SUMMARY AND GENERAL DEDUCTIONS
The foregoing chapters comprise a connected narrative of the history
of volcanic action in the area of the British Isles during the vast
succession of ages from the early Archæan dawn down to the latest
eruptions of Tertiary time. In this final chapter I propose to present
a brief summary of the facts of largest import and widest interest
which this protracted history has placed before us, together with a
statement of deductions which may be drawn from them regarding the
nature and progress of volcanism in the evolution of the globe.
1. Among the broad features which soonest arrest attention in such a
survey is the geographical position of the theatre of this volcanic
activity. In the distribution of volcanoes at the present time we are
familiar with their tendency to range themselves along continental
borders or in oceanic islands. The volcanic energy so conspicuous in
the geological history of Britain has shown itself along the western
or Atlantic margin of the European continent. When the eruptions have
not been actually on the land itself, they have taken place within
the shallow tracts near the land, where the lavas and tuffs have been
interstratified with sediments derived from the adjacent coasts.
Moreover the volcanic rocks in Britain are ranged along the greatest
length of the group of islands, in a general north and south line, from
the south of Devonshire to the far Shetlands. It is on the western
side of the country that they occur. East of a line drawn from Berwick
by Leicester to Exeter, although the geological formations, ranging
from the Carboniferous Limestone to the latest Pleistocene deposits,
are there abundantly exposed to view, they include no contemporaneous
volcanic rocks.
2. A second and still more remarkable feature in the geological history
of Western Europe is the persistence of volcanic activity along the
site of the British Isles. Evidence has been brought forward in these
volumes that from the primeval time vaguely termed Archæan, onward
to that of the older Tertiary clays and sands of the south-east of
England--that is to say, through by far the largest part of geological
history, as chronicled in the stratified crust of the globe--this
long strip of territory continued to be intermittently a theatre of
volcanic action. Every great division of Palæozoic time was marked by
volcanic eruptions, sometimes over tracts hundreds of square miles in
area and on a colossal scale. After a long period of quiescence during
the Mesozoic ages, the renewed outbreak of volcanic energy in older
Tertiary time, so marked over the western half of Europe, reached its
maximum of development along the Atlantic border, from the north of
England and Ireland through the chain of the Inner Hebrides to the
Faroe Islands, Iceland and Greenland.
3. Not only has there been a remarkable persistence of volcanic
activity over the comparatively limited area of the British Isles,
viewed as a whole, but if we examine the different parts of this area
we perceive that many of them, of relatively restricted extent, have
been the sites of a recrudescence of volcanic action, again and again,
through a vast succession of geological periods. While the whole region
has been in different quarters and at different times affected, there
have been districts where the volcanic fires have been rekindled after
long intervals of quiescence, the new vents being opened among or near
to the sites of earlier volcanoes. In the south-west of England, for
example, the Middle Devonian tuffs and diabases were succeeded in the
Carboniferous period by the eruptions of the Culm-measures, and in the
very same tracts came last of all the lavas and tuffs of the Permian
conglomerates. Still more astonishing is the record of volcanic energy
in the south of Scotland, where, within a space of not many hundred
square miles, there are the chronicles of the Arenig, Llandeilo and
Bala eruptions of the Southern Uplands, the huge piles of lavas and
tuffs of the Lower Old Red Sandstone, the long succession of the
plateaux and then of the puys of the Carboniferous period, the groups
of tuff-cones of the Permian period, and, lastly, the numerous dykes
connected with the Tertiary volcanoes.
While some portions of the region have been specially liable to
exhibitions of volcanic action, others have continuously escaped. Some
of these "horsts," or stationary and unaffected blocks of country, have
been surrounded by or have risen close to the borders of this volcanic
district, yet have maintained their immunity through a long series of
ages. Thus the Central Highlands of Scotland, though they were flanked
on the south and south-west by the active volcanoes of the Old Red
Sandstone, and again on the south by those of Carboniferous time,
had no vents opened on their surface after the metamorphism of their
schists. Still more striking perhaps is the immunity of the Southern
Uplands. Though they were in large measure surrounded by the volcanoes
of the Lower Old Red Sandstone, then by those of the Calciferous
Sandstones and Carboniferous Limestone, and though they looked down on
the Permian eruptions of Ayrshire and Nithsdale, which spread streams
of lava and showers of ash along their flanks, these hills formed a
solid block that seems to have resisted perforation by the volcanic
funnels. Again, the tracts covered with Carboniferous Limestone in
England and Ireland almost entirely escaped from invasion by volcanic
eruptions.
