THE ACID ROCKS
Their Petrography--Their Stratigraphical Position and its
Analogies in Central France
We now come to the examination of another distinct phase of volcanic
action during Tertiary time in Britain. The igneous rocks that have
been under consideration in the foregoing chapters, whether poured
out at the surface or injected below ground, have been chiefly of
basic, partly indeed, like the peridotites, of ultra-basic character.
Some, however, have shown an andesitic or intermediate composition.
Reference has also been made to the probable eruption of acid rhyolites
in the long interval between the outflow of the lower and the upper
basalts in Antrim. But we now encounter a great series, decidedly acid
in composition, in the more largely crystalline members of which the
excess of silica is visible to the eye in the form of free quartz.
While there is a strong contrast in chemical composition between this
series and the rocks hitherto under discussion, there are also marked
differences in structure and mode of occurrence. Like the gabbros,
all the masses of acid rock now visible appear to be intrusive. They
have been injected beneath the surface, and therefore record for us
subterranean rather than superficial manifestations of volcanic action.
The existence of rocks of this class in the midst of the basic
masses has long been recognized. They were noticed by Jameson, who
described the hills between Loch Sligachan and Broadford as composed
of "a compound of felspar and quartz, or what may be called a
granitel, with occasional veins of pitchstone."[359] Macculloch gave a
fuller account of the same region, and classed the rocks as chiefly
"syenite" and "porphyry."[360] In Antrim, also, even in the midst of the
basalt-tableland, masses of "pitchstone-porphyry "pearlstone-porphyry,"
"clay-porphyry," and "greystone" were observed and described.[361] In
more recent years Professor Zirkel has given a brief account of the
so-called "syenite and porphyry" of Mull and Skye,[362] and the late
Professor Von Lasaulx fully described the "trachyte" or rhyolite of
Antrim.[363]
[Footnote 359: _Mineralogical Travels_, ii. 90.]
[Footnote 360: _Western Isles_, see the descriptions of Skye, Mull and
Rum.]
[Footnote 361: Berger, _Trans. Geol Soc._ iii. (1816), p. 190; Portlock,
_Journ. Geol. Soc. Ireland_, vol. i. (1834), p. 9.]
[Footnote 362: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871), pp.
54, 77, 84, 88.]
[Footnote 363: Tschermak's _Min. und Petrog. Mittheilungen_, 1878, p.
412. The chemical composition of this rock and its place among the
rhyolites had already been determined by E. T. Hardman from analysis,
_Journ. Geol. Soc. Ireland_, vol. iii. (1871), p. 32.]
This interesting series of rocks embraces a greater variety of
petrographical characters than any other portion of the British
Tertiary volcanic rocks. On the one hand, it presents thoroughly
vitreous masses, some of which in their colour, lustre and microscopic
structure remind us of recent obsidians. On the other hand, it affords
coarsely crystalline compounds, to which no other name than granite
can be assigned, and which, did we not know their geological position,
might almost be classed with some of the most ancient eruptive rocks.
Between these two extremes abundant gradations may be found, including
beautiful spherulitic rocks, felsites and rhyolites.
In dealing with such a series of intrusive rocks, we again encounter
the difficulty of reaching certainty as to their relative dates of
eruption, since in each case all that can usually be affirmed is that
the intrusive mass is younger than that into which it is injected. It
is quite possible that protrusions of acid rocks occurred at intervals
during the accumulation of the basic masses, as may perhaps be inferred
from the rhyolite-tuffs and conglomerates of Antrim and from the
occurrence of fragments of siliceous lavas in the gravels near the base
of the basalt-plateau of Mull, and in the agglomerates of that island
as well as of other districts.[364] It is probable, therefore, that at
the time when the basalts of the plateaux were emitted, there existed,
within reach of volcanic explosions, masses of granophyric, felsitic
or rhyolitic rocks, fragments from which were shot up the funnels of
discharge. That portions of these rocks were actually intruded into
the basalt-sheets before the building up of the plateaux was completed
appears to be proved in Antrim. Elsewhere, however, no evidence has
yet been obtained of any such intrusion until after the close of the
plateau-period. On the contrary, in every case where the relative
ages of the rocks can be fixed, the acid are younger than the basic
protrusions.
[Footnote 364: Reference may also again be made to the agglomerates of
Strath, Skye, which contain in some parts abundant fragments of acid
rocks that closely resemble some of the masses of granophyre which
disrupt these agglomerates.]
The only known exceptions to this rule are the latest basalt-dykes.
Hence, while amid the large and varied series of acid rocks, which
no doubt represents a wide interval of time, some may belong to
comparatively early epochs in the protracted volcanic period, the
actual available evidence places the emission of these rocks, as a
whole, towards the end of the volcanic history. This evidence I shall
bring, forward in full detail, since it necessitates an abandonment of
what has been the general belief in regard to the relative ages of the
rocks.
i. PETROGRAPHY OF THE ACID ROCKS
The classification of the rocks which best harmonizes the
field-evidence and the detailed study of their mineralogical
composition, is one that arranges these volcanic protrusions into two
series. In the one, the orthoclase is sanidine, and the rocks range
from the most vitreous pitchstone through perlitic and spherulitic
varieties to rhyolite ("quartz-trachyte"). In the other series, which
embraces by far the largest proportion of the whole, the orthoclase is
always turbid, and in this respect as well as in many others the rocks
remind us rather of ancient eruptive masses than of those which have
appeared in Tertiary time. They range from flinty felsitic varieties,
which are obviously devitrified glasses, through different textures of
quartz-porphyry into granophyre, and finally into granite. As I have
been unable to recognize any essential difference of structure and
composition between these acid Tertiary rocks and those of far earlier
geological time, I give them the names which no petrographer would
hesitate to apply to them if they were of Palæozoic age. It has long
appeared to me that these rocks furnish conclusive evidence of the
misleading artificiality of any petrographical nomenclature in which
relative antiquity is made an essential element of discrimination.
_Granite._--That true granites form part of the Tertiary volcanic
series of the British Isles has now been completely established. They
occur as bosses and sills which have been intruded into the gabbros
and all older rocks. They are thus proved not only to belong to the
Tertiary period, but to one of the latest phases of its volcanic
history. But besides these granites, the relative age of which can be
definitely fixed, there occur others which, standing alone and at some
distance from the basaltic plateaux, can only be inferentially classed
in the Tertiary series. To this group belong the granite masses of the
Isle of Arran and the Mourne Mountains in north-eastern Ireland.
Taking first the unquestionably Tertiary granites which occur as
bosses and intrusive sheets, we have to note that the more coarsely
crystalline granophyres are hardly to be distinguished externally from
granite. As the dark ferro-magnesian constituent of these rocks was
generally believed to be hornblende, they were called by the older
petrographers "syenite"; that is, granite with hornblende instead of
mica. The peculiar micropegmatitic groundmass, which constitutes the
distinguishing feature of the granophyres, may occasionally be observed
so reduced in amount as only to appear here and there between the
other minerals, which are grouped in a granitic structure. From this
condition, one step further carries us into a true granite, from which
all trace of the granophyric character has disappeared. Such gradations
may be traced even within short distances in the same boss of rock.
Thus, in the hornblende-biotite-granite boss of Beinn-an-Dubhaich,
Skye, a thoroughly granitic arrangement of the component minerals is
observable in the centre, while a specimen taken from near the edge
on the shore of Camas Malag shows the development of a granophyric
groundmass. But, though the large bosses are usually somewhat coarsely
crystalline in the centre, and tend to assume finer felsitic textures
around their borders, as was observed long ago by Oeynhausen and Von
Dechen,[365] the granitic structure is sometimes exhibited even at the
very edge, and not only so, but in the dykes that protrude from the
bosses into the surrounding rocks. Thus the Beinn-an-Dubhaich mass, at
its margin in Camas Malag, sends a vein into the surrounding limestone,
but though more close-grained than the main body of the rock, this vein
is neither felsitic nor granophyric, but truly granitic in structure.
[Footnote 365: Karsten's _Archiv_, i. p. 89.]
So far as I have observed, the true granites contain a brown mica and
also a little hornblende, both visible to the naked eye, but generally
somewhat decomposed. These rocks are thus hornblende-biotite-granites
(amphibole-granitites of Rosenbusch). They may be defined as
medium-grained aggregates of quartz, orthoclase (also plagioclase),
biotite and hornblende, with sometimes magnetite, apatite, epidote and
zircon. Dr. Hatch found that in some instances (Beinn-an-Dubhaich) the
quartz contains minute inclusions (glass?), bearing immovable bubbles
with strongly-marked contours; while in others (Beinn-na-Chro, Skye)
this mineral is full of liquid inclusions with bubbles, sometimes
vibratile, sometimes fixed. He remarked that the quartz and felspar
have consolidated almost simultaneously, but that in some instances
(Marsco, Glen Sligachan) there are isolated roughly idiomorphic
crystals, of a white, less turbid orthoclase, which belong to a
slightly earlier consolidation than that of the more kaolinized felspar
of the rest of the rock.
The granite of the island of Arran, in the Birth of Clyde, which is
here included in the Tertiary volcanic series, has long been recognized
as consisting of two distinct portions, an eastern or coarse-grained,
and a western or fine-grained variety. The latter sends veins into the
former. These granites contain orthoclase, plagioclase, quartz and dark
mica, the quartz being often idiomorphic with respect to the felspar,
and a tendency towards a micropegmatitic structure being sometimes
observable. A distinguishing characteristic of the Arran granite is
the cavernous or drusy structure which it presents, the cavities being
often lined with well-crystallized orthoclase and smoky quartz.[366] The
granite of the Mourne Mountains in Ireland closely resembles that of
Arran. Its druses, with their beautifully terminated minerals, have
long been well known.
[Footnote 366: See Mr. Teall's _British Petrography_, p. 328.]
_Microgranite._--This term is applied to certain intrusive masses,
which megascopically may be classed with the quartz-porphyries
and felsites, but which microscopically are found to possess a
holocrystalline granitic groundmass of quartz and orthoclase, through
which are scattered porphyritic crystals of the same two minerals,
sometimes also with plagioclase, augite, magnetite or apatite. Rocks of
this type do not appear to be abundant. They occur as dykes and bosses,
but occasionally also as sheets. I have collected them from Skye, Rum
and Ardnamurchan.
_Granophyre._--Under this name may be grouped the large majority of
the acid rocks which play an important part in the geology of the West
of Scotland. They are typically developed in the islands of Mull and
Skye. Generally pale grey or buff in colour, they range in texture
from the true granites, into which, as above stated, they graduate,
to exceedingly close-grained varieties like the felsites of Palæozoic
formations. In the great majority of them the micrographic intergrowth
of quartz and felspar, known as micropegmatite, is their conspicuous
structure, and even constitutes most of their substance. They may
thus be classed generally as granophyres, in the sense in which this
term is employed by Rosenbusch, but without his limitation of it to
pre-Tertiary rocks.
The specific gravity of these rocks has been determined from a series
of specimens by Mr. A. Harker to range from about 2·3 among the
felsites to 2·7 among the granites. No chemical analyses of these rocks
have yet been made, but they have been subjected to microscopical
examination, and their general structure and composition are now known.
The typical granophyre of the Inner Hebrides outwardly closely
resembles an ordinary granite of medium grain, in which the component
dull felspar and clear quartz can be readily distinguished by the naked
eye. Throughout all the varieties of texture there is a strong tendency
to the development of minute irregularly-shaped drusy (miarolitic)
cavities, which here and there give a carious aspect to the rock.
That these cavities, however, are part of the original structure
of the rock, and are not due to mere weathering, is shown by the
well-terminated crystals of quartz and felspar which project into them.
On a small scale, it is the same structure so characteristic of the
granite of the Mourne Mountains and of parts of that of Arran.
Examined under the microscope, a normal specimen of the granophyre of
the Western Isles presents a holocrystalline groundmass, which fills
all the interspaces between the crystals of earlier consolidation.
This groundmass consists of an aggregate of clear quartz and turbid
orthoclase, arranged as micropegmatite, but also in more or less
idiomorphic crystals. In some parts, the two dominant minerals are
grouped in alternate parallel fibres, diverging from the surface of the
enclosed crystals, which are thus more or less completely surrounded by
a radially fibrous mass. The felspathic portion of the micropegmatite
which usually surrounds the orthoclase crystals, when viewed between
crossed Nicols, is found to extinguish simultaneously with the central
crystal.[367] In other parts, the felspar forms a kind of network, the
meshes of which are filled up with quartz. Through the groundmass,
besides the clear quartz and dull orthoclase, some ferro-magnesian
or other additional constituent is generally distributed, but
usually somewhat decomposed. In certain varieties Dr. Hatch found an
abundant brown mica, as in the rock at Camas Malag, Skye. In others,
a pyroxene occurs, which he observed in minute greenish grains,
sometimes completely enclosed in the quartz. In a third variety, the
dark constituent is hornblende, the most remarkable example of which
is one to be seen at Ishriff, in the Glen More of Mull, where the
ferro-magnesian mineral takes the form of long dirty-green needles,
conspicuous on a weathered surface of the rock. A fourth variety is
distinguished by containing plagioclase in addition to or instead of
orthoclase. In the rock of the sheet forming Cnoc Carnach, near Heast,
in Skye, Dr. Hatch observed both orthoclase and plagioclase scattered
through a fine micropegmatitic groundmass, and in a part of the boss at
Ishriff he found the rock to be composed mainly of plagioclase, in a
micropegmatitic groundmass of quartz and felspar, with a few scattered
grains of a pale brown augite and grains of magnetite. A fifth variety
is marked by the prominence of the crystals of quartz and felspar of
earlier consolidation, and by the fineness of grain in the surrounding
micropegmatitic groundmass, whereby a distinct porphyritic structure
is developed. Rocks of this kind are megascopically like ordinary
quartz-porphyries. Still another variety has been detected by Mr. Teall
in the rock of Meall Dearg, at the head of Glen Sligachan, Skye, in
which, besides irregular patches which may represent decayed biotite,
and others which are possibly ilmenite, the rare mineral riebeckite is
present.[368]
[Footnote 367: Mr. Teall, _Quart. Journ. Geol. Soc._ vol. 1. (1894) p.
219. See also his _British Petrography_, p. 327.]
[Footnote 368: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 219.]
_Felsite._--The close-grained rocks into which the ordinary granophyres
frequently graduate may be conveniently grouped under the general name
of Felsite. They differ in no essential feature from the felsites of
the Palæozoic formations. They are more particularly developed, as
might be expected, in those places where the conditions have been most
favourable for rapid cooling, while the more coarsely crystalline
granophyres occur where the material may be supposed to have
consolidated most slowly. Where the acid magma has been injected into
chinks and fissures so as to take the form of veins or dykes, it is
sometimes felsitic, sometimes granophyric, in texture. Along the margin
of large bosses, like those of Mull and Skye, it frequently though
not invariably has assumed a fine texture, with even spherulitic and
flow-structures. But in the centre of large bosses it usually appears
as coarse granophyre or as granite.
The felsites vary in texture from flinty or horny to dull
finely-granular, and in colour from white through shades of grey,
buff and lilac, to black, generally with porphyritic felspars and
blebs of quartz. Where these porphyritic enclosures increase in size
and number, the rocks cannot be distinguished externally from ancient
quartz-porphyries. In general the groundmass of these rocks has been
completely devitrified. But in some dykes enough of the glassy base
remains to show their original vitreous condition. A gradation can thus
be traced from thoroughly glassy pitchstone into completely lithoid
felsites and crystalline granophyres.
A characteristic feature of the felsitic varieties of acid rock is
their flow-structure, which they often display in great perfection.
Sometimes, indeed, this structure has been so strongly developed as to
cause the rock to weather along the planes of flow and to break up into
thin slabs.
Many of these rocks also present admirably developed spherulitic
structures, varying from microscopic minuteness up to large round or
egg-shaped balls nearly two inches in diameter, and often distributed
in lines along those of flow-structure. They likewise exhibit a
frequent development of micropegmatite. No line indeed can be drawn
between these felsites and the granitoid varieties, for the same
characteristic granophyric intergrowth of felspar and quartz runs
through them all.
_Pitchstone._--This name is applied to the glassy varieties apart from
their chemical composition, and specially denotes the possession of
a vitreous structure. Some of the rocks to which it has been applied
are probably glassy varieties of andesite, others are dacites, while
some may be as acid as the most acid felsites and granophyres. The
pitchstones are found in veins or dykes which traverse different
geological formations up to and including the great granophyre bosses
of the Inner Hebrides. They vary in colour from a deep jet-black or
raven-black to a pale bottle-green, and in lustre from an almost
glassy obsidian-like to a dull resinous aspect. Occasionally they
assume a felsitic texture, owing to devitrification, and also a finely
spherulitic structure. Some varieties appear to the naked eye to be
perfectly homogeneous, others become porphyritic by the appearance of
abundant sanidine crystals.
The microscopic structure of the British pitchstones has not yet been
fully worked out. The beautiful feathery microlites of the Arran dykes,
first made known by David Forbes, and subsequently described by Zirkel,
Allport and others, are well known objects to geological collectors.
Dr. Hatch, in whose hands I placed my tolerably large collection of
specimens and their thin slides, furnished me with some preliminary
notes on the slides, from which the following generalized summary is
compiled.
At the one end of the pitchstone group we have a nearly pure glass,
with no microlites, and only a few scattered crystals of sanidine,
quartz, augite or magetite. The glass in thin slices is almost
colourless, but generally inclines to yellow, sometimes to dark-grey.
Some varieties of the rock are crowded with microlites, in others these
bodies are gathered into groups, the glass between which is nearly
free from them. Among the minerals that have been observed in this
microlitic form are sanidine, augite, hornblende (forming the beautiful
green feathery or fern-like aggregates in the Arran pitchstones, Fig.
3) and magnetite. Sometimes the rudimentary forms appear as globulites,
or as belonites, but more commonly as dark trichites. Among the more
definite mineral forms are grains of sanidine, quartz and augite. The
porphyritic crystals are chiefly sanidine, augite and magnetite, but
plagioclase occasionally occurs. The development of spherulites is well
seen in a few of the slides, and occasionally perlitic structure makes
its appearance.
The interesting rhyolitic areas of Antrim include several varieties of
pitchstone. One of these is described by Professor Cole as "a glassy
pyroxene-rhyolite, on the verge of the rhyolitic andesites." Another is
a blue-black porphyritic obsidian.[369]
[Footnote 369: _Scientif. Trans. Roy. Dublin Soc._ vol. vi. (ser. ii.)
1896, p. 77.]
_Rhyolite (Quartz-Trachyte)._--This rock has been abundantly erupted
in north-east Ireland, where it rises in occasional bosses among the
plateau-basalts.[370] It is best exposed at the Tardree and Carnearny
Hills, where it has long been quarried. Its petrographical characters
at that locality were described by Von Lasaulx as those of a typical
quartz-trachyte rich in tridymite, and containing large crystals
of glassy sanidine, isolated narrow laths of plagioclase (probably
andesine), grains of smoky-grey quartz, partly bounded by dihexahedral
faces, and a few scattered flakes of a dark-coloured mica. The
groundmass is microgranitic, and under a high power is resolvable
into a confused aggregate of minute microlites of felspar, with
interstitial quartz-granules.[371] More recently a detailed investigation
of the petrography of the Antrim rhyolites has been conducted by
Professor Cole, who has called attention to their remarkable varieties
of structure, ranging from perfect volcanic glass to a thoroughly
lithoidal texture, and exhibiting flow, perlitic and spherulitic
structures.[372]
[Footnote 370: Fragments of acid rock were detected by Prof. Cole in the
gravel among the Ardtun basalt of Mull, as already noticed on p. 212.]
[Footnote 371: Tschermak's _Min. und Pet. Mittheil._ 1878, p. 412.]
[Footnote 372: _Scientif. Trans. Roy. Dublin Soc._ vol. vi. (ser. ii.)
1896, p. 77. This paper gives an excellent account of the microscopical
character and mineralogical and chemical compositions of these rocks.]
Intrusive masses of rhyolite are also found in the Carlingford region.
One of these, seen at Forkhill, is a velvet-black almost resinous rock
with abundant quartz and felspar, and sometimes displaying beautiful
flow-structure. It will be more particularly described in Chapter
xlvii. Some of the acid dykes and sills of the Inner Hebrides are
varieties of rhyolite. No undoubted example has yet been observed of a
superficial rhyolite-lava, though such not improbably appeared in the
interval between the lower and upper basalts of Antrim.
ii. STRATIGRAPHICAL POSITION.--ANALOGIES FROM CENTRAL FRANCE
In the history of opinion regarding the relative position of the
Tertiary eruptive rocks, no feature is so remarkable as the universal
acceptance of the misconception regarding the place of the acid
protrusions. In tracing this mistake to its source, we find that
it probably arose from the fact that along their line of junction
the granitoid masses generally underlie the basic. This order of
superposition, which would usually suffice to fix the age of two groups
of stratified rocks, is obviously not of itself enough to settle the
relative epochs of two groups of intrusive rocks. Yet it has been
assumed as adequate for this purpose, and hence what can be proved
to be really the youngest has been placed as the oldest part of the
Tertiary volcanic series.
Macculloch, who showed that his "syenites" and "porphyries" had invaded
the Secondary strata of the Inner Hebrides, and must therefore be of
younger date than these, left their relations to the other igneous
rocks of the region in a curiously indefinite position. He was disposed
to regard them all as merely parts of one great series; and seems to
have thought that they graduate into each other, and that any attempt
to discriminate between them as to relative age is superfluous. Yet
he evidently felt that the contrasts of topography which he described
could hardly fail to raise the question of whether rocks so distinct
in outward form did not differ also in relative antiquity. But he
dismissed the question without answering it, remarking that if there
is any difference of age between the two kinds of rock, "there appears
no great prospect of discovering it."[373] He records an instance of
a vein of "syenite" traversing the "hypersthene rock" in the valley
of Coruisk. "If this vein," he says, "could be traced to the mass of
syenite, it might be held a sufficient ground of judgment, but under
the present circumstances it is incapable of affording any assistance
in solving the difficulty."[374] Instead, however, of being a solitary
instance, it is only one of hundreds of similar intrusions which can be
connected with the general body of granitic and granophyric masses, and
which put the relative ages of the several groups of rock beyond any
further doubt.
[Footnote 373: _Western Islands_, i. p. 368; see also pp. 488, 575, 578.]
[Footnote 374: _Op. cit._ p. 370.]
Boué, who knew the geology of some of the extinct volcanic regions of
Europe, recognized the similarity of the Scottish masses to those of
the Continent, and classed the acid rocks as "trachytes." He saw in
each of the volcanic areas of the West of Scotland a trachytic centre,
and supposed that the more granitoid parts might represent the centres
in the European trachytic masses. He traced in imagination the flow of
the lava-streams from these foci of volcanic activity, distinguishing
them as products of different epochs of eruption, among the last
of which he thought that the trachytic porphyries might have been
discharged. He admitted, however, that his restoration could not be
based on the few available data without recourse to theoretical notions
drawn from the analogy of other regions.[375]
[Footnote 375: _Essai Géologique sur l'Écosse_, pp. 291, 322, 327.]
In the careful exploration of the central region of Skye made by Von
Oeynhausen and Von Dechen, these able observers traced the boundary
between the "syenite" and the "hypersthene rock"; and as they found
the former lying underneath the latter, they seem naturally to have
considered it to be the older protrusion of the two.[376] Principal
Forbes came to a similar conclusion from the fact that he found the
dark gabbro always overlying the light-coloured felspathic masses.[377]
Professor Zirkel also observed the same relative position, and adopted
the same inference as to the relative age of the rocks.[378] Professor
Judd followed these writers in placing the acid rocks before the basic.
He supposed the granitoid masses to form the cores of volcanic piles
probably of Eocene age, through and over which the protrusions of
gabbro and the eruptions of the plateau-basalts took place.[379]
[Footnote 376: Karsten's _Archiv_, i. p. 82. It will be shown in later
pages that the apparent infraposition of the granophyre is often
deceptive, the real junction being vertical.]
[Footnote 377: _Edin. New Phil. Jour._ xl. (1846) p. 84.]
[Footnote 378: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871) pp. 90,
95. He says that the gabbro seems to be the younger rock, so far as
their relations to each other can be seen.]
[Footnote 379: _Quart. Jour. Geol. Soc._ xxx. (1874) p. 255.]
The evidence for the posteriority of the acid rocks will be fully
detailed in later pages. Before entering upon its consideration,
however, I would remark that the uprise of the British granophyres
presents so many points of resemblance to that of the trachytes and
phonolites among the basalt-plateaux of Auvergne and the Velay in
Central France, that a brief account of the acid protrusions of these
regions may be suitably given here as an introduction to the account of
those of the Inner Hebrides. A succession of stages in the progress of
denudation allows us to follow the gradual isolation and dissection of
the French volcanic groups. The youngest examples occur in the chain of
cones and craters, in the region of the Puy de Dôme. These may be of
Pleistocene, or even of more recent date. Older and more deeply eroded
than these are the numerous domes and cones in the territory of Haute
Loire. Yet more ancient and still more stupendously denuded come the
bosses, sills and dykes of Britain. Nevertheless, the geologist, by
the methods so admirably devised by Desmarest, may follow the chain of
relationship through these different regions and trace a remarkable
continuity of structure. The younger rocks serve to illustrate the
original condition of the more ancient, while the latter, by their
extensive denudation, permit points of structure to be seen which in
the former are still concealed.
No feature in the interesting volcanic district of Auvergne has
attracted more attention than the trachytic protrusions.[380] Rising
conspicuously along the chain of puys, they claim notice even from
a distance owing to the topographical contrast which their pale
rounded domes offer to the truncated, crater-bearing cones of dark
cinders around them. They consist of masses of a pale variety of
trachyte (domite), which in ground-plan present a circular or somewhat
elliptical outline. They vary in size from the nearly circular dome of
the Grand Sarcoui, which measures about 400 yards in diameter, to the
largest mass of all--that of the Puy de Dôme, which extends for some
1500 yards from north to south with a breadth varying from 500 to 800
yards. They are likewise prominent from their height; in the Puy de
Dôme they form the highest elevation of the whole region (1465 metres),
and even in the less conspicuous hills they rise from 500 to 600 feet
above the surrounding plateau.
[Footnote 380: The admirable Map and Memoirs of Desmarest on Auvergne
are classics in geology. Scrope's work, vol. i. p. 45, gives still
the best published account of this district. See also the work of
Lecoq (_ibid._). The results of more detailed petrographical research
regarding the rocks will be found in the essays of M. Michel Lévy
(_Bull. Soc. Géol. France_, 1890, p. 688) and in the Clermont sheet of
the Geological Survey Map of France (Feuille, 166). A bibliography of
the district up to the year 1890 is given in the volume of the _Bull.
Soc. Géol. France_ just cited, p. 674.]
Five such dome-shaped protrusions of trachyte have made their
appearance among the cinder-cones in a space of about five English
miles in length by about two miles in extreme breadth. Though opinions
have varied as to the mode of formation of these domes, there has been
a general agreement that their present topographic contours cannot be
far from the original outlines assumed by the masses at the time of
their production. The position of the trachyte bosses among the puys
serves to show that they were not deep-seated masses which have been
entirely uncovered by denudation, but were essentially superficial,
and were protruded to the surface at various points along the plateau
in the midst of already existing cinder-cones. In some cases, they have
risen on or near the position of the vents of these cones. Thus the Puy
de Chopine is half encircled by the crater of the Puy de la Goutte,
and the Grand Sarcoui stands in a similar relation to the fragmentary
crater-wall of the Petit Sarcoui.
M. Michel Lévy, in pointing out the superficial character of the
domitic protrusions, has forcibly dwelt on the evidence that these
rocks have undergone a comparatively trifling denudation, and that
they could never have extended much beyond their present limits.[381] As
Scrope pointed out, they were obviously protruded in a pasty condition,
not flowing out in streams like the other lavas of the district, but
consolidating within their chimneys and rising from these in rounded
domes.
[Footnote 381: _Op. cit._ p. 711.]
[Illustration: Fig. 344.--Section through the Puy de la Goutte and Puy
de Chopine.
1, Mica-schist; 2 2, Granite; 3 3, Tuffs; 4, Trachyte; 5, Basalt dyke.]
Undoubtedly denudation, cannot have left them altogether unaffected,
but must have removed some amount of material from their surface.
There is reason to believe that the material so removed may have been
in large part of a fragmental character, and that it was under a
covering of loose pyroclastic debris that the upward termination of the
trachyte column assumed its typical dome-form. Thus in the crater-wall
of the Puy de la Goutte, layers of buff-coloured trachytic tuff dip
gently away from the central domite mass of the Puy de Chopine. That
this material was thrown out from the vent previous to the uprise of
the domite may be inferred from the way in which the latter rock has
obliterated the northern half of the crater. The relations of the rocks
are somewhat obscured by talus and herbage, but when I last visited
the locality in the spring of 1895 the structure seemed to me to be as
expressed in the accompanying diagram (Fig. 344).[382]
[Footnote 382: Compare M. Michel Lévy, _ibid._]
The relative date of the protrusion of the trachytic domes cannot
be very precisely defined. There can, indeed, be no doubt that it
belongs to a late phase of the volcanic history. It came long after the
outpouring of the older basaltic plateaux, of which large fragments
emerge from beyond the limits of the younger lavas on both sides of the
great ridge of the puys, and not only long after that outpouring, but
even after the widespread sheets of basalt had been deeply trenched by
valleys and isolated into outliers capping the hill-tops. Yet there is
good evidence also that the uprise of the comparatively acid trachytes
was not the last volcanic episode of the district. The abundance of
dark slags and fragments of basalt lying on the domite hills shows that
discharges of more basic detritus occurred after these hills had taken
their place in the landscape.
Since the latest eruptions, a gradual alteration of the topographical
features by denudation has been slowly but continuously going on. The
Grand Sarcoui, possibly from having originally had a considerable
covering of fragmentary material, shows least the effects of this
waste. Its remarkably regular form, like that of an inverted cauldron
(the "Chaudron," as it is called in the district), presents, in a
distant view, a smooth grassy surface which slopes steeply down into
the great volcanic plain. But on a nearer examination these declivities
are found to be seamed with trenches which the rain-storms of centuries
have dug out. The covering of loose debris has been largely washed
away, though many fragments of dark slag are still strewn over the
slopes, and the scars are now being cut into the domite below. A more
advanced stage of decay may be seen on the Puy de Dôme, where, from
greater elevation and exposure, the domite is already deeply gashed
by gullies and ravines, while the slopes below are strewn with its
detritus.
The region of the Velay displays on a far more extensive scale the
protrusion of trachytic and phonolitic bosses, but as its volcanic
history goes back beyond the time of the Puys of Auvergne, its
volcanic monuments have consequently been more extensively affected
by denudation.[383] A series of basaltic eruptions forming extensive
sheets can there be traced, the oldest dating from Miocene time, the
youngest coming down to the age of the mammoth, cave-bear and early
man. During this prolonged outpouring of basic lavas there were several
intervals during which materials of a more acid nature--trachytes and
phonolites--were erupted. These rocks occur partly as extensive tracts,
covering five or six square miles, like those of the Mezenc, the Megal,
the Pic de Lizieux, and the Rand, and partly in isolated conical or
dome-shaped prominences, sometimes only a few hundred feet in diameter.
Upwards of one hundred distinct eruptions of phonolite have been
observed in the Velay. Even in the tracts where they cover the largest
space, several prominent eminences may usually be observed, not unlike
in general shape the isolated cones and domes of Auvergne. In these
wider areas there appears to be evidence of the outcome of the lava
from one or more vents, either as superficial streams or as underground
intrusive sheets. M. Boule has expressed his opinion that most of the
masses of trachyte and phonolite have been the result of local and
limited eruptions, the pasty rock having risen in and accumulated
around its pipe, without flowing far in any direction. A section across
one of these masses would present a somewhat mushroom-shaped form.[384]
[Footnote 383: In addition to the work of Scrope, the student of this
important volcanic district will find an invaluable guide in the Le
Puy Sheet (No. 186) of the Geological Survey Map of France, and in the
_Bulletins_ of the Survey, particularly those by MM. Termier and Boule,
No. 13 (1890) and No. 28 (1892).]
[Footnote 384: _Bull. Carte. Géol. France_, No. 28 (tome iv.) p. 125.]
That fragmentary ejections accompanied the protrusion of these rocks,
though probably on a very limited scale, is shown by the occasional
survival of portions of trachyte tuff around them. One of the most
notable of these deposits occurs in the hollow between the Suc du
Pertuis and the next dome to the south. It consists of fine and coarse,
trachytic detritus, which in one place is rudely bedded and appears to
dip away from the phonolite dome behind it at an angle of 30°. This
material and its inclination are what might be expected to occur round
an eruptive vent, and may be compared with those of the crater-wall
of the Puy de la Goutte in relation to the domite boss of the Puy de
Chopine.
The denudation of Velay has undoubtedly advanced considerably further
than that of the Puys of Auvergne. The pyroclastic material which may
have originally covered the domes of trachyte and phonolite has been
in great part swept away. The surrounding rocks, too, both aqueous and
igneous, have been extensively removed from around the necks of more
enduring material. Hence the trachyte and phonolite bosses stand out
with so striking a prominence as to arrest the eye even for a distance
of many miles.
[Illustration: Fig. 345.--View of the Huche Pointue and Huche Platte
west of Le Pertuis.
The cone is one of the trachytic domes, while the flat plateau to the
left is a denuded outlier of the basalt sheets.]
There cannot be any doubt that these necks have pierced the older
basalts, and therefore belong to a later epoch in the volcanic history.
The approximately horizontal sheets of basalt have been deeply eroded
and reduced to mere fragments, and in some instances their existing
portions owe their survival to the protection afforded to them by the
immense protrusions of more acid material. But there is here, as well
as in Auvergne, evidence of the uprise of a later more basic magma, for
sheets of basalt are found overlying some parts of the trachytes and
phonolites.
While the external forms of these Velay necks recall with singular
vividness the features of many more ancient necks in Britain, an
examination of the internal structure of some of them affords some
further interesting points of resemblance. The slabs into which, by
means of weathering along the joints, the rock is apt to split up are
sometimes arranged with a general dip outwards from the centre of the
hill, so that their flat surfaces roughly coincide with the hillslopes.
In other cases the peculiar platy structure, so characteristic of
phonolite, is disposed vertically or dips at a steep angle into the
hill, so that the edges of the slabs are presented to the declivities,
which consequently become more abrupt and rugged.
Though none of the volcanic series in Auvergne or the Velay is so acid
in composition as the more acid members of the Tertiary volcanic series
of Britain, the manner in which the trachytes and phonolites of the
French region make their appearance presents some suggestive analogies
to that of the corresponding rocks in this country. We see that they
were erupted long after the outpouring of extensive basaltic plateaux,
that they belonged to successive epochs of volcanic activity, that they
were protruded in a pasty condition to the surface, where, more or less
covered with fragmentary ejections, they terminated in dome-shaped
hills or spread out to a limited distance around the vents, and
lastly, that they were succeeded by a still later series of more basic
eruptions, which completed the long volcanic history. We shall see in
the following pages how closely the various stages in this complex
record of volcanic activity may be paralleled in the geological records
of Tertiary time in Britain.[385]
[Footnote 385: The phonolite necks of Bohemia, which form so prominent
a feature in the Tertiary geology of that country, might likewise be
cited here in illustration of the acid domes and bosses of the British
Isles.]