volcanoes, I have no doubt that many which have been reckoned of that
number, will be found to derive their origin more directly from the
fire of the mineral regions. The basaltic rocks of the Vivarais, I am
well persuaded, belong to this class; and I conclude that they do so,
not only from the account of them given by Mr Strange, but from the
description of Faujas himself, who, though under the influence of the
opposite theory, seems very fair and accurate in his description of
phenomena. The most unequivocal mark of real whinstone rock, and of
a formation in the strictest sense mineral, is where veins of that
kind of rock intersect the strata. Now, in a letter to Buffon, on the
streams of lava found in the interior of certain calcareous rocks in
the lower Vivarais, Faujas describes what can be accounted nothing
else but a vein or dike of whinstone, accompanied with several of
its most remarkable and characteristic appearances: "Figurez-vous un
courant de lave, de la nature du basalte noir, dur et compacte, qui a
percé à travers les masses calcaires, et s'est fait jour dans quelques
parties, paroissant et disparoissant alternativement: Cette coulée
de matière volcanique s'enfonce sous une partie de la ville, bâtie
sur le rocher; elle reparoit dans la cave d'un maréchal, se cache et
se montre encore de temps en temps en descendant dans le vallon, &c.
Ce qu'il y a d'admirable, c'est que la lave forme deux branches bien
extraordinaires, dont l'une s'éleve sur la crête du rocher, tandis que
l'autre coupe horizontalement de grands bancs calcaires escarpés, qui
sont à découvert, et bordent le chemin.
"Quels efforts n'-a-t-il pas fallu pour forcer cette lave se prendre
une telle direction, et se percer cette suite de rochers calcaires?
Si cette longue coulée de lave avoit eu 200 ou 300 toises de largeur,
je ne serois pas surpris qu'un torrent de matière en fusion de ce
volume eut pu produire, des effets extraordinaires et violens; _mais
figurez-vous, Monsieur, que dans les endroits les plus larges, elle n'a
tout-au-plus qu'environ 12 ou 15 pieds; elle n'en a que 3 ou 4 dans
certaines parties_".[126]
[Footnote 126: Volcains Eteints du Vivarais, p. 328, &c.]
This narrow stream is to be traced across the strata for more than a
league and a half; and the whole appeared to Faujas so marvellous,
that he says he almost doubted the testimony of his senses. He would
have done much better, however, to have doubted the conclusions of his
theory; for it was by them that the phenomena before him were rendered
so mysterious and incredible. While he continued to regard what is
described above as a stream of melted lava, which had descended from
the top of one mountain, and climbed up the sides of the opposite, like
water in a conduit pipe, piercing occasionally through vast bodies of
solid rock, it is no wonder that he considered as marvellous what is
indeed physically impossible. Had his belief in the volcanic theory
permitted him to see in all this, not a superficial current, but one of
indefinite depth, he would have beheld the object divested, not of what
was curious and interesting, but of what was incredible or absurd, and
reduced to the same class of things with mineral veins. That it belongs
really to this class, and is no more than a vein or dike of whinstone,
intersecting the strata to an unknown depth, and most probably, like
other veins, communicating with the mineral regions, cannot be doubted
by any one who has studied the subject of basaltine rocks, through
any other medium than the volcanic theory. The ramifications which
run from it into the calcareous rock, contrived, Faujas says, just as
if on purpose to perplex mineralogists, is one of the well known and
characteristic appearances of basaltic veins.
242. It can hardly be doubted, that the lava described by the same
author as heaving up a mass of granite,[127] and including pieces of
it, is a rock of real whinstone. The same may be said of many others;
and, though I pretend not to affirm that there is nothing volcanic in
the Vivarais, I must say, that nothing decidedly volcanic appears in
the description of that country, but many things that are certainly of
a very different origin.
[Footnote 127: Volcains Eteints du Vivarais, fol. p. 365, &c.]
In the present state of geological science, a skilful mineralogist
could hardly employ himself better, than in traversing those ambiguous
countries, where so much has been ascribed to the ancient operation
of volcanic fire, and marking out what belongs either clearly to the
erupted or unerupted lavas, and what parts are of doubtful formation,
containing no mark by which they may be referred to the one of these
any more than the other. Such a work would contribute very materially
to illustrate the natural history of the earth.
243. One of the most ingenious attempts to support the volcanic theory,
is the system of _submarine volcanoes_, imagined by the celebrated
mineralogist DOLOMIEU. The phenomenon that led to this hypothesis,
was what he had observed in the hills near Lisbon, and still more
remarkably in those of the Val di Noto in Sicily, where the basaltine
rocks had regular strata incumbent on them, and in some cases
interposed or alternated with them.[128] It seemed from this evident,
that the strata were of later formation than the stone on which they
rested; and as they must, on every supposition, be held to be deposited
by water, it was concluded, that the lava which they covered had been
thrown out by volcanoes at the bottom of the sea; that the strata had
afterwards been deposited on this lava; and that, in some cases, there
had been frequent alternations of these eruptions and depositions.[129]
[Footnote 128: Mémoire de Deodate de Dolomieu, sur les Volcains
Eteints du Val di Noto, en Sicile. Journal de Phys. tom. xxv. (1784.
Septembre.) p. 191.]
[Footnote 129: Near Vizini, in the Val di Noto, Dolomieu tells us,
that he counted eleven beds, alternately calcareous and volcanic, in
the perpendicular face of a hill, which at a distance appeared like a
piece of cloth, striped black and white; _ubi supra_. He has since made
similar observations in the Vicentine and in Tyrol. Journal de Phys.
tom. xxxvii. (1790), partie 2, p. 200.]
244. Though this hypothesis does certainly deliver the system of
the Volcanists from one great difficulty, it is itself liable to
insurmountable objections. I shall just mention some of the principal.
1. The regular and equidistant strata that we often see covering the
tops of whinstone or basaltic rocks, could not have been deposited in
the oblique and very much inclined position which they now occupy.
This is remarkable in the strata which cover the basaltic rock of
Salisbury _Craig_, near Edinburgh, at its northern extremity. The
strata are very regular, and must have been deposited in a plane nearly
horizontal; yet the surface of the basaltes on which they now rest is
very much inclined, dipping rapidly to the north-east. The necessity of
a horizontal deposition in strata, which, though not now horizontal,
have their planes nearly parallel to one another, has been proved at §
38.
2. If there is any truth in the principles established above, even the
strata themselves have not been consolidated without the action of
fire. By Dolomieu's system, therefore, the consolidation of the strata
which cover the basaltes is not accounted for.
3. There are no means furnished by the hypothesis of submarine
volcanoes for bringing the basalt, and the strata which cover it,
above the level of the sea. If it is said that the waters of the sea
have been drained off, the objections are all incurred that have been
stated at § 37.[130] If it is said, that the rocks themselves have been
elevated by a force, impelling them upwards, we say, that the existence
of such a force, when admitted, furnishes another means of explaining
the whole phenomenon, namely, that of the injection of melted matter
among the strata, the same that is used in the Huttonian Theory.
[Footnote 130: Dolomieu adopts this supposition; he thinks, that the
surface of the sea must have been formerly 500 or 600 toises above its
present level. _Ibid._ p. 196.]
4. The phenomena of basaltic veins are not in the least explained by
the hypothesis of submarine volcanoes. That hypothesis, then, even
if the foregoing objections were removed, does not serve to explain
all the facts respecting the rocks of this genus, and wants, of
consequence, one of the most important characters of a true theory.
It must be allowed, however, that it makes a considerable approach to
such a theory, and that the submarine volcanoes of Dolomieu, have an
affinity to the unerupted lavas of Dr Hutton.
245. Though in these remarks I have endeavoured to expose the errors
of the volcanic system, I cannot but consider that system as coming
infinitely nearer to the truth than the Neptunian. It has the merit
of distinguishing an order of rocks, which bears no marks of aqueous
formation, and in which the crystallized, sparry, or lava-like
structure, bespeaks their primeval fluidity, and refers their origin
to fire. The Neptunian system, on the other hand, strives to confound
the most marked distinction in the mineral kingdom, and to explain
the formation, both of the stratified and unstratified rocks, by
the operation of the same element. Though chargeable with this
inconsistency, it has become the prevailing system of geology; and the
arguments which support it are therefore entitled to attention.
246. It will no doubt be thought singular, that the same mineralogist,
whom we have just seen exerting his ingenuity in defence of the
volcanic system, should now appear equally strenuous in defence of the
Neptunian. Though Dolomieu contends for the volcanic origin of some
basaltic rocks, he does not admit that all basaltes is volcanic, nor
even all of igneous formation. Thus he states, that he had examined at
Rome some of the most ancient monuments of art, executed in basaltes,
brought from Upper Egypt, and that he could discover no mark of the
action of fire in any of them.[131]
[Footnote 131: Journal de Physique, tome xxxvii. (1790,) partie 2, p.
193.]
On the contrary, he found that some of them consisted of green
basaltes, which changes its colour to a bronze, when exposed even to a
moderate heat, and which therefore, he argues, can never have endured
any strong action of fire.
The answer to this argument is very plain, if we admit the effects
ascribed by Dr Hutton to the compression which necessarily takes place
in the mineral regions. If indeed the heat in those regions resembled
exactly that of our fires at the surface, it would not be easy to deny
the above conclusion, which therefore certainly holds good against
the volcanic origin of the Egyptian basaltes. But there is no reason
why, under strong compression, the colouring matter of these stones
might not be fixed, and indestructible by heat, though it can be easily
volatilized or consumed when such compression is removed. This argument
then is against the volcanic; but not against what has been called the
_Plutonic_ formation of basaltes.
247. As to the other marks of fire which Dolomieu sought for and did
not find in the above mentioned stones, we are not exactly informed
in what they consisted. If the crystallized or spathose texture that
belongs to this description of stones was wanting, the specimens were
not to be considered as of the real basaltic or whinstone genus,
whatever their name or history may seem to indicate. If they did
possess that texture, they had the only mark of an igneous origin that
could be expected, supposing that origin to have been in the bowels of
the earth. No part, therefore, of the observations of this ingenious
mineralogist, can be considered as inconsistent with the theory of
basaltic rocks which has been laid down above.
248. Bergman had before reasoned on this subject precisely in the same
manner, but from better data, as the stones from which he derived
his argument were in their native place: "Trap," says that ingenious
author, (that is whinstone,) "is found in the stratified mountains
of West Gothland, in a way that deserves to be described. The lower
stratum, which is several Swedish miles in circuit, (10-1/2 of these
miles make a degree,) is an arenaceous stone, horizontal, resting on
granite, and having its particles agglutinated by clay. The stratum
above this is calcareous, full of the petrifactions of marine animals,
and above this is the trap. These three kinds of rock compose the
greater part of the mountains just mentioned, though there are some
other beds, particularly very thin beds of marl and of clay, which
separate the middle stratum, both from that which is under it and
over it, and are frequently so penetrated with bitumen that they
burn in the fire. This schistus is black; when burnt it becomes red,
and afterwards, when washed with water, affords alum. How can it be
supposed," he adds, "that the trap has ever been violently heated,
while the shistus on which it is incumbent retains its blackness, which
however it loses by the action even of a very weak fire?"[132]
[Footnote 132: Bergman de Productis Volcaniis, Opuscula, tom. iii. p.
214, &c.]
The answer to this argument is already given. The reasoning, as
in the former instance, is conclusive only against the action of
volcanic fire, or fire at the surface; but not against the action of
heat deep in the bowels of the earth, and under the pressure of the
superincumbent ocean. In such a situation, the bituminous schistus
might be in contact with the melted basalt, and yet there might be no
evaporation of the volatile, nor combustion of the inflammable parts.
It does not, however, always happen, that the bituminous substances, or
substances alterable by fire, which are found in contact with basaltes,
are without any mark of having endured the operation of fire. Instances
in which such operation is apparent are given above, § 30; and more
will be added in the conclusion of this note.
249. The same mineralogist founds another argument for the aqueous
formation of whin or trap on the existence of that stone in the form
of veins, included in primeval rocks: "Invenitur hoc saxum (trap) in
Suecia pluribus locis, sæpeque in montibus primævis, angustas implens
venas, adeo subtilis structuræ, ut particulæ sint impalpabiles, et, dum
niger est, genuinum efficit lapidem Lydium. In hisce montibus, nulla
adsunt ignis subterranei vestigia."[133]
[Footnote 133: Opuscula, _ubi supra_.]
The phenomena here described, namely, a vein of compact whinstone
traversing a primary rock, is, without doubt, as incapable of being
explained by the operation of a volcano, as it is by that of aqueous
deposition. It is, however, a most complete proof of the original
softness of the substance of which the veins consist, and affords one
of the strongest possible arguments for such an operation of fire as is
supposed in the present theory. The main arguments, therefore, which
have been proposed as subversive of the igneous origin of basaltes, are
only subversive of their formation by one modification of fire, viz.
of fire acting near the surface; and thus the weapons which directly
pierce the armour of the Volcanist, and inflict a mortal wound, are
easily turned aside by the superior temper of the _Plutonic_ mail.
250. An argument founded on facts very similar to some of the
preceding, and leading to the same conclusion, is employed by the
mineralogist to whom the Neptunian system owes its chief support.
Werner, in his observations on volcanic rocks and on basaltes, has
rested his proof of the aqueous formation of the latter, on their
interposition between beds of stone in mountains regularly stratified,
and obviously formed by water. He describes an instance of this in the
basaltic hill of _Scheibenberg_; and the facts, though most of them are
not uncommon, are highly deserving of attention. Near the top of this
hill, and above the basaltic rock which composes the body of it, he
tells us, that there was a sand-pit; a circumstance which he appears
to consider as not a little singular. It was, however, at the bottom of
the hill, that he met with the appearances which chiefly attracted his
notice: "First," says he; "or lowest, was a thick bank of quartzy sand,
above that a bed of clay, then a bed of the argillaceous stone called
wacke; and upon this last rested the basaltes." "When I saw," adds he,
"the three first beds running almost horizontally under the basaltes,
and forming its base; the sand becoming finer above, then argillaceous,
and at last changing into real clay, as the argil was converted into
wacke in the superior part; and, lastly, the wacke into basaltes; in a
word, when I found a perfect transition from pure sand to argillaceous
sand, from the latter to a sandy clay, and from this sandy clay,
through many gradations, to a fat clay, to wacke, and at last basaltes,
I was irresistibly led to conclude, that the basaltes, the wacke, the
clay, and the sand, are all of one and the same formation; and that
they are all the effect of a chemical precipitation during one and the
same submersion of this country."[134]
[Footnote 134: "Combien je fus surpris de voir en arrivant au fond,
un epais _banc de sable quartzeux_, puis au-dessus une _couche
d'argile_, enfin une couche de la pierre argileuse nommée _Wacke_, et
sur celle-ci reposer le _basalte_. Quand je vis les trois premiéres
couches s'enfoncer _presqu' horizontalement sous le basalte_, et former
ainsi sa _base_; le sable devenir plus fin au-dessus, puis argileux,
et se changer enfin en vraie argile, comme l'argile se convertissoit
en wacke dans sa partie supérieure; et finalement la wacke en basalte;
en un mot, de trouver ici une _transition parfaite_ du _sable pur_ au
_sable argileux_, de celui-ci à _l'argile sablonneuse_, et de _l'argile
sablonneuse_, par plusieurs gradations, à l'argile _grasse_, à la
_wacke_ et enfin au _basalte_.
"A cette vue, je fus sur le champ et irrésistiblement entrainé à
penser, (comme l'auroit été sans doute tout connoisseur impartial
frappé des conséquences de ce phénomène;) je fus, dis je,
irrésistiblement entrainé aux id es suivantes: Ce _basalte_, cette
_wacke_, cette _argile_, et ce _sable, sont d'une seule et même
formation_; ils sont tous l'effet d'une _precipitation par voie humide_
dans une seule et même submersion de cette contrée; les eaux qui la
couvroient alors transportoient d'abord le _sable_, puis deposoient
l'_argile_, et changoient peu-à-peu leur précipitation en _wacke_, et
enfin en vraie _basalte_.--Journal de Physique, tom. xxxviii. (1791,)
Partie i. p. 415.]
First, as to the sand on the top of this basaltic hill, it is most
probably the remains of certain sandstone strata that originally
covered the basaltic part, but are now worn away. We are therefore to
consider this as an instance of a basaltic rock, interposed between
strata that are undoubtedly of marine origin. In this, however, there
is nothing inconsistent with Dr Hutton's theory of basaltes; on the
contrary, it is one of the principal facts on which that theory is
founded. It has indeed been argued by some mineralogists, that bodies
thus contiguous must owe their origin to the same element, and that
a mineral substance cannot be of more recent formation than that
which lies above it. But the maxim, that a fossil must have the same
origin with those that surround it, does not hold, unless they have a
certain similarity of structure. It is, for instance, the want of this
similarity, that authorizes us to assign different periods of formation
to mineral veins, and to the rocks in which they are included.
In a succession of strata, no one can doubt, that the lowest were the
first formed, and the others in the order in which they lie; but, when
between two strata of sandstone or of limestone we find an intermediate
rock, so different as to resemble lava, and to have nothing schistose
or stratified in its composition, the same instrument cannot be
supposed to have been employed in the formation of both; nor is there
any reason why we may not suppose, that the intermediate body was
interposed between the other two, by some action subsequent to their
formation. It was thus that Dolomieu concluded, when he saw a lava-like
stone interposed between calcareous strata in the Val di Noto, that,
though contiguous, these two rocks could not possibly be of the same
formation; and thus far it is certain, that every unprejudiced observer
must agree with him.
251. But the circumstance on which Werner seems to lay the greatest
stress, is the gradual transition from the sand to the basalt, through
the intermediate steps of clay and wacke; this gradual transition he
considers as a direct proof, that they are all of the same formation.
A gradual transition of one body into another, can only be said to
take place, when it is impossible to define their common boundary,
or to determine the line where the one begins and the other ends.
Now, if this be the proper notion of gradual transition, I must say,
that after much careful examination, I have never seen an instance,
in which such a transition takes place between whinstone and the
contiguous strata. The _line_ of separation, though in some places
less evident than in others, has, on the whole, been marked out with
great precision; and, though the stones have been firmly united, or,
as one may say, welded one upon another, yet, when a fresh fracture
was obtained, the stratified and unstratified parts have rarely failed
to be distinguished. The fresh fracture is indeed often necessary, for
many species of whinstone get by decomposition a granulated texture at
the surface, so as hardly to be distinguished from real sandstone.
Some of the kinds of primary schistus also, particularly the
argillaceous, when much indurated, have in their structure a
considerable resemblance to whinstone; they are slightly granular, or
laminated, and have a tendency to a sparry texture. Where it happens
that this sort of schistus and whinstone are contiguous, it is natural
to expect, that their common boundary will be traced with difficulty,
and in many parts will be quite uncertain. Still, however, if a careful
examination is made; if the effects of accidental causes are removed;
and, above all, if the more ambiguous instances are compared with the
more decisive, and interpreted by them, though single specimens may be
doubtful, we will hardly ever find that any uncertainty remains with
respect to entire rocks.
252. This general fact, which I state on much better authority than
that of my own observations, viz. on those of Dr Hutton, is not given
as absolutely without exception. The theory of whinstone which has
been laid down here, leads us indeed to look for some such exceptions.
It is certain, that the basis of whinstone, or the material out of
which it is prepared by the action of subterraneous heat, is clay in
some state or other, and probably in that of argillaceous schistus.
It follows, of consequence, that argillaceous schistus may by heat be
converted into whinstone, or the material out of which is prepared by
the action of subterraneous heat, is clay in some state or other, and
probably in that of argillaceous schistus. It follows, of consequence,
that argillaceous schistus may by heat be converted into whinstone.
When, therefore, melted whinstone has been poured over a rock of such
shistus, it may, by its heat, have converted a part of that rock into
a stone similar to itself; and thus may now seem to be united, by
an insensible gradation, with the stratum on which it is incumbent;
and phenomena of this kind may be expected to have really happened,
though but rarely, as a particular combination of circumstances seems
necessary to produce them. Hence it is evident, that stones may
graduate into one another, without being of the same formation; and
that it is fallacious to conclude, from the insensible transition
of one kind of rock into another, without any other circumstance of
affinity, that they have both the same origin.
I am disposed, therefore, to make some limitation to what is said
in § 72, where I have expressed an absolute incredulity as to such
transitions as are here referred to. The great skill and experience
of the mineralogist who has described the strata at Scheibenberg, do
not allow us to doubt of his exactness, though some of the appearances
are such as decomposition and wearing might well enough be supposed to
produce. The fairest way is to take Mr Werner's observations just as
they are given us, and to try whether they cannot be explained without
the assistance of his theory. In effect, the wacken which he describes,
rests, it would seem, on an unconsolidated bed of clay; and it may be
supposed, that a part of this bed has been converted into wacken by
the heat of the incumbent mass, and has thus produced the apparent
gradation from the one substance to the other. As the appearances of
the rocks of Scheibenberg seem to be considered by Werner as furnishing
a very strong, and even an unexpected confirmation of his system, I
cannot help thinking, that an explanation of them, on the principles
of Dr Hutton, without any straining or forcing of those principles,
contributes not a little toward extending the empire of the latter over
all the phenomena of geology.
253. Another fact, which has been much infilled on of late, in proof
of the aqueous formation of basaltic rocks, is that shells are found
in them. Of the reality of this fact, however, or at least of the
instances hitherto produced, great doubts I think may be reasonably
entertained. The specimens of the supposed basaltes, with shells
included in them, that are chiefly relied on, are found at Portrush in
Ireland, a rocky promontory to the westward of the Giant's Causeway,
and separated from it by a considerable body of calcareous strata.
Some of these specimens were brought to Edinburgh about a year ago,
and were supposed, I believe, to contain an irrefragable proof of the
Neptunian origin of the basaltic promontory where they were found. I
went to see these specimens in company with Lord Webb Seymour and Sir
James Hall; and, on examining them carefully, we were all of opinion,
that the stones which contained the shells, or the impressions of the
shells, were no part of the real basaltes. They were all very compact,
and had all more or less of a siliceous appearance, such as that of
chert; they had nothing of a sparry or crystallized structure; their
fracture was conchoidal, and but slightly uneven. In two of them,
one of which bore the impression of a _cornu ammonis_, the schistose
texture might be distinctly perceived. A specimen which accompanied
them, but in which there was no shell, served very exactly to explain
the relation between these stones and the true basaltes. Part of this
specimen was a true basalt, and the rest a sort of hornstone, exactly
the same with that in which the shells were, and not unlike the jasper
that is under the whinstone of Salisbury Crag, and in contact with
it; so that on the whole it was evident, that the rock containing the
shells is the schistus or stratified stone, which serves as the base of
the basaltes, and which has acquired a high degree of induration, by
the vicinity of the great ignited mass of whinstone.
This solution of the difficulty has since been confirmed by
observations made on the spot by Dr Hope, who discovered two or three
alternations of the basaltic rock, with the beds of the schistus in
which the shells are contained.
254. This also explains some observations of Spallanzani, made in
the island of Cerigo, on the coast of Greece, the Cythæra of the
ancients.[135] The base of that island is limestone; but it abounds
also in unstratified rocks, which the Italian naturalist supposes to
be of volcanic origin; but which, if I mistake not, we would regard
as whinstone, or perhaps porphyry; and they are said to contain
oyster-shells and pectinites of a large size, perfectly mineralized.
These petrifactions, however, Spallanzani says, are not contained
in the lava that has actually flowed, but in stones which have only
endured a slighter action of fire. Without the commentary afforded by
the Portrush specimens, it would be difficult to make out any thing
very precise from this description. By help of the information derived
from those specimens, we may conclude, that the condition of the
shells in them, and in the rocks of Cerigo, is perfectly alike; and
that, in both cases, the shells are involved in parts of the rock which
are truly stratified, but which have been, in some degree, assimilated
to the basaltes by the heat which they have endured. Spallanzani would
probably have used exactly the same terms which he employs in speaking
of Cerigo, if he had been required to describe the petrified shells at
Portrush.
[Footnote 135: Journal de physique, tom. xlviii. (1798,) p. 278.]
255. In the instances just mentioned, the petrified marine objects are
not found in the real whinstone; but if they were found in it, when it
borders on stratified rocks containing such objects, the thing would
not be at all surprising, nor furnish any argument against the igneous
consolidation of the stone. If a torrent of melted matter was poured
in among the strata, by a force which at the same time broke up and
disordered those strata, nothing could be more natural, than that this
matter should contain fragments of them, and of the objects peculiar to
them.
In one instance, mentioned by Mr Strange, this seems actually to have
taken place. In the Veronese, a country remarkable for a mixture of
limestone strata, containing marine objects, with volcanic or basaltine
hills, he assures us, that he had seen a mass of stone, which had
evidently concreted from fusion, in which the marine fossil bodies,
originally, as he supposes, contained in the strata, were perfectly
distinguishable, though variously disfigured.[136] It may be, that
in this, as in the foregoing examples, it was not real basaltes, or
real lava, which contained the shells, but the conterminal rock; but,
supposing it to be as Mr Strange represents it, there appears to be
no inconsistency between the phenomenon, and the igneous origin of
the rock in which the shells were included. Here, however, it should
be remarked, that the presence of great pressure, to prevent the
conversion of the shells into quicklime, seems absolutely necessary;
and that the phenomenon of these basaltic petrifactions, requires the
application of heat to have been deep under the surface of the earth.
[Footnote 136: Phil. Trans. 1775, p. 25.]
256. The phenomena we have been considering, have been selected as the
most unfavourable to the igneous origin of basaltic rocks; and we have
seen, that when duly examined, they are not at all inconsistent with
it. We are now to take a view of some appearances, that seem quite
irreconcilable with the aqueous formation of these rocks.
Where whinstone rocks are found in masses, bounded by the strata,
and insulated among them, they subject the Neptunian system to great
difficulties. For, supposing it true that this stone may be produced by
the precipitation and crystallization of mineral substances dissolved
in water, yet it seems unaccountable, that this effect has been so
local and limited in extent, as often to be confined to an irregular
figure of a few acres, while, all round, the substances deposited have
had no tendency to crystallization, and have been formed into the
common secondary strata. The rock of Salisbury _Craig_, for instance,
is a mass of whinstone, having a perpendicular face eighty or ninety
feet high toward the west, and extending from north to south with
a circular sweep about 900 yards. The whole of this rock rests on
regular beds of secondary sandstone, not horizontal, but considerably
depressed toward the north-east: the rock is loftiest in the middle,
and decreases in thickness toward each end, terminating at its northern
extremity in a kind of wedge. It is covered at top, toward that
extremity, with regular beds of sandstone, perfectly similar to those
on which it is incumbent; and it is not improbable, that this covering
formerly extended over the whole.
Now, what cause can have determined the column of water, which rested
on the base at present occupied by this rock, to deposit nothing but
the materials of whinstone, while the water on the south, west, and
north, was depositing the materials of arenaceous and marly strata?
Wherefore, within this small space, was the precipitate every where
_chemical_, to use the language of Werner, while close to it, on either
side, it was entirely _mechanical?_ Why is there, in this case, no
gradation? and why is a mere mathematical line the boundary between
regions where such different laws have prevailed? Whence also, we may
ask, has the basaltic deposit been abruptly terminated toward the west,
so as to produce the steep face which has just been mentioned? The
operation of currents, or of any motion that can take place in a fluid,
will furnish no explanation whatever of these phenomena; yet they are
phenomena far from being peculiar to a single hill; they are among the
most general and characteristic appearances in the natural history of
whinstone mountains; and a geological theory which does not account for
them, is hardly entitled to any consideration.
257. The basaltic rock, just described, is also covered, at least
partly, with strata perfectly similar to those that lie under it.
Now, it appears altogether unaccountable, that after the water had
done depositing the materials of the whin on the spot in question,
the former order was so quickly resumed, and a deposition of sand,
and of the other materials of the strata, took place just as before.
All this is quite unintelligible; and the principles of the Neptunian
system seem here to stand as much in need of explanation, as any of the
appearances which they are intended to account for.
258. The unequal thickness, and great irregularity in the surface
of the whinstone mass, here treated of, and of many rocks of the
same kind, is also a great objection to the notion of their aqueous
formation. This seems to have been perceived by Werner, in the instance
of the rocks formerly mentioned; and he endeavours to explain it, by
supposing, that much of these rocks has been destroyed by waste and
decomposition, so that an irregularity of their surface, and want of
correspondence has been given to them, which they did not originally
possess. In the instance of Salisbury _Craig_, however, we have a
proof, that the great irregularity of surface, and the inequality of
thickness, do not always arise from these causes. The thinnest part
of that rock, toward its northern extremity, is still covered by the
strata in their natural place, and has been perfectly defended by them
from every sort of wearing and decay. The cuneiform shape, therefore,
which this rock takes at its extremities, and the great difference of
its thickness at them and in the middle, is a part of its original
constitution, and can be attributed to nothing casual, or subsequent to
its consolidation.
The same may be said of many other basaltic rocks, where an inequality
of thickness, most unlike to what belongs to aqueous deposits, is known
to exist in beds of whinstone that are still deep under the surface.
Thus the toadstone of Derbyshire, even where it has a thick covering of
strata over it, has been found, by the sinking of perpendicular shafts,
to vary from the thickness of eighteen yards to more than sixty, within
the horizontal distance of less than a furlong. Nothing of this kind
is ever found to take place in those beds of rock which are certainly
known to originate from aqueous deposition, and no character can more
strongly mark an essential difference of formation.
259. We have had frequent occasion to consider the characters of
those masses of whinstone which are so often found interposed between
stratified rocks. These have been found in general very adverse to the
Neptunian system; and two of them which yet remain to be mentioned,
are even more so than any of the rest.
Where a bed or tabular mass of whinstone is interposed between strata,
and wherever an opportunity offers of seeing its termination, if the
strata under it are not broken, it may be remarked, that they do not
abut themselves bluff and abrupt against the whin. On the contrary,
if we mark the course of the stratum which covers the whinstone, and
of that which is the base of it, we shall find they converge toward
one another, the interposed mass growing thinner and thinner, like a
wedge. When the latter terminates, the two former come in contact, and
have no stratum interposed between them. Thus the roof and base of the
whinstone rock are contiguous beds, that appear as if they had been
lifted up and bent, and separated by an interposed mass. Had the whole
been an effect of simultaneous deposition, the regular strata must have
been abruptly terminated by the whin, like two courses of different
forts of masonry where they meet with one another.
260. From this wedge-form of the whinstone masses, and in general from
the irregularity of their surfaces, another conclusion follows, similar
to the preceding, and one which has been already mentioned. Where the
surface of the interposed mass is greatly inclined to the horizon, the
strata which rest on this inclined plane, are nevertheless as exactly
parallel to that plane, and to one another, as if they were really
horizontal. It is certain, therefore, that they were not deposited on
the same inclined plane on which they now rest; for, if so, they would
have been still nearly horizontal, and by no means parallel to the
inclined side of the whinstone. This follows from the nature of aqueous
deposition, as already explained.
We have a remarkable instance of the phenomenon here referred to, in
the rock of Salisbury _Craig_, of which mention has been so often made,
and in which almost every circumstance is united, that can serve to
elucidate the natural history of basaltic rocks. The north end of that
rock is in the figure of a wedge, with its inclined side considerably
steep, and covered by strata of grit, perfectly regular, and parallel
to the surface on which they lie. The inspection of them will convince
any one, that they were not deposited by the water, on a bottom so
highly inclined as that on which they now rest. They are of a structure
very schistose; their layers very thin; so that any inaccuracy of
their parallelism would be readily observed. The appearances of the
horizontal deposition of these strata, are indeed so clear, and so
impossible to be misunderstood, that the followers of the Huttonian
system would not risk much, if they were to leave the whole theory of
whinstone to the decision of this single fact, and should agree to
abandon that theory altogether, if the Neptunists can shew any physical
or statical principle, on which the deposition now described can
possibly have been made; or will point out the rule, by which nature
has given a structure so nicely stratified to arenaceous beds deposited
on a surface so highly inclined. If no such principle can be pointed
out, though we cannot conclude that the Huttonian Theory is true, we
certainly may conclude that the Neptunian is false.
261. Proofs of the igneous formation of whinstone, still more direct,
are derived from the induration of the contiguous strata; from their
disturbance when interfered by veins of whinstone; and from the
charring of the coal which happens to be in contact with these veins.
These are considered above at § 66, 67, &c.; and it is particularly
taken notice of at § 66, that pieces of sandstone are sometimes found
as if floating in the whinstone, and, at the same time, greatly altered
in their texture. One of the best and most unequivocal instances of
this sort which I have seen, is to be found on the south side of
_Arthur's Seat_, near Edinburgh. The rock which composes the upper
part of the hill, on that side, is a whinstone breccia, such as we have
many examples of, and, I believe, very much resembling what is called
a _lava brecciata_ by the volcanic geologists. The stony fragments
included in this compound mass, are for the greater part rounded;
and some of them are of whinstone, others of porphyry, strongly
characterized by rectangular maculæ of feldspar, and many seem to be
of sandstone, but so considerably altered, as to leave it at least
disputable whether they really are so or not. In one part, however,
where the face of the rock is nearly perpendicular, a narrow ridge is
seen standing out from the rest, and of a different colour, being more
entirely covered with moss than the rock round about it, and, as may
be presumed from that circumstance, less liable to decomposition. On
examination I found, that this ridge does not consist of whinstone, but
of a very hard and highly consolidated sandstone. It appears to be the
edge of a stratum, of the thickness of about nine or ten inches, and of
the height of fifteen or sixteen feet. It is not perfectly straight,
but slightly waved, its general direction being nearly vertical; and
it is on both sides firmly embraced by the whinstone. When broken, it
appears that this sandstone resembles in colour, and in every thing
but its greater consolidation, and more vitreous structure, the common
grit found at the bottom of the hill, and over all the adjacent plain.
262. If all these circumstances are put together, there appears but
one conclusion that can be drawn from them. We have here the manifest
marks of some power which could lift up this fragment of rock from its
native place, distant at least several hundred yards from its present
situation, place it upright on its edge, encompass it with a solid
rock, of a nature quite heterogeneous to itself, and bestow on it, at
the same time, a great addition of solidity and induration. If the mass
in which this stone is now imbedded, be supposed to have been once in
fusion, and forcibly thrown up from below, invading the strata, and
carrying the fragments along with it, the whole phenomena now described
admit of an explanation, and all the circumstances accord perfectly
with one another; but, without this supposition, they are so many
separate prodigies, which have no connection with one another, nor with
any thing that is known. It is indeed impossible, that the effects of
motion and heat can be more clearly expressed than they are here, or
the subject in which these powers resided more distinctly pointed out.
263. The preceding facts being susceptible but of one interpretation,
are on that account extremely valuable. The phenomena of Salisbury
_Craig_, near the same place, are almost equally free from ambiguity.
The basaltic rock which forms that precipice, rests on arenaceous or
marly strata; and these, in their immediate contact with the former,
afford an instance of what is mentioned § 67, namely, the conversion
of the strata in such situations into a kind of petrosilex, or even
jasper. The line which separates the one rock from the other, is,
at the same time, so well defined, as, in the eyes even of the most
determined Neptunist, to exclude all idea of insensible gradation.
264. The same rock affords some remarkable instances of the disturbance
of the strata contiguous to the whinstone. The beds of the former are
bent upwards in several places; and, at one in particular, form an
arch, with its convexity downward, so as to make it evident, that the
force which produced this bending was directed from below upwards.
265. It is, however, where whinstone takes the form of veins,
intersecting the strata, that the induration of the latter is most
conspicuous. The coast of Ayrshire, and the opposite coast of Arran,
exhibit these veins in astonishing variety and abundance. The strata
are, in many instances, so _reticulated_ by the veins, and intersected
at such small distances, that it seems necessary to suppose, that the
fissures in them were hardly sooner made than filled up. This at least
is true, if the veins are to be accounted all of the same formation;
and, in the greatest number of instances by far, there is no mark of
the one being posterior to the other.
266. The induration of the sides of these veins, in some cases, has
been such, that the sides have become more durable than the vein
itself; so that the whinstone has been worn away by the washing of the
waves, and has left the sides standing up, with an empty space, like
a _ditch_, between them. One of these I remarked on the south side of
Brodick Bay, in Arran, which, where it met the face of an abrupt cliff
was not less than forty or fifty feet in depth.
267. I shall pass over whatever argument might be drawn in favour
of our system, from the slender ramifications of the veins, and
the varieties of their sizes, from a few inches to many fathoms in
diameter, and also from the connection which they often appear to have
with the great tabular masses of basaltes; and shall only add a few
remarks on the charring of coal in the vicinity of veins or masses
of whinstone. The connection between the charring of coal and the
presence of whinstone, was first observed by Dr Hutton; and, as far as
opportunities of verifying the observation have yet occurred, appears
to be a fact no less general than it is curious and interesting. In the
coal mines of Scotland, it certainly holds remarkably, particularly in
those about Saltcoats in Ayrshire, where a whinstone dike is known to
stretch across the whole of the coal country, and to be every where
accompanied with blind or uninflammable coal. At Newcastle, dikes of
the same kind are met with, and one, in particular, in what is called
the _Walker_ Colliery, has proved the action of subterraneous fire, to
the satisfaction of mineralogists nowise prejudiced in favour of the
Huttonian system.
The coal found under basaltes, in the Island of Sky, has been already
mentioned, § 140. To what was said concerning the fibrous structure of
the parts of that fossil in immediate contact with the whin, it may be
added, that it is also charred in those parts, so as to have hardly
any flame when it is burnt, though further down it is of the nature of
ordinary coal. Indeed, if there be any truth in Mr Kirwan's general
remark, that it is common to find wood coal under basaltes, it must
be understood to arise from this, that the coal in contact with the
basaltes is frequently charred, and its fibrous structure, by that
means, rendered more visible.
268. It has been objected to the supposition of coal having its
bituminous part driven off by the heat of the whinstone, that this
ought not, on Dr Hutton's principles, to happen in the mineral
regions. But it may be replied, as has been done above, that the local
application of heat might certainly produce this effect, and might
drive off the volatile ports from a hotter to a colder part of the
same stratum. The bitumen has not been so volatilized and expanded as
entirely to escape from the mineral regions; but it has been expelled
from some parts of a mass, only to be condensed and concentrated in
others. This supposition coincides exactly with the appearances.
269. The native or fossil coke which accompanies whinstone, has been
distinguished into two varieties. The first is the most common, in
which, though the coal is perfectly charred, it is solid, and breaks
with a smooth and shining surface. The second is also perfect charcoal,
but is very porous and spongy. This substance is much rarer than the
other. Dr Hutton mentions an instance of it at the mouth of the river
Ayr, where there is a whinstone dike.[137] I had the satisfaction of
visiting it along with him. It was in the bed of the river, below the
high water mark; the specimens had the exact appearance of a _cinder_.
[Footnote 137: Theory of the Earth, vol. i. p. 611.]
In the banks of the same river, some miles higher up, he found a piece
of coal, belonging to a regular stratum, involved in whinstone, and
extremely incombustible. It consumed very slowly in the fire, and
deflagrated with nitre like plumbago. This be considered as the same
fossil which has been described under the name of _plombagine_. Near
it, and connected with the same vein of whinstone, was a real and
undoubted plumbago.
From these circumstances he also concluded, that plumbago is the
extreme of a gradation, of which fossil-coal is the beginning, and is
nothing else than this last reduced to perfect charcoal This agrees
with the chemical analysis, which shows plumbago to be composed of
carbon, combined with iron.
In confirmation of this theory, he mentions a specimen, in his
possession, of steatical whinstone, from Cumberland, containing nodules
of a very perfect and beautiful plumbago; and he also takes notice of a
mine of this last, in Ayrshire, which, on the authority of Dr Kennedy,
who has examined it with great care, I can state as being contained,
or enveloped in whinstone; and I hope the public will soon be favoured
with a particular description of this very interesting spot, by the
same ingenious and accurate observer.
270. Thus the mineralogical and chemical discoveries agree in
representing coal, blind coal, plombagine, plumbago, as all
modifications of the same substance, and as exhibiting the same
principle, carbon, in a state of greater or less combination. As the
last and highest term of this series should be placed the _diamond_;
but we are yet unacquainted with the matrix of this curious fossil,
and its geological relation to other minerals. When known, they will
probably give to this substance the same place in the geological, as in
the chemical arrangement: in the mean time, it is hardly necessary to
remark, how well all the preceding facts agree with the hypothesis of
the igneous formation of whinstone, and how anomalous and unconnected
they appear, according to every other theory.
271. Notwithstanding all this accumulated and unanswerable evidence
for the igneous formation of basaltes, a great objection would still
remain to our theory, were it not for the very accurate and conclusive
experiments concerning the fusion of this fossil, referred to above, §
75. A strong prejudice against the production of any thing like a real
stone by means of fusion, had arisen, even among those mineralogists,
who were every day witnesses of the stony appearance assumed by
volcanic lava. They still maintained, on the authority of their own
imperfect experiments, that nothing but glass can ever be obtained
by the melting of earths or of stones, in whatever manner they are
combined.
An ingenious naturalist, after describing a block of basaltes, in which
he discovered such appearances, as inclined him to admit its igneous
consolidation, rejects that hypothesis, merely from the imaginary
inability of fire to give to any substance a stony character: "Quelque
mélange,"says he, "de terres que l'on suppose, quelque soit le degré
de feu que l'on imagine, quelque soit le tems que l'on emploie, il est
très certain que l'on n'obtiendra pas, par le seul fluide igné, ni
basalte, ni rien qui lui ressemble."[138]
[Footnote 138: Journal de Phys. tom. xlix. (1799,) p. 36.]
Sir James Hall's experiments have completely demonstrated the contrary
of what is here asserted: they have added much to the evidence of the
Huttonian system; and, independently of all theory, have narrowed the
circle of prejudice and error.
NOTE XV. § 83.
_On Granite._
1. _Granite Veins._
272. It is said above, § 77, that granite is found in unstratified
masses, and in veins. In the former of these conditions, it constitutes
entire mountains, and forms the central ridge of many of the greatest
chains that traverse the surface of the earth. It is the granite of
this kind that has been most generally described by travellers and
mineralogists. The veins have not been so much attended to, though
they are of peculiar importance for ascertaining the relation between
granite and other fossils.
273. Though Dr Hutton was the first geologist who explained the nature
of granite veins, and who observed with attention the phenomena
which accompany them, he is not the first who has mentioned them. M.
Besson found veins of this kind in the Limoges, in an argillaceous
schistus, and unconnected, as far as appeared, with any large mass of
granite.[139]
[Footnote 139: Journal de Phys. tom. xxix. p. 89.]
Saussure met with granite veins in the Valorsine, but did not see them
distinctly. He ascribed them to infiltration.[140] The date of this
observation is in 1776: He afterwards discovered similar appearances at
Lyons.[141]
[Footnote 140: Voyage au Alpes, tom. i. § 598, 599.]
[Footnote 141: _Ibid._ § 601.]
Werner also, in enumerating the substances of which veins are formed,
reckons granite as one of them.
274. Veins of granite may be considered as of two kinds, according as
they are connected, or not connected apparently with any large mass of
granite, it is probable, that these two kinds of veins only differ in
appearance, and that both are connected with masses of the same rock,
though that connection is visible in some instances, and invisible in
others. The distinction, however, whatever it be with respect to the
thing observed, is real with respect to the observer; and, as it is
right, in a description of facts, to avoid every thing hypothetical, I
shall speak of these veins separately.
275. Veins of granite, having no communication, so far as can be
discovered, with any mass of the same rock, are found in the Western
Islands of Scotland, peculiarly in that of Coll, where they traverse
the beds of gneiss and hornblende schistus, which compose the main
body of the island. They are sometimes several fathoms in thickness,
obliquely intersecting the planes of the strata just mentioned, which
are nearly vertical. In these veins the feldspar is predominant; it is
very highly crystallized, and of a beautiful flesh colour. Many smaller
veins are also to be met with in the same place; but no large mass of
granite is found, either in this or the adjacent island of Tiree.
276. The Portsoy granite, of which mention has been already made, §
80, also constitutes a vein or dike, traversing a highly indurated
micaceous schistus, about a mile to the eastward of the little town
of Portsoy, and not visibly connected with any large mass of the same
kind. More dikes than one of this granite have been observed near the
same spot.
A similar granite is likewise found inland, in the neighbourhood of
Huntly, about eighteen miles south of Portsoy; but whether in the shape
of a vein or a mass, I have not been able to learn.
277. Veins of granite are also frequent in Cornwall, where they are
known by the name of _lodes_, the same name which is applied in that
country to metallic veins. The granite veins frequently intersect
the metallic, and are remarkable for producing shifts in them, or
for throwing them out of their natural direction. The mineral veins,
particularly those that yield copper and tin, run nearly from east to
west, having the same direction with the beds of the rock itself, which
is a very hard schistus. The granite lodes, as also those of porphyry,
called _elvan_ in Cornwall, are at right angles nearly to the former;
and it is remarked, that they generally heave the mineral veins, but
that the mineral veins seldom or never heave the cross-veins. In this
country, therefore, the veins of granite and porphyry are posterior in
formation to the metallic veins. These veins of granite may perhaps be
connected with the great granitic mass that runs longitudinally through
Cornwall, from Dartmoor to the Land's End. This much is certain, that
their directions in general are such, that, if produced, they would
intersect that mass, nearly at right angles.
278. The granite veins in Glentilt, where Dr Hutton made his first
observations on this subject, are not, I believe, visibly connected
with any large mass of the same rock.[142] The bed of the river Tilt,
in the distance of little more than a mile, is intersected by no
less than six very powerful veins of granite, all of them accompanied
with such marks of disorder and confusion in the strata, as indicate
very strongly the violence with which the granite was here introduced
into its place. These veins very probably belong to the great mass
of granite which is known to form the central ridge of the Grampians
further to the north; but they are several miles distant from it, and
the connection is perhaps invisible in the present state of the earth's
surface.
[Footnote 142: Trans. Royal Society Edin. Vol iii. p. 77, &c.]
279. The second kind of granite vein, is one which proceeds visibly
from a mass of that rock, and penetrates into the contiguous strata.
The importance of this class of veins, for ascertaining the relation
between granite and other mineral bodies, has been pointed out, §
82; and by means of them it has been shown, that the granite, though
inferior in position, is of more recent formation than the schistus
incumbent on it; and that the latter, instead of having been quietly
deposited on the former, has been, long after its deposition and
consolidation, heaved up from its horizontal position, by the liquid
body of granite forcibly impelled against it from below.
It has been alleged, in order to take off the force of the argument
derived from granite veins, that these veins are formed by
infiltration, though, to give any probability to this supposition,
it would be necessary to show, that water is able to dissolve the
ingredients of granite; and even if this could be done, the direction
which the veins have, in many instances, rising up from the granite,
is a proof, as remarked § 82, that they cannot be the effect of
infiltration.
Another objection has been thrown out, namely, that the veins here
referred to are not of true granite, according to the definition
which mineralogists have given of that substance. The force of a
fact, however, is not to be lessened by a change of names, or the
use of arbitrary definitions. The general fact is, that the granitic
mass, and the vein proceeding from it, constitute one continuous,
and uninterrupted body, without any line of separation between them.
The geological argument turns on this circumstance alone; and it is
no matter whether the rock be a syenite, a granitelle, or a real
granite. The phenomenon speaks the same language, and leads to the same
conclusion, whatever be the technical terms the mineralogist employs in
describing it.
280. It must, however, be admitted, that a difference of character is
often to be observed between the granite mass and the veins proceeding
from it; sometimes the substances in the latter are more highly
crystallized than in the former; sometimes, but more rarely, they are
less crystallized, and, in some instances, an ingredient that enters
into the mass seems entirely wanting in the vein. These varieties,
for what we yet know, are not subject to any general rule; but they
have been held out as a proof, that the masses and the veins are not
of the same formation. It may be answered, that a perfect similarity
between substances that, on every hypothesis, must have crystallized
in very different circumstances, is not always to be looked for; but
the most direct answer is, that this perfect similarity does sometimes
occur, insomuch that, in certain instances, no difference whatsoever
can be discovered between the mass and the vein, but they consist of
the same ingredients, and have the same degree of crystallization. Some
instances of this are just about to be remarked.
281. A strong objection to the supposed origin of granitic veins
from infiltration, and indeed to their formation in any way but by
igneous fusion, arises from the number of fragments of schistus, often
contained, and completely insulated in those veins. How these fragments
were introduced into the fissures of the schistus, and sustained till
they were surrounded by the matter deposited by water, is very hard
to be conceived; but if they were carried in by the melted granite,
nothing is more easily understood.
The following are some of the places where the phenomena of granite
veins may be distinctly seen.
282. The island of Arran, remarkable for collecting into a very small
compass a great number of the most interesting facts of geology,
exhibits many instances of the penetration of schistus by veins of
granite. A group of granite mountains occupies the northern extremity
of the island, the highest of which, Goatfield, rises nearly to the
height of 3000 feet, and on the south side is covered with schistus
to the height of 1100. From thence, the line of junction, or that
at which the granite emerges from under the schistus, winds, so far
as I was able to observe, round the whole group of mountains, with
many wavings and irregularities, rising sometimes to a greater, and
descending sometimes to a much lower level, than that just mentioned.
Along this line, particularly on the south, wherever the rock is laid
bare, and cut into by the torrents, innumerable veins of granite are to
be seen entering into the schistus, growing narrower as they advance
into it; and being directed, in very many cases, from below upwards,
they are precisely of the kind which the infiltration of water could
not produce, even were that fluid capable of dissolving the substances
which the vein consists of. From this south face of the mountain, and
from the bed of a torrent that intersects it very deeply, Dr Hutton
brought a block of schistus, of several hundred weight, curiously
penetrated by granite veins, including in them many insulated fragments
of the schistus.
From this point, the common section of the granite and schistus
descends towards the west side of the mountain, and is visible at the
bottom of a deep glen, (Glen-Rosa,) which detaches Goatfield from the
hills farther to the west. The junction is laid bare at several places
in the bed of the river which runs in the bottom of this glen; and in
all of them exhibits, in a greater or less degree, the appearances of
disturbance and violence which have accompanied the injection of the
granite veins. Many circumstances render this spot interesting to a
geologist, and, among others, an intersection of the granite, a little
above its junction with the schistus, by a dike or vein of very compact
whinstone.
The same line of junction is found on the opposite, or north-east, side
of the mountain, where it is intersected by another little river, the
Sannax, which on this side determines the base of the mountain. This
junction is no less remarkable than the other two.
The island of Arran contains, I have no doubt, many other spots where
these phenomena are to be seen; but I have had no opportunity of
observing them, nor do I find that Dr Hutton met with any others in his
visit to this island.
283. Another series of granite veins is found in Galloway, which was
first discovered by Dr Hutton and his friend Mr Clerk, and afterwards
more fully explored by Sir James Hall and Mr Douglas, the present Earl
of Selkirk. The two last traced the line of separation between a mass
of granite and the schistus incumbent upon it, all round a tract of
country, about eleven miles by seven, extending from the banks of Loch
Ken westward; and in all this tract they found, "that wherever the
junction of the granite with the schistus was visible, veins of the
former, from fifty yards, to the tenth of an inch in width, were to be
seen running into the latter, and pervading it in all directions, so
as to put it beyond all doubt, that the granite of these veins, and
consequently of the great body itself, which was observed to form with
the veins one uninterrupted mass, must have flowed in a soft or liquid
state into its present position."[143] I have only farther to add, that
some of these veins are remarkable for containing granite, not sensibly
different, in any respect, from the mass from which they proceed.
[Footnote 143: Trans. Royal Society Edin. vol. iii. p. 8.]
284. In Invernessshire, between Bernera and Fort Augustus, the same
phenomena occur on the north side of Loch Chloney, where some granite
mountains rise from under the schistus. In travelling near this place,
Lord Webb Seymour and myself were advertised of our approach to a
junction of granite and schistus, by finding among the loose stones on
the road many pieces of schistus, intersected with veins of feldspar
and granite. We walked along this junction for more than a mile; and
toward the east end, where the road leaves it, we saw, in the bed of
a stream that runs into Loch Chloney, many beautiful specimens of
granitic veins pervading the schistus, and branching out into very
minute ramifications.
285. The last instance I have to mention from my own observation, is
at St Michael's Mount in Cornwall. That mount is entirely of granite,
thrust up from under a very hard micaceous schistus, which surrounds
it on all sides. At the base of it, on the west side, a great number
of veins run off from the granite, and spread themselves like so many
roots fixed in the schistus: they are seen at low water. In the smaller
veins, the granite is of very minute, though distinct parts; in the
larger, it is more highly crystallized, and is undistinguishable from
the mass of the hill.
Besides the above, Cornwall probably affords many other instances of
the same kind, which I have not had an opportunity to examine. Such
instances may in particular be looked for at the Land's End, where a
promontory, consisting of a central part of granite, and covered by
a micaceous schistus on both sides of it, is cut transversely by the
sea coast, and the contact of the granite and schistus of course twice
exposed to view.
286. Scotland also affords other examples of granite veins, and some
of them have been actually described. Mr Jameson has taken notice of
some which he saw in the bottom of the river Spey, at Glen Drummond, in
Badenach, and has represented them in an engraving.[144] They traverse
the strata in various directions, and inclose pieces of the micaceous
schistus; and, from the great number of loose blocks which he found,
exhibiting portions of such veins, it is probable, that they are very
numerous in this quarter. The same mineralogist mentions some instances
of similar veins in the Shetland Isles.[145]
[Footnote 144: Mineralogy of the Scottish Isles, vol. ii. p. 3.]
[Footnote 145: _Ibid._ p. 216.]
In Ross-shire, Sir George Mackenzie has observed a great variety of
granite veins, some of them of large size. One of them, in particular,
not far from Coul, when first discovered, was supposed to be a
single mass, rising from under the schistus; but, on a more careful
examination, has been found to be a part of a great system of veins
which intersects the micaceous schistus of this tract in various
directions.
287. The granite veins are not the only proof that this stone is more
recent than some other productions of the mineral kingdom. Specimens
of granite are often found, containing round nodules of other stones,
as, for example, of gneiss or micaceous schistus. Such is the specimen
of granite containing gneiss, which Werner himself is said to be in
possession of, and to consider as a proof, that the schistus is of
greater antiquity than the granite. Such also seemed to me some pieces
of granite, which I met with in Cornwall, near the Land's End; and
others which I saw in Ayrshire, in loose blocks, on the sea coast
between Ayr and Girvan. It is impossible to deny that the containing
stone is more modern than the contained. The Neptunists indeed admit
this to be true, but allege, that all granite is not of the same
formation; and that, though some granite is recent, the greater part
boasts of the highest antiquity which belongs to any thing in the
fossil kingdom. This distinction, however, is purely hypothetical; it
is a fiction contrived on purpose to reconcile the fact here mentioned
with the general system of aqueous deposition, and has no support from
any other phenomenon.
2. _Granite of Portsoy._
288. The granite of Portsoy is one of the most singular varieties of
this stone, and is remarkable for this circumstance, that the feldspar
is the substance which has assumed the figure of its proper crystal,
and has given its form to the quartz, so that the latter is impressed
both with the acute and obtuse angles belonging to the rhombic figure
of the former. The angular pieces of quartz thus moulded on the
feltspar, and ranged by means of it in rows, give to this stone the
appearance of rude alphabetical writing.
Now, Dr Hutton argued, that substances precipitated from a solution,
and crystallizing at liberty, cannot be supposed to impress one another
in the manner here exemplified; and that they could do so only when
the whole mass acquired solidity at the same time, or at the same time
nearly.[146] Such simultaneous consolidation can be produced in no way
that we know of, but by the cooling of a mass that has been in fusion.
[Footnote 146: Theory of the Earth, vol. i. P. 104.]
289. A granite, brought from Daouria by M. PATRIN, and described by him
in the Journal de Physique for 1791, p. 295, under the name of _pierre
graphique_, seemed to Dr Hutton to have so great a resemblance to the
granite of Portsoy, that he ventured to consider them both as the same
stone, and as both containing quartz moulded on feltspar.[147] It
should seem, however, from further explanations, which M. Patrin has
since given, but Dr Hutton was mistaken in his conjecture, and that, in
the _pierre graphique_ of the former mineralogist, the quartz gives its
form to the feltspar, preserving in its crystals their natural angle of
120 degrees[148] It is impossible, I think, to doubt of the accuracy
of this statement; and the graphical stone of Portsoy must therefore
be admitted to differ materially from that of Daouria. They are not,
however, without some considerable affinity, besides that of their
outward appearance; for, though the quartz in the former is generally
moulded on the feltspar, the feldspar is also occasionally impressed by
the quartz, and sometimes even included in it. They may be considered
as varieties of the same species of granite; and the _pierre graphique_
of Corsica is probably a third variety, different from them both.
[Footnote 147: Trans. Royal Society Edin. vol. iii. p. 83.]
[Footnote 148: Journal Britainnique, (of Geneva,) 1798, vol. viii.
Sciences et Arts, p. 78.]
290. It would seem, however, that all these stones lead exactly to
the same conclusion. M. Patrin describes his specimen as containing
quartz crystals, that are for the most part only _cases_, having their
interior filled with feltspar "Le feltspath en masse contient des
crysteaux quartzeux, qui n'ont le plus souvent que la carcasse, et dont
l'interieur est rempli de feltspath; souvent il manque à ces carcasses
quelques unes de leurs faces, et souvent la section de cette pierre
dans un sens transversal aux crysteaux, presente une suite de figures
qui sont des portions d'hexagones, et qui ne resemblent pas mal à des
caractères Hebraiques."[149]
[Footnote 149: Journal Britannique, _Ibid._]
These imperfect hexagonal cases of quartz, filled with feldspar,
certainly indicate the crystallization of substances, which all
assumed their solidity at the same time, and, in doing so, constrained
the figures of one another. To use the words of Dr Hutton, "whether
crystallizing quartz inclose a body of feltspar, or concreting feltspar
determine the shape of fluid quartz, particularly if we have, as is
here the case, two solid bodies including and included, it amounts to
a demonstration, that those bodies have concreted from a fluid state
of fusion, have not crystallized, in the manner of salts, from a
solution."[150]
[Footnote 150: Trans. Royal Society Edin. _ubi supra_, p. 84.]
291. The quartz in granite so generally receives the impressions of
all the other substances, particularly of the feldspar and schorl,
and appears to be so passive a body, that it has been doubted by some
mineralogists, whether in this stone it ever assumes its own figure,
except where cavities afford room for its crystallization. But it
is certain that, beside the Daourian granite just mentioned, there
are others, in which the quartz is completely crystallized. Of this
sort are some specimens, found in a granite vein on the west side of
the hill of St Agnes, in Cornwall. The vein traverses the primitive
schistus, of which that hill consists, from south to north nearly:
the stone is much decomposed, and the feldspar in general is almost
reduced to the state of clay. In this decomposed mass, quartz crystals
are found, having the shape of double hexagonal pyramids, perfectly
regular and complete. The side of the hexagon, which is the base of the
two opposite pyramids, varies from half a tenth to a tenth of an inch
in length, and is the same with the altitude of each of the pyramids.
In some few specimens, the two pyramids do not rest on the same base,
but are separated by a very short, though regular, hexagonal prism.
The surfaces of these crystals are rough, and somewhat opaque, with
slender spiculæ of schorl frequently traversing them. This roughness
is occasioned by slight furrows on the surface of the crystal, very
regularly disposed, and parallel to one another, being without doubt
impressions from the thin plates of the feldspar, which surrounded
the crystal, and slightly indented it. They very much resemble some
impressions, remarked by Dr Hutton in the granite of Portsoy, and
ascribed by him also to a similar cause. He has represented these in
his Theory of the Earth, vol. i. plate ii. fig. 4. The action and
reaction of two crystallizing bodies, hardly admits of a stronger and
more unequivocal expression, than in these two instances.
Where the granite was little decomposed, the quartz was not easily
disengaged from the mass it was imbedded in, and often broke in pieces
before it could be extricated. The crystallization of the quartz,
therefore, would not have been discovered, but for the decomposition of
the feltspar; and it is probable, that similar crystallizations exist
in many granites where they are not perceived.
292. Some mineralogists are inclined to think, that the regular
crystallization of quartz is to be found only in what they call
secondary granites, or in those that are of a formation subsequent
to the great masses which constitute the granite mountains. It is
indeed true, that in the instances given here, both from Cornwall and
Daouria, the granites containing quartz crystals are from veins that
intersect the primary schistus, and are therefore, on every hypothesis,
of a formation subsequent to that schistus. But it does not follow
from thence, that they are less ancient than the great masses of
unstratified granite; with these last they are most probably coëval,
nor can there be any reason for thinking the crystallization of quartz
a mark of more recent formation than that of feltspar.
3. _Stratification of Granite._
293. What are the various modes in which granite exists, is a question
not absolutely decided among mineralogists. 1. That it exists as a
schistose stone of a fissile texture, in gneiss and _veined granite_,
is on all hands admitted, though in this state the name of granite
is generally withheld from it. 2. That it exists often without any
indication of a fissile texture, and altogether unstratified, is
likewise acknowledged. 3. That it is found in veins, intersecting
the strata, has been shown above. The only mode of its existence
subject to dispute, is that in which it is said to be stratified in
its outward configuration, but not schistose in its texture. On this
point mineralogists do not perfectly agree: Dr Hutton did not think
that this was a state in which granite ever appears. When not schistose
in its structure, he supposed it to be unstratified altogether; and
he considered it as a body which, like whinstone, was originally in
a state of igneous fusion, and, in that condition, injected among
the strata. The school of Werner, on the other hand, maintain, that
granite, if not always, is generally stratified, and disposed in beds,
sometimes horizontal, though more frequently vertical, or highly
inclined.
In forming an opinion where there are great authorities on opposite
sides, a man must trust chiefly to his own observations, and ought
to esteem himself fortunate if these lead to any certain conclusion.
Mine incline me to differ from Dr Hutton, on the one hand, and from
the Neptunists, on the other, as they convince me, that granite does
form strata where it has no character of gneiss; and, at the same
time, induce me to suspect, that the stratification ascribed by the
Neptunists to the granite mountains, is, in many instances, either an
illusion, or at least something very different from what, in other
stones, is accounted stratification.
294. The first example I ever saw of granite that was stratified,
and yet had no character of gneiss, was at Chorley Forest, in
Leicestershire. The greater part of that forest has for its base a
hornstone schistus, primary and vertical; and, on its eastern border,
particularly near Mount Sorrel, are beds of granite, holding the same
direction with those of the schistus. The stone is a real granite;
it has nothing in its internal structure of a schistose or fissile
appearance; and its beds, which it is material to remark, are no
thicker than those of the hornstone strata in the neighbourhood. This
granite is remarkable, too, for being close to the secondary sandstone
strata; I did not see their contact, but traced them within a small
distance of one another; so that I think it is not likely that any body
of rock intervenes. At the same time that I state my belief of this
rock of granite being in regular strata, I must acknowledge, that a
very intelligent mineralogist, who viewed these rocks at the same time,
and whose eye was well practised in geological observation, remained in
doubt concerning them.
295. Another instance of a real granite, disposed in regular beds, but
without any character of gneiss, is one which I saw in Berwickshire,
in Lammermuir, near the village of Priestlaw. The little river of
Fassnet cuts the beds across, and renders it easy to observe their
structure. The beds are not very thick; they run from about S.S.W. to
N.N.E. like the schistus on either side of them. I was in company with
Sir James Hall when I saw these rocks; we examined them with a good
deal of attention, and traced them for more than a mile in the bed of
the river; and, if I mistake not, our opinions concerning them were
precisely the same.
296. What exists in two instances may exist in many, and, after these
observations, I should be guilty of great inconsistency, in refusing
to assent to the accounts of Pallas, De Luc, Saussure, and many other
mineralogists, who so often represent granite as formed into strata.
In some cases, however, it is certain, that the stratification they
describe is extremely unlike that in the two instances just mentioned,
and indeed very unlike any thing that is elsewhere known by the name
of stratification. For example, the stratification must be very
ambiguous, and very obscurely marked, that was not discovered till
after a series of observations, continued for more than twenty years,
by a very skilful and distinguishing mineralogist. Yet such undoubtedly
is the stratification of Mont Blanc, and of the granite mountains in
its neighbourhood, as it escaped the eyes of Saussure, in the repeated
visits which he made to them, during a period of no less extent than
has just been mentioned. It was not till near the conclusion of those
labours, to which the geologists of every age will consider themselves
as highly indebted, that, having reached the summit of Mont Blanc, he
perceived, or thought that he perceived, the stratification of the
granite mountains. The _Aiguilles_ or Needles which border the valley
of Chamouni, and even Mont Blanc itself, appeared to be formed of vast
tabular masses of granite, in position nearly vertical, and so exactly
parallel, that he did not hesitate to call them by the name of strata.
Till this moment, these same mountains, viewed from a lower point,
had been regarded by him as composed of great plates of rock, nearly
vertical indeed, but applied, as it were, round an axis, and resembling
the leaves of an artichoke;[151] and the fissures by which they are
separated from one another, had been considered as effects of waste and
degradation. "But now," (says he, speaking of the view from the top of
Mont Blanc,) "I was fully convinced, that these mountains are entirely
composed of vast plates of granite, perpendicular to the horizon, and
directed from N.E. to S.W. Three of these plates, separated from each
other, formed the top of the _Aiguille du Midi_, and other similar
plates, decreasing gradually in height, compose its declivity to the
south."[152]
[Footnote 151: Voyages aux Alpes, tom. ii. § 910, &c.]
[Footnote 152: Voyages aux Alpes, tom. iv. § 1996.]
297. Saussure was so strongly impressed with the appearances of what
he accounted regular stratification, such as water only can produce,
and such as must have been in the beginning horizontal, that, placed
as he now was, on one of the highest points of the earth's surface, he
formed the bold conception, that the summit on which he was standing
had been once buried under the surface, to the depth at least of half
the diameter of the mountain, and horizontally distant from its present
place by a line not less than the whole height of the mountain; the
granite beds which compose that mountain, having been raised by some
enormous power from their horizontal position, and turned as on an
axis, till they were brought into the vertical plane. In this notion,
which suits so well with the nature of mountains really composed of
vertical strata, and which does credit to the extent of Saussure's
views, it is wonderful that he did not see the overthrow of the
geological system be had adopted, which is provided with no means
whatsoever of explaining these great effects.
Such, then, were the ideas suggested to Saussure, by viewing the
mountains of the Alps from the highest of their summits. His great
experience, his accurate knowledge of the objects before him, and the
power he had acquired of dissipating those illusions, to which, in
viewing mountainous tracts, the eye is peculiarly subject, all conspire
to give great weight to his opinion. Yet, as this opinion is opposed
by that which he himself had so long entertained, before it can be
received with perfect confidence, it will require to be verified by new
observations. It seems certain, that the beds of rock here described,
differ from all ordinary strata, both horizontal and vertical, in the
circumstance of their vast thickness, three of them being so large as
to form the main body of a mountain. Their parallelism cannot easily
be ascertained; and they have at best but a very slight resemblance to
such beds as water is known to produce.
298. Their parallelism is difficult to be ascertained; for, on account
of the magnitude and inaccessibility of the objects, it is impossible
to place the eye in any situation, where it shall not be much nearer to
one part of the planes whereof the parallelism is to be estimated, than
to another. Indeed, one can perceive a cause which may have rendered
the parallelism of the plates of granite which compose the _aiguilles_,
more accurate in appearance than in reality, when viewed from a point
so elevated as the summit of Mont Blanc. For, even on the supposition
that the comparison of those plates to leaves of artichokes was just,
and that the planes of their separation converged toward one another,
in ascending to the top, when they were viewed from a point more
elevated than that top, this convergency would be diminished, and, by
the force of the perspective, might even be converted into parallelism.
We cannot at present ascertain what effect this cause of deception may
have actually produced.