We thus learn that even within comparatively restricted regions some
portions of the terrestrial crust have been areas of weakness, liable
to serve again and again as lines of escape for volcanic energy, while
close to them other portions of greater solidity have been persistently
left intact.
4. The sites of volcanic vents in all the geological systems wherein
they occur in Britain have not usually been determined by any obvious
structure in the rocks now visible. They comparatively seldom depend
on ascertainable lines of fault, even when faults, probably already
existent, occur in their near neighbourhood. This independence, to
which, however, there are occasional marked exceptions, comes out more
particularly in the coal-fields pierced by vents, for mining operations
have there revealed the positions of many more faults than can be
traced at the surface. If the sites of the vents have been fixed by
dislocations or lines of weakness in the terrestrial crust, these must
generally lie below the formations now visible at the surface.
There is one striking connection between the sites of the vents and
ancient topographical features to which frequent reference has been
made in the foregoing chapters. All through the long volcanic history,
as far back as such features can be traced, we see that orifices of
discharge for the erupted materials have been opened along low grounds
and valleys rather than on ridges and hills. The great central hollow
of the Scottish midlands was a depression even as long ago as the time
of the Lower Old Red Sandstone, and though it has probably been several
times since then filled up, and more or less completely effaced, its
ancient features have been partially revealed by extensive denudation.
This vast depression, 40 miles broad, between the Highland mountains
on the one side and the Southern Uplands on the other, was the chief
centre of volcanic activity in western Europe during the latter half of
Palæozoic time. The vents of the Old Red Sandstone, Carboniferous and
Permian series are scattered all over it, but few or none of them are
to be found on the high grounds that bound it. Again, in Tertiary time,
the great outpouring of lava took place in the hollow that lay between
the ridge of the Outer Hebrides and the mainland of Scotland. This wide
and long tract of low ground was buried under upwards of 3000 feet of
lava and tuff, but these materials were erupted from fissures and vents
within its own border and not from the mountains on either side.
But perhaps the most conspicuous example of any in which the vents keep
to the valleys is that supplied by the Permian necks of Nithsdale and
the neighbouring glens. These depressions are as old as Permian, and
even as Carboniferous time, but they appear to be entirely hollows of
erosion; at least they have yielded no evidence that their direction
has been determined by lines of fault. The chain of vents can be
followed from the lowlands of Ayrshire up to the base of the Southern
Uplands, down the wide valley cut by the Nith in these hills and up
some of the tributary valleys, and though the volcanoes continued for
some time in vigorous eruption, not a trace of any contemporary vent
has yet been met with on the surrounding hills.
While the position of volcanic vents in lines of valley may be
generally due to guiding lines of fissure in the crust underneath,
either within or below the rocks visible at the surface, there may
sometimes be conditions in which other dominant causes come into play.
The curious coincidence between variations in the upper limit of
dykes and inequalities in the configuration of the overlying ground,
suggest that where the subterranean magma has ascended to within a
comparatively short distance from the surface, a difference of a few
hundreds or thousands of feet in the depth of overlying rock, such as
the difference of height between the bottom of a valley and the tops
of the adjacent hills, may determine the path of escape for the magma
through the least thickness of overarching roof.
5. Volcanic phenomena cannot be regarded as a mere isolated and
incidental feature in the physics of the globe. During the short
time within which man has been observing the operations of existing
volcanoes, he has hardly yet had sufficient opportunity of watching
how far they can be correlated with other terrestrial movements. Nor,
when he endeavours to trace some such connection among the records of
the geological past, has he yet collected materials enough to furnish
a sufficiently broad and firm basis of comparison. One formidable
obstacle is presented by the difficulty in determining chronological
equivalents in separated groups of rock. Geologists have tried to
discover whether the volcanoes of some particular period or region
were in any way connected with such geological changes as extensive
plication, dislocations of the crust, or elevation of mountain-chains.
In regard to the volcanic history of Britain, various possible
relations of this kind obviously suggest themselves. Thus the division
of geological time comprised within the Lower Silurian period was
undoubtedly an interval of considerable terrestrial disturbance in
western Europe. The unconformabilities and overlaps in the series
of formations belonging to that period, the frequent conglomerates,
the great and often rapid changes in the thickness and lithological
characters of the strata, all point to instability of land-surface and
sea-floor. During these oscillations a prolonged and widespread series
of volcanic eruptions took place. The volcanic manifestations began
in Cambrian time and continued in intermittent activity till towards
the close of the deposition of the Lower Silurian formations. It is
certainly a significant fact that the Upper Silurian deposits, in
their lithological characters, present a strong contrast to those that
preceded them. They point, on the whole, to quiet sedimentation, during
an interval of comparative calm in the terrestrial crust. With this
evidence of tranquillity there is, over almost the whole of the British
Isles, an entire absence of any trace of renewed volcanic activity.
With the exception of the Dingle lavas and tuffs, in the extreme west
of Ireland, not a single undoubted instance is yet known of an Upper
Silurian volcano.
After the deposition of the Upper Silurian rocks an interval of great
terrestrial disturbance ensued, and these rocks over a large part of
Britain were intensely plicated and crushed. The movements, continued
into the period of the Lower Old Red Sandstone, were, in their later
stages, accompanied or, at least, followed by the vast outpourings of
lava which now cover so much of the tracts of Old Red Sandstone in
Scotland and Ireland.[438]
[Footnote 438: _Trans. Geol. Soc. Edin._ vol. ii. part iii. (1874).]
In proportion as the volcanic energy was vigorous, widespread and
long-continued, we may expect it to have been connected with important
terrestrial movements affecting extensive regions of the earth. The
Tertiary volcanic history seems to afford a remarkable instance of this
connection. A wide area of the European continent is dotted over with
old centres of volcanic activity which were in eruption at successive
epochs throughout the Tertiary period. Of all these centres the most
important was that of the north-western basalt-plateaux, where floods
of lava were discharged over many thousand square miles from Ireland
to Greenland. The geological date of these outpourings probably
coincides with the last great orographic movements that gave to the
mountain-chains of Europe their latest elevation and dimensions.
But without entering into what must be for the present a field of
speculation, we can be assured of one important fact in the connection
of ancient volcanoes with movements of the terrestrial crust. A study
of the records of volcanic action in Britain proves beyond dispute that
the volcanoes of past time have been active on areas of the earth's
surface that were sinking and not rising. We usually associate volcanic
action with elevation rather than subsidence, and there are certainly
abundant proofs of such elevation around active or recently extinct
volcanoes. Many of the active vents of the present time, like Vesuvius
and Etna, began with submarine eruptions and have been gradually
upraised into land. It may be, however, that such uprise is merely a
temporary incident, and that if we could survey the whole geological
period of which human history chronicles so small a part, we might find
that subsidence, and not upheaval, is ultimately the rule over volcanic
areas.
Be this as it may, there can be no question that with the one solitary
exception of the Tertiary volcanoes, which were terrestrial and not
submarine, all the British vents were carried down and eventually
buried under aqueous sediments. Even the Tertiary lava-fields have in
many places sunk down below sea-level since their eruptions ceased.
That there are any Palæozoic volcanic rocks now visible at the surface
is obviously due to subsequent movements not immediately connected with
their original conditions of eruption, and to gigantic denudation. The
amount of subsidence which followed on a volcanic episode was sometimes
enormous, even within the same geological period, as one may see by
observing the prodigious piles of sedimentary material heaped over
the lavas and tuffs of Arenig time, or over those of the Lower Old
Red Sandstone. I do not wish to maintain that the downward movement
was necessarily a consequence of volