of
_Salisbury Crag_, near Edinburgh.
This similarity of the strata that cover the masses of whinstone,
to those that serve as the base on which they rest, and again the
dissimilitude of both to the interposed mass, are facts which I think
can hardly receive any explanation, on the principles of the Neptunian
theory. If these rocks, both stratified and unstratified, are to be
regarded as productions of the sea, the circumstances would require to
be pointed out, which have determined the whinstone, and the beds that
are all round it, to be so extremely unlike in their structure, though
formed at the same time, and in the immediate vicinity of one another;
as also those circumstances, on the other hand, which determined the
stratified deposits above and below the whinstone, to be precisely
the same, though the times of their formation must have been very
different. The homogeneous substances, thus, placed at a distance, and
the heterogeneous brought so closely together, are phenomena equally
unaccountable, in a theory that ascribes their origin to the operation
of the same element, and that necessarily dates their formation
according to the order in which they lie, one above another.
72. If, indeed, in these instances, the gradation were insensible, as
some have asserted it to be, between the strata and the interposed
mass, so that it was impossible to point out the line where the one
ended and the other began, whatever difficulties we might perceive in
the Neptunian theory, we should find it hard to substitute a better
in its room. But the truth seems to be, that, in the cases we are now
treating of, no such gradation exists; and that, though where the two
kinds of rock come into contact a change is often observed, by the
strata having acquired an additional degree of induration, yet the line
of separation is well defined, and can be precisely ascertained. This
at least is certain, that innumerable specimens, exhibiting such lines
of separation, are to be met with; and wherever care has been taken
to obtain a fresh fracture of the stone, and to remove the effects of
accidental causes, even where the two rocks are most firmly united,
and most closely assimilated, I am persuaded that no uncertainty has
ever remained as to the line of their separation. For these reasons,
it seems probable that the gradual transition of basaltes into the
adjoining strata, is in all cases imaginary, and is, in truth, a mere
illusion, proceeding from hasty and inaccurate observation.
73. Another remarkable fact in the natural history of the whinstone
rocks, remains yet to be mentioned, and with it I shall conclude the
argument, as far as these rocks are concerned.
Some of the species of whinstone are the common matrices of agates and
chalcedonies, which lie inclosed in them in the form of round nodules.
The original fluidity of these nodules is evinced by their figured, and
sometimes crystallized structure, and indeed is so generally admitted,
that the only question concerning them is, whether this fluidity
was the effect of heat or of solution. To answer this question, Dr
Hutton observes, that the formation of the concentric coats, of which
the agate is usually composed, has evidently proceeded from the
circumference toward the centre, the exterior coats always impressing
the interior, but never the reverse. The same thing also follows from
this other fact, that when there is any vacuity within the agate, it is
usually at the centre, and there too are found the regular crystals,
when any such have been formed. It therefore appears certain, that the
progress of consolidation has been from the circumference inwards, and
that the outward coats of the agate were the first to acquire solidity
and hardness.
74. Now, it must be considered that these coats are highly
consolidated; that they are of very pure siliceous matter, and are
utterly impervious to every substance which we know of, except light
and heat. It is plain, therefore, that whatever at any time, during
the progress of consolidation, was contained within the coats already
formed, must have remained there as long as the agate was entire,
without the least possibility of escape. But nothing is found within
the coats of the agate save its own substance; therefore, no extraneous
substance, that is to say no solvent, was ever included within them.
The fluidity of the agate was therefore simple, and unassisted by any
menstruum.
In this argument, nothing appears to me wanting, that is necessary to
the perfection of a physical, I had almost said of a mathematical,
demonstration. It seems, indeed, to be impossible that the igneous
origin of fossils could be recorded in plainer language, than by the
phenomenon which has just been described.
75. The examination of particular specimens of agates and chalcedonies,
affords many more arguments of the same kind, which Dr Hutton used to
deduce with an acuteness and vivacity, which his friends have often
listened to with great admiration and delight.[22] These, however,
must be passed over at present; and I have only further to remark,
that a series of the most interesting experiments, instituted by Sir
JAMES HALL, and published in the Transactions of the Royal Society of
Edinburgh,[23] has removed the only remaining objection that could
be urged against the igneous origin of whinstone. This objection is
founded on the common observation, that when a piece of whinstone or
basaltes is actually melted in a crucible, on cooling, it becomes
glass, and loses its original character entirely; and from thence it
was concluded, that this character had not been originally produced
by fusion. The experiments above mentioned, however, have shown, in
the most satisfactory manner, that melted whin, by _regulated_ or by
slow cooling, is prevented from assuming the appearance of glass, and
becomes a stony substance, hardly to be distinguished from whinstone or
lava.
[Footnote 22: NOTE XVI.]
[Footnote 23: vol. v. p. 43.]
The experiments of another ingenious chemist, Dr KENNEDY, have shown,
that whinstone contains mineral alkali, by which, of course, its fusion
must have been assisted.[24] Dr Hutton used to ascribe its fusibility,
in a great measure at least, to the quantity of iron contained in it:
both these causes have no doubt united to render it more easily melted
than the ordinary materials of the strata.
[Footnote 24: Trans. R. S. Edin. vol. v. p. 85.]
76. In a word, therefore, to conceive aright the origin of that class
of unstratified rocks, distinguished by the name of whinstone, we must
suppose, that long after the consolidation of the strata, and during
the time of their elevation, the materials of the former were melted
by the force of subterraneous heat, and injected among the rents and
fissures of the rocks already formed. In this manner were produced
the veins or dikes of whinstone; and, where circumstances allowed
the stream of melted matter to diffuse itself more widely, tabular
masses were formed, which were afterwards raised up, together with the
surrounding strata, above the level of the sea, and have been since
laid open by the operation of those causes that continually change and
waste the surface of the land.
These unstratified rocks are not, however, all the work of the same
period; they differ evidently in the date of their formation, and it is
not unusual, to find tabular masses of one species of whin, intersected
by veins of another species. Indeed, of all the fossil bodies which
compose the present land, the veins of whin appear to be the most
recently consolidated.[25]
[Footnote 25: NOTE XIV.]
Porphyry may so properly be regarded as a variety of whin,
distinguished only by involving crystallized feldspar, that, in a
geological sketch like the present, it is hardly entitled to a separate
article. Like the other kinds of whin, it exists both in veins and
in tabular masses, having, no doubt, an origin similar to that which
has just been described. Porphyry, however, has the peculiarity of
being rarely found in any but the primary strata; it seems to be the
whinstone of the old world, or at least that which is of the highest
antiquity in the present. It no where, I believe, assumes a columnar,
or basaltic appearance, of any regularity; but this is also true of
many other varieties of whin, of all, indeed, except the most compact
and homogeneous. These differences are not so considerable as to
require our entering into any particular detail concerning the natural
history of this fossil.
3. _Granite_.
77. The term Granite is used by Dr Hutton to signify an aggregate
stone, in which quartz, feldspar, and mica are found distinct from
one another, and not disposed in layers. The addition of hornblende,
schorl, or garnet, to the three ingredients just mentioned, is not
understood to alter the _genus_ of the stone, but only to constitute a
specific difference, which it is the business of lithology to mark by
some appropriate character, annexed to the generic name of granite.
The fossil now defined exists, like whinstone and porphyry, both in
masses and in veins, though most frequently in the former. It is like
them unstratified in its texture, and is regarded here, as being also
unstratified in its outward structure.[26] One ingredient which is
essential to granite, namely, quartz, is not contained in whinstone;
and this circumstance serves to distinguish these _genera_ from one
another, though, in other respects, they seem to be united by a chain
of insensible gradations, from the most homogeneous basaltes, to
granite the most highly crystallized.
[Footnote 26: Those rocks that consist of the ingredients here
enumerated, if they have at the same time a schistose texture, or a
disposition into layers, are properly distinguished from granite, and
called Gneiss, or Granitic Schistus. But it has been questioned whether
a stone does not exist composed of these ingredients, and destitute
of a schistose texture, but yet divided into large beds, visible in
its external form. Dr Hutton supposes such a stone not to exist, or at
least not to constitute any such proportion of the mineral kingdom, as
to entitle it to particular consideration, in the general speculations
of geology.
Whether this supposition is perfectly correct, may require to be
farther considered: this, however, is certain, that a rock, in all
respects conformable to it, composes a great proportion of what are
usually called the granite mountains. See NOTE XV.]
78. Granite, it has been just said, exists most commonly in masses; and
these masses are rarely, if ever, incumbent on any other rock: they are
the basis on which others rest, and seem, for the most part, to rise
up from under the ancient, or primary strata. The granite, therefore,
wherever it is found, is inferior to every other rock; and as it also
composes many of the greatest mountains, it has the peculiarity of
being elevated the highest into the atmosphere, and sunk the deepest
under the surface, of all the mineral substances with which we are
acquainted.
Notwithstanding the circumstance of not being alternated with
stratified bodies, which constitutes a remarkable difference between
granite and whinstone, the affinity of these fossils is such as to make
the similarity of their origin by no means improbable. Accordingly,
in Dr Hutton's theory, granite is regarded as a stone of more recent
formation than the strata incumbent on it; as a substance which has
been melted by heat, and which, when forced up from the mineral
regions, has elevated the strata at the same time.
79. That granite has undergone a change from a fluid to a solid
state, is evinced from the crystallized structure in which some of
its component parts are usually found. This crystallization is
particularly to be remarked of the feldspar, and also of the schorl,
where there is any admixture of that substance, whether in slender
spiculæ, or in larger masses. The quartz itself is in some cases
crystallized, and is so, perhaps, more frequently than is generally
supposed. The fluidity of granite, in some former period of its
existence, is so evident from this, as to make it appear singular that
it should ever have been considered as a fossil that had remained
always the same, and one, into the origin of which it was needless to
inquire. If the regular forms of crystallization are not to be received
as proofs of the substance to which they belong having passed from
a fluid to a solid state, neither are the figures of shells and of
other supposed petrifactions, to be taken as indications of a passage
from the animal to the mineral kingdom; so that there is an end of
all geological theories, and of all reasonings concerning the ancient
condition of the globe. To an argument which strikes equally at the
root of all theories, it belongs not to this, in particular, to make
any reply.
80. We shall, therefore, consider it as admitted, that the materials
of the granite were originally fluid; and, in addition to this, we
think it can easily be proved, that this fluidity was not that of the
elements taken separately, but of the entire mass. This last conclusion
follows, from the structure of those specimens, where one of the
substances is impressed by the forms which are peculiar to another.
Thus, in the Portsoy granite,[27] which Dr Hutton has so minutely
described, the quartz is impressed by the rhomboidal crystals of the
feldspar, and the stone thus formed is compact and highly consolidated.
Hence, this granite is not a congeries of parts, which, after being
separately formed, were somehow brought together and agglutinated; but
it is certain that the quartz, at least, was fluid when it was moulded
on the feldspar. In other granites, the impressions of the substances
on one another are observed in a different order, and the quartz gives
its form to the feldspar. This, however, is more unusual; the quartz
is commonly the substance which has received the impressions of all
the rest; and the spiculæ of schorl often shoot both across it and the
feldspar.
[Footnote 27: Theory of the Earth, vol. i. p. 104.]
The ingredients of granite were therefore fluid when mixed, or at least
when in contact with one another. Now, this fluidity was not the effect
of solution in a menstruum; for, in that case, one kind of crystal
ought not to impress another, but each of them should have its own
peculiar shape.
81. The perfect consolidation of many granites, furnishes an argument
to the same effect. For, agreeably to what was already observed, in
treating of the strata, a substance, when crystallizing, or passing
from a fluid to a solid state, cannot be free from porosity, much
less fill up completely a space of a given form, if, at the same
time, any solvent is separated from it; because the solvent so
separated would still occupy a certain space, and, when removed by
evaporation or otherwise, would leave that space empty. The perfect
adjustment, therefore, of the shape of one set of crystallizing bodies,
to the shape of another set, as in the Portsoy granite, and their
consolidation into one mass, is as strong a proof as could be desired,
that they crystallized from a state of simple fluidity, such as, of all
known causes, heat alone is able to produce.
82. This conclusion, however, does not rest on a single class of
facts. It has been observed in many instances, that where granite and
stratified rocks, such as primary schistus, are in contact, the latter
are penetrated by veins of the former, which traverse them in various
directions. These veins are of different dimensions, some being of the
breadth of several yards, others of a few inches, or even tenths of an
inch; they diminish as they recede from the main body of the granite,
to which they are always firmly united, constituting, indeed, a part of
the same continued rock.
These phenomena, which were first distinctly observed by Dr Hutton, are
of great importance in geology, and afford a clear solution of the two
chief questions concerning the relation between granite and schistus.
As every vein must be of a date posterior to the body in which it is
contained, it follows, that the schistus was not super-imposed on
the granite, after the formation of this last. If it be argued, that
these veins, though posterior to the schisti, are also posterior to
the granite, and were formed by the infiltration of water in which the
granite was dissolved or suspended; it may be replied, _1mo_, That the
power of water to dissolve granite, is a postulatum of the same kind
that we have so often, and for such good reason, refused to concede;
and, _2do_, That in many instances the veins proceed from the main
body of the granite _upwards_ into the schistus; so that they are in
planes much elevated in respect of the horizon, and have a direction
quite opposite to that which the hypothesis of infiltration requires.
It remains certain, therefore, that the whole mass of granite, and the
veins proceeding from it, are coeval, and both of later formation than
the strata.
Now, this being established, and the fluidity of the veins, when they
penetrated into the schistus, being obvious, it necessarily follows,
that the whole granite mass was also fluid at the same time. But this
can have been brought about only by subterraneous heat, which also
impelled the melted matter against the superincumbent strata, with such
force as to raise them from their place, and to give them that highly
inclined position in which they are still supported by the granite,
after its fluidity has ceased. Thus a conclusion, rendered probable by
the crystallization of granite, is established beyond all contradiction
by the phenomena of granitic veins.[28]
[Footnote 28: NOTE XV.]
83. With the granite, we shall consider the proof of the igneous origin
of all mineral substances as completed. These substances, therefore,
whether stratified or unstratified, owe their consolidation to the same
cause, though acting with different degrees of energy. The stratified
have been in general only softened or penetrated by melted matter,
whereas the unstratified have been reduced into perfect fusion.
84. In this general conclusion we may distinguish two parts, which, in
their degree of certainty, differ perhaps somewhat from one another.
The first of these, and that which stands highest in point of evidence,
consists of two propositions; namely, that the fluidity which preceded
the consolidation of mineral substances was SIMPLE, that is, it did not
arise from the combination of these substances with any solvent; and,
next, that after consolidation, these bodies have been raised up by an
expansive force acting from below, and have by that means been brought
into their present situation. These two propositions seem to me to be
supported by all the evidence that is necessary to constitute the most
perfect demonstration.
85. The other part of the general conclusion, that fire, or more
properly heat, was the cause of the fluidity of these mineral bodies,
and also of their subsequent elevation, is not perhaps to be considered
as a truth so fully demonstrated as the two preceding propositions;
it is, no doubt, a matter of THEORY; or a portion of one of those
invisible chains by which men seek to connect in the mind the state
of nature that is present, with the states of it that are past; and
participates of that uncertainty from which our reasonings concerning
such causes as are not direct objects of perception, are hardly ever
exempted. That it participates of this uncertainty in a very slight
degree, will, however, be admitted, when it is considered that the
cause assigned has been proved sufficient for the effect; that the same
is not true of any other known cause; and that this theory accounts,
with singular simplicity and precision, for a system of facts so
various and complex, as that which is presented by the natural history
of the globe.
86. Neither can it be said that the existence of subterraneous heat is
a principle assumed without any evidence, but that of the geological
facts which it is intended to explain: on the contrary, it is proved by
phenomena within the circle of ordinary experience, namely, those of
hot springs, volcanoes, and earthquakes. These leave no doubt of the
existence of heat, and of a moving and expansive power, in the bowels
of the earth; so that the only questions are, at what depth is this
power lodged? to what extent, and with what intensity, does it act?
That it is lodged at a very considerable depth, is rendered probable by
the permanency of some of the preceding phenomena: from the earliest
times many fountains have retained their heat to the present day; and
volcanoes, though they become extinguished at length, have a very
long period allotted for their duration. The cause of earthquakes is
certainly a force that resides very deep under the surface, otherwise
the extent of the concussion could not be such as has been observed in
many instances.
87. The intensity of volcanic fire, is another circumstance that
favours the opinion of its being seated deep under the surface. That
this intensity is considerable, is certain from the experiments made
by Sir James Hall on the fusibility of whinstone and lava; from which
it appears, that the lowest temperature in which either of these
stones melt, is about 80° of Wedgewood's pyrometer. Some mineralogists
have indeed affirmed, that lava is melted, not by the intensity of
the heat applied to it, but in consequence of a certain combination
formed between it and bituminous substances, in a manner which they do
not attempt to explain, and which has indeed no analogy to any thing
that is known. That a hypothesis, formed in such direct opposition to
the most obvious principles of inductive reasoning, should have been
imagined by a philosopher who had examined the phenomena of Etna and
Vesuvius with much attention, and described them with great accuracy
and truth, is more wonderful than that it should have been adopted by
mineralogists, whose views of nature may have been confined within a
cabinet or a laboratory. It is, however, a hypothesis, which, having
never had any support but from other hypotheses, hardly merited the
direct refutation that it has received from the experiments just
mentioned.
88. But, if the intensity of volcanic heat be such as is here
stated, it will be found very difficult to account for a fire of
such activity, and of such long continuance in the same spot, by any
decomposition of mineral substances near the surface. In the place
where this combustion is supposed to exist, it must be remembered,
that there is no fresh supply of materials to replace those that have
been consumed, and that, therefore, the original accumulation of these
materials in one spot, must have been very unlike any thing that has
ever been observed concerning the disposition of minerals in the bowels
of the earth.
89. If, on the other hand, we ascribe the phenomena of volcanoes to
the central heat, the account that may be given of them is simple, and
consistent with itself. According to all the appearances from which the
existence of such heat has been inferred above, it is of a nature so
far different from ordinary fire, that it may require no circulation
of air, and no supply of combustible materials to support it. It is
not accompanied with inflammation or combustion, the great pressure
preventing any separation of parts in the substances on which it acts,
and the absence of that elastic fluid without which heat seems to have
no power to decompose bodies, even the most combustible, contributing
to the unalterable nature of all the substances in the mineral regions.
There, of consequence, the only effects of heat are fusion and
expansion; and that which forms the nucleus of the globe may therefore
be a fluid mass, melted, but unchanged by the action of heat.
90. If, from the confines of this nucleus, we conceive certain fissures
and openings to traverse the solid crust, and to issue at the surface
of the earth, the vapours ascending through these may in time heat the
sides of the tubes through which they pass to a vast distance from the
lower extremities. It is, indeed, difficult to fix the limit to which
this distance may extend, on account of the great difference between
the rate at which heat moves when it has a fluid for its vehicle, and
when it is left to make its way alone through a solid body. In the
present case, the supply of heat is rapid, as being made by a vapour
ascending through a tube of solid rock; and the dissipation of it slow,
as arising from its transmission through the rock. The waste of heat is
therefore small, compared with the supply, and grows smaller at every
given point, the longer the stream of heated vapour has continued to
flow. Such a stream, therefore, though it may at first be condensed
within a small distance of its source, will in time reach higher
and higher, and may at last be able to carry its heat to an immense
distance from the place of its original derivation. Thus, it is easy
to conceive, that vapours from the mineral regions may convey their
heat to reservoirs of water near the surface of the earth, and may in
that manner produce hot springs, and even boiling fountains, like those
of Rycum and Geyser.
91. When, instead of a heated vapour, melted matter is thrown up
through the _shafts_ or _tubes_, which thus communicate with the
mineral regions, veins of whinstone and basaltes are formed in the
interior of the earth. When the melted matter reaches to the surface,
it is thrown out in the form of lava, and all the other phenomena of
volcanoes are produced.
Lastly, where melted matter of this kind, or vapours without being
condensed, have their progress obstructed, those dreadful concussions
are produced, which seem to threaten the existence even of the earth
itself. Though terrible, therefore, to the present inhabitants of the
globe, the earthquake has its place in the great system of geological
operations, and is part of a series of events, essential, as will more
clearly appear hereafter, to the general order, and to the preservation
of the whole.
Such, according to this theory, are the changes which have befallen
mineral substances in the bowels of the earth; and though different
for the stratified and unstratified parts of those substances, they
are connected together by the same _principle_, or explained by the
same _cause_. It remains to consider that part of the history of both
which describes their changes after their elevation to the surface;
and here we shall find new causes introduced, which are more directly
the subjects of observation, than those hitherto treated of; causes,
also, which act on all fossils alike, and alike prepare them for their
ultimate destination.