299. The observations of Saussure concerning the stratification
of granite, are not, however, in all instances, liable to these
objections; and it seems to be on much less exceptionable grounds
that he pronounces the granite of St Gothard to be stratified. The
gneiss and micaceous schistus which constitute the lower part of that
mountain, are succeeded by a granite without any schistose appearance,
but divided into large plates, exactly parallel to the beds of the
former gneiss. These he regards as real strata. On studying them in
detail, he says, considerable irregularities were to be observed, but
not greater than in the case of limestone or micaceous schistus.[153]
It may be inferred from this, that these plates of granite are not so
thick but that they admit of comparison with beds that are known with
certainty to be of aqueous formation, and I am therefore disposed to
believe, that the granite of St Gothard, in this part at least, is
stratified. The transition from gneiss to granite en masse, is not
uncommon, as Saussure has observed in other instances, and as we are
just about to consider more particularly.
[Footnote 153: Voyages aux Alpes, tom. iv. § 1830.]
300. In the mountains of our own country, some difficulties concerning
the stratification of granite have also occurred. In Arran, for
instance, the mountain of Goatfield, which I have mentioned above
as affording an instance of granite sending out many veins into
the schistus, and rivetted, as it were, by means of them to the
superincumbent rock, when I visited it, with, a view of verifying
on the spot the interesting observations which Dr Hutton had there
made, appeared to me to be without any vestige of stratification in
its granitic part, as did also the whole group of mountains to which
it belongs. It was, therefore, not without a good deal of surprise,
that I lately read, in an account of that island, by a very accurate
and ingenious mineralogist, that Goatfield consists of stratified
granite.[154] The impression which the appearance of that mountain
made on my mind, is just the reverse; and though I saw large tabular
masses, sometimes nearly vertical, separated by fissures, they
appeared to be much too irregular, too little extended in length and
height, and vastly too much in thickness, to be reckoned the effects
of stratification. For all this, I would by no means be understood
to set my observations in opposition to those of Mr Jameson. In my
visit to Arran, I did not direct my inquiries much toward this point;
the general appearance of the rocks did not suggest the necessity of
doing so, and I was not perfectly aware how much the stratification of
granite had been insisted on by some mineralogists; so that I applied
myself entirely to study some other of the interesting phenomena which
this little island offers in so great abundance. I therefore carry
my confidence in the appearances which seemed to indicate a want of
stratification in the granite of Arran no further than to remain
sceptical both as to Mr Jameson's conclusions and my own, till an
opportunity shall occur of verifying the one or the other by actual
observation.
[Footnote 154: Mineralogy of the Scottish Isles, vol. i. p. 35, 36.]
301. The stratification of granite, though it made no part of Dr
Hutton's system, does by no means embarrass his theory with any new
difficulty. Rocks, of which the parts are highly crystallized, are
already admitted as belonging to the strata, and are exemplified in
marble, gneiss, and veined granite. In the two last, we have not only
stratification, but a schistose, united with a crystallized structure,
and the effects of deposition by water, and of fluidity by fire, are
certainly no where more singularly combined. The stratification of
these substances is therefore more extraordinary than even that of the
most highly crystallized granite. Neither the one nor the other can be
explained but by supposing, that while such a degree of fluidity was
produced by heat, as enabled the body when it cooled to crystallize,
the whole mass was kept in its place by great pressure acting on all
sides, so that the shape was preserved as originally given to it by
the sea. As we cannot, however, suppose, that the intensity of the
heat, or the fusibility of the substance through all the parts of a
stratum, were precisely the same, we may expect to find in the same
stratum, or in the same body of strata, that in some parts the marks
of stratification are completely obliterated while in others they
remain entire. It is thus that _veined granite_, of what I think should
be called granitic schistus, often graduates into granite _in mass_,
that is, granite without any schistose or fissile texture. Saussure
says, that to be veined or not veined, is an affection of granite,
that seems, in many cases, accidental;[155] as, in the midst of rocks
of that substance, most clearly fissile, large portions appear without
any vestige of stratification. Of this phenomenon, which is frequent
in the Alps, instances are also to be met with in the granite rocks
of Scotland, and the adjacent isles; and I know that Dr Hope, in a
mineralogical excursion which he lately made among the Hebrides,
observed many interesting and curious examples of it. Indeed, when
rocks were so much fused as to crystallize, and so compressed, at the
same time, as to remain stratified, they were evidently on the verge of
change; two opposite forces were very nearly balanced, and each carried
as far as it could go without entirely overcoming the other; so that
a small alteration in the conditions may have made a great alteration
in the effects. Hence a sudden transition from a stratified to an
unstratified texture, which is only found in rocks highly crystallized,
and such as have endured the most violent action of the mineralizing
powers.
[Footnote 155: Voyages aux Alpes, tom. iv. § 2143.]
302. Now, though the stratification of granite, or the mixture of the
stratified with the unstratified rocks of that genus, is not only
reconcilable with the principles of the Huttonian geology, but might
even have been deduced as a corollary from those principles, before
it was actually observed, it may be considered as inconsistent with
the theory of granitic veins that has just been given. A stratum,
though soft or fluid, could not invade the surrounding strata with
violence, nor send out veins to penetrate into them. It might, if,
strongly compressed by another stratum less fluid than itself, fill up
any fissures or cracks that were in that other, but this would hardly
produce, such large veins, and of such considerable length, as often:
penetrate from the granite into the schistus, nor could it give rise
to any appearance of disturbance. If, therefore, veins were found
proceeding from such stratified granite as that of Chorley Forest or
Lammermuir, I should think, that the explanation of them was still a
_desideratum_ in geology. The Neptunian theory of infiltration would
indeed be as applicable to them as to any other veins; for it is but
little affected by the condition of the phenomena to be explained.
Indeed, it is very difficult to set any limits to the explanations
which this theory affords; and it would certainly puzzle a Neptunist,
to assign any good reason why infiltration has not produced veins of
one schistus running into another, or veins of schistus running into
granite, as well as of granite running into, schistus. He will find it
a hard task to restrain the activity of his theory, and to confine its
explanations to those things that really exist.
303. As the Huttonian system cannot boast of theories of equal
versatility, it would be not a little embarrassed to account for veins
of great magnitude proceeding from a rock distinctly stratified, and
accompanied with marks of having disturbed the rocks through which they
pass. I am, however, inclined to believe, that this embarrassment will
never occur; and that the granite veins do not proceed from the rocks
that are really stratified, but from such as have never been deposited
by water, and where the appearances of stratification, if there are
any, are altogether illusory. This anticipation, however, requires to
be verified by future observation; and it remains to be seen, whether
granitic veins ever accompany real granitic strata, or are peculiar to
those in which the appearances of regular beds are either ambiguous,
or are entirely wanting. The decision of this question is an object
highly worthy of the attention of geologists.
304. An argument, directed at once against the igneous origin and
unstratified nature of all granite, is given in a work already
mentioned. "If granite had flowed from below, how does it happen, that,
after it had burst through the strata of micaceous schistus, &c. it did
not overflow the neighbouring country? If this hypothesis were true,
Mont Blanc could never have existed."[156]
[Footnote 156: Mineralogy of the Scottish Isles, vol. ii. p. 166.]
A theory is never more unfairly dealt with, than when those parts
are separated which were meant to support one another, and each left
to stand or fall by itself. This, however, is precisely what is done
in the present instance; for Dr Hutton's theory of granite would
not deserve a moment's consideration, if it were so inartificially
constructed, as to suppose that granite was originally fluid, and yet
to point out no means of hindering this fluid from diffusing itself
over the strata, and settling in a horizontal plane. The truth is, that
his theory, at the same time that it conceives this stone to have been
in fusion, supposes it to have been, in that state, injected among the
strata already consolidated; to have heaved them up, and to have been
formed in the concavity so produced, as in a mould. Thus Mont Blanc,
supposing that it is unstratified, is understood to consist of a mass
that was melted by subterraneous heat under the strata, and being
impelled upwards by a force, that may stand in some comparison with
that which projected the planets in their orbits, heaved up the strata
by which it was covered, and in which it remained included on all sides.
305. The covering of strata, thus raised up, may have been burst
asunder at the summit, where the curvature and elevation were the
greatest; but the melted mass underneath may have already acquired
solidity, or may have been sustained by the beds of schistus incumbent
on its sides. This schistus, forming the exterior crust, was
immediately acted on by the causes of waste and decomposition, which
have long since stripped the granite of a great part of its covering,
and are now exercising their power on the central mass. That even Mont
Blanc itself, as well as other unstratified mountains, was once covered
with schistus, will appear to have in it nothing incongruous, when we
consider the height to which the schistus still rises on its sides,
or in the adjacent mountains; and when we reflect, that, from the
appearances of waste and degradation which these mountains exhibit, it
is certain, that the schistus must have reached much higher than it
does at present.
It is obvious, therefore, that when the corresponding parts are brought
together, and placed in their natural order, no room is left for the
reproach, that this system is inconsistent with the _existence_ of
granite mountains. I have no pleasure in controversial writing; and,
notwithstanding the advantages which a weak attack always gives to a
defender, I cannot but regret, that Dr Hutton's adversaries have been
so much more eager to refute than to understand his theory.
[Illustration]
306. A remark which Dr Hutton has made on the quantity of granite that
appears at the surface, compared with that of other mineral bodies,
has been warmly contested. Having affirmed, that the greater part of
rocks bear marks of being formed from the waste and decomposition of
other rocks, he alleges that granite, (a stone which does not contain
such marks) does not, for as much as appears from actual observation,
make up a tenth, nor perhaps even a hundredth part of the mineral
kingdom.[157] Mr Kirwan contends, that this is a very erroneous
estimate, and that the quantity of granite visible on the surface, far
exceeds what is here supposed.[158] The question is certainly of no
material importance to the establishment of Dr Hutton's theory: it is
evident, too, that an estimation, which varies so much as from a tenth
to a hundredth part, cannot have been meant as any thing precise; yet
it may not be quite superfluous to show, that the truth probably lies
nearer to the least than the greatest of the limits just mentioned.
[Footnote 157: Theory of the Earth, vol. i. p. 211.]
[Footnote 158: Geol. Essays, p. 480.]
307. Though granite forms a part, generally the central part, of all
the great chains of mountains, it usually occupies a much less extent
of surface than the primary schistus. Thus in the Alps, if a line be
drawn from Geneva to Ivrea, it will be about eighty-five geographical
miles in length, and will measure the breadth of this formidable
chain of mountains, at the place of its greatest elevation. Now, from
the observations of Saussure, who crossed the Alps exactly in this
direction, it may be collected, that less than nine miles of this line,
or not above a tenth part of it, in the immediate vicinity of Mont
Blanc, is occupied by granite.
308. In some sections of the Alps, no granite at all appears. Thus, in
the route from Chambery to Turin, across Mont Cenis, which measures by
the road not less than ninety miles, no granite is found, at least of
that kind which is distinctly in mass, and different from gneiss or
veined granite.[159]
[Footnote 159: Voyages aux Alpes, tom. iii. § 1190, &c.]
309. In some other places of the same mountains, the granite is more
abundant. A line from the lake of Thun, along the course of the Aar,
and over the mountains to the upper end of Lago Maggiore, crosses a
very elevated tract, and passes by the sources of the Rhone, the Rhine,
and the Tessino, which last runs into the Po. A good deal of granite is
discovered here, in the mountains of Grimsel and St Gothard; but by far
the greater part of it is the veined granite, the granite in mass being
confined chiefly to the north side of the Grimsel. Both together do not
occupy more than one-third of the line, and therefore the latter less
than one-sixth.
310. The essay on the mineralogy of the Pyrenees, by the Abbé PALASSO,
contains a mineralogical chart of those mountains. From this chart I
have found, by computation, that the granite does not occupy one-fifth
of the horizontal surface on the north side of the ridge, reckoning
from one end of it to the other. Indeed, many great tracts, even of the
central parts of the Pyrenees, contain no granite whatsoever; and not a
few of the highest mountains consist entirely of calcareous schistus.
A large deduction should be made from the fraction 1/5 on account of
the substances unknown, which, from the construction of the chart, are
often confounded with the granitic tract.
311. I might add other estimations of the same kind, all confessedly
rude and imperfect, but still conveying, by means of numbers, a better
idea of the limit to which our knowledge approximates, than could be
done simply by words; and, on the whole, it would appear, that if we
state the proportion of granite to schistus to be that of one to four,
we shall certainly do no injustice to the extent of the former.
It remains to form a rough estimate from maps, and from the accounts
of travellers, of what proportion of the earth's surface consists
of primary, and what of secondary rocks. After supplying the want
of accurate measurement by what appeared to me the most probable
suppositions, I have found, that about 1/18 of the surface of the old
continent may be conceived to be occupied by primitive mountains; of
which, if we take one-fifth, we have 1/90 for the part of the surface
occupied by granite rocks, which differs not greatly from the least of
the two limits assigned by Dr Hutton.
312. In estimating the granite of Scotland, Dr Hutton has certainly
erred considerably in defect,[160] and Mr Kirwan, who always differs
from him, is here nearest the truth; though he is right purely by
accident, as the information on which he proceeds is vague and
erroneous.
[Footnote 160: Dr Hutton in this case no doubt made a very loose
estimate. He says, the granite does not perhaps occupy more than a
500dth part of the whole surface. The whole surface of Scotland is not
much more than 23,000 geographical miles, the 500dth part of which is
exactly 46; and this is exceeded by the granite in Kirkcudbrightshire
done, as may be gathered from what is said § 283.]
The places in Scotland where granite is found, are very well known; but
the extent of some of the most considerable of them is not accurately
ascertained. In the southern parts, except the granite of Galloway,
which is found in two pretty large insulated tracts, there is no
other of any magnitude. The granite of the north extends over a large
district. If we suppose a line to be drawn, from a few miles south
of Aberdeen to a few miles south of Fort William, it will mark out
the central chain of the Grampians in its full extent, passing over
the most elevated ground, and by the heads of the largest rivers,
in Scotland. Along this line there are many granite mountains, and
large tracts in which granite is the prevailing rock. There are,
however, large spaces also in which no granite appears, though, if we
were permitted to speak theoretically, and if the question did not
entirely relate to a matter of observation, we might suppose, that,
in no part of this central ridge is the granite far from the surface,
notwithstanding that in some places it may be covered by the schistus.
313. A great part of the Grampian mountains is on the south side of the
line just mentioned, but hardly any granite is found in this division
of them, except such veins as those of Glentilt. On the north side of
the line, the granite extends in various directions; and, if from Fort
William a line is drawn to Inverness, the quadrilateral figure, bounded
on two sides by these lines, and on the other two by the sea, will be
found to contain much granite, and many districts consisting entirely
of that stone. This is in fact the great granite country of Scotland:
it is a large tract, containing about 3170 square geographical
miles, or about a seventh part of the whole: but the proportion of
it occupied by granite cannot at present be ascertained with any
exactness, nor will, till some mineralogist shall find leisure to
examine the courses of the great rivers, the Dee, the Spey, &c. which
traverse this country. If we call it one-fourth of the whole surface,
its extent is certainly not underrated, and will amount to 790 square
miles nearly; to which adding 150, as a very full allowance for all the
other granite contained in Scotland, exclusive of the isles, we shall
have 940 square miles, between a twenty-fourth and twenty-fifth part of
the surface of the whole.
This computation, it must be observed, aims at nothing precise, but I
think it is such, that a more accurate survey would rather diminish
than increase the proportion assigned in it to the granite rock.
314. This result may perhaps fall as much short of Mr Kirwan's notion,
as it exceeds the estimate made by Dr Hutton. If it shall not, and if
the former has, in this instance, come nearest the truth, it cannot
be ascribed to the accuracy of his information, or the soundness of
the principles which directed his research. Mr WILLIAMS, whom he
quotes, was a miner, of great skill and experience in some branches
of his profession, to which, if he had confined himself, he might
have written a book full of useful information. What he says on the
subject of granite, is, in the main I believe just; but it is far too
general to authorize the conclusion which Mr Kirwan derives from it.
Dr Ash, for whose judgment I have great respect, cannot, I think, have
meant, when he used the expression granitic rocks, to describe granite
strictly so called. He says, in the passage quoted by Mr Kirwan, that
"from Galloway, Dumfries, and Berwick, there is a chain of mountains,
commonly schistose, but often also granitic." Now, the fact is, that
the great belt of primary rock, here alluded to, which traverses the
south of Scotland, consists of vertical schistus of various kinds;
but except in Galloway, and again in Lammermuir, near Priestlaw, it
appears, as already mentioned, to contain no granite whatsoever. If the
German mineralogist quoted by Mr Kirwan, when he says that the Grampian
mountains consist of micaceous limestone, gneiss, porphyry, argillite,
and granite, alternating with one another, means only to affirm that
all these stones are found in the Grampians, he is certainly in the
right, and the catalogue might easily be enlarged; but, if he either
means to say, that these are nearly in equal abundance, or that the
granite is commonly found in strata alternating with other strata, I
must say, that these are propositions quite contrary to any thing
I have ever seen or heard of those mountains. But it is probable
that this is not meant, and that the fault lies in understanding
the expressions much too literally. Mr Kirwan accuses Dr Hutton of
not knowing where to look for the granite; not aware of how much,
notwithstanding any error committed in the present estimate, he was
skilled in the art of mineralogical observation; an art, which those
who have not practised do not always know how to appreciate. But,
however imperfect Mr Kirwan's knowledge of this subject has been,
he has here had the good fortune to correct a mineralogist of very
superior information. The mere disposition to oppose is not always
without its use: no man is in every thing free from error, and, to
controvert indiscriminately all the opinions of any individual, is an
infallible secret for being sometimes in the right.
NOTE XVI. § 100.
_Rivers and Lakes._
315. Rivers are the causes of waste most visible to us, and most
obviously capable of producing great effects. It is not, however, in
the greatest rivers, that the power to change and wear the surface of
the land is most clearly seen. It is at the heads of rivers, and in the
feeders of the larger streams, where they descend over the most rapid
slope, and are most subject to irregular or temporary increase and
diminution, that the causes which tend to preserve, and those that tend
to change the form of the earth's surface, are farthest from balancing
one another, and where, after every season, almost after every flood,
we perceive some change produced, for which no compensation can be
made, and something removed which is never to be replaced. When we
trace up rivers and their branches toward their source, we come at
last to rivulets, that run only in time of rain, and that are dry, at
other seasons. It is there, says Dr Hutton, that I would wish to carry
my reader, that he may be convinced, by his own observation, of this
great; fact, _that the rivers have, in general, hollowed out their
valleys_. The changes of the valley of the main river are but slow;
the plain indeed is wasted in one place, but is repaired in another,
and we do not perceive the place from whence the repairing matter has
proceeded. That which the spectator sees here, does not therefore
immediately suggest to him what has been the state of things before
the valley was hollowed out. But it is otherwise in the valley of the
rivulet; no person can examine it without seeing, that the rivulet
carries away matter which cannot be repaired, except by wearing away
some part of the surface of the place upon which the rain that forms
the stream is gathered. The remains of a former state are here visible;
and we can, without any long chain of reasoning, compare what has been
with what is at the present moment. It requires but little study to
replace the parts removed, and to see nature at work, resolving the
most hard and solid masses, by the continued influences of the sun and
atmosphere.[161] We see the beginning of that long journey, by which
heavy bodies travel from the summit of the land to the bottom of the
ocean, and we remain convinced, that, _on our continents, there is no
spot on which a river may not formerly have run_.[162]
[Footnote 161: Theory of the Earth, vol. ii. 294.]
[Footnote 162: _Ibid._ p. 296.]
316. The view thus afforded of the operations, in their nascent state,
which have shaped out and fashioned the present surface of the land,
is necessary to prepare us for following them to the utmost extent
of their effects. From these effects, the truth of the proposition,
that rivers have cut and formed, not the beds only, but the whole of
the valleys, or rather system of valleys, through which they flow, is
demonstrated on a principle which has a close affinity to that on which
chances are usually calculated, § 99. In order to conceive rightly the
course of a great river, and the communication subsisting between the
main trunk and its remotest branches, let us take the instance of the
Danube and cast our eyes on one of the maps constructed by MARSIGLI,
for illustrating the natural history of that great river.[163] When
it is considered, that over all the vast and uneven surface, which
reaches from the Alps to the Euxine, and from the mountains of Crapack
to those of Hæmus, a regular communication is kept up between every
point and the line of greatest depression, in which the river flows, no
one can hesitate to acknowledge, that it is the agency of the waters
alone which has opened them a free passage through all the intricacies
of this amazing labyrinth. In effect, suppose this communication to be
interrupted, and that some sudden operation of nature were to erect a
barrier of mountains to oppose the Theise or the Drave, as they rolled
their waters to the Danube. From this what could possibly result, but
the damming up of those rivers till their waters were deep, or high
enough to find a vent, either under the bases or over the tops of the
opposing ridge. Thus there would be formed immense lakes and immense
cataracts, which, by filling up what was too low, and cutting down what
was too high, would in time restore such a uniform declivity of surface
as had before prevailed. Just so in the times that are past, whatever
may have been the irregularities of the surface at its first emerging
from the sea, or whatever irregularities may have been produced in
it by subsequent convulsions, the slow action of the streams would
not fail in time to create or renew a system of valleys communicating
with one another, like that which we at present behold. Water, in all
circumstances, would find its way to the lowest point; though, where
the surface was quite irregular, it would not do so till after being
dammed up in a thousand lakes, or dashed in cataracts over a thousand
precipices. Where neither of these is the case; and where the lake and
the cataract are comparatively rare phenomena; there we perceive that
constitution of a surface, which water alone, of all physical agents,
has a tendency to produce; and we must conclude, that the probability
of such a constitution having arisen from another cause, is, to the
probability of its having arisen from the running of water, in such a
proportion as unity bears to a number infinitely great.
[Footnote 163: Histoire du Danube, tom. i. tab. 34.]
317. The courses of many rivers retain marks that they once consisted
of a series of lakes, which have been converted into dry ground, by the
twofold operation of filling up the bottoms, and deepening the outlets.
This happens, especially, when successive terraces of gravelly and
flat land are found on the banks of a river, § 100. Such platforms, or
_haughs_ as they are called in this country, are always proofs of the
waste and _detritus_ produced by the river, and of the different levels
on which it has run; but they sometimes lead us farther, and make it
certain, that the great mass of gravel which forms the successive
terraces on each side of the river, was deposited in the basin of a
lake. If, from the level of the highest terrace, down to the present
bed of the river, all is alluvial, and formed of sand and gravel, it
is then evident, that the space as low as the river now runs must have
been once occupied by water; at the same time, it is dear, that water
must have stood, or flowed as high at least, as the uppermost surface
of the meadow. It is impossible to reconcile these two facts, which are
both undeniable, but by supposing a lake, or body of stagnant water,
to have here occupied a great hollow, (which by us must be held as
one of the original inequalities of the globe, because we can trace
it no farther back,) and that this hollow, in the course of ages, has
been filled up by the gravel and alluvial earth brought down by the
river, which is now cutting its channel through materials of its own
depositing. There is no great river that does not afford instances
of this, both in the hilly part of its course, and where it descends
first from thence into the plain. Were there room here for the minuter
details of topographical description, this might be illustrated by
innumerable examples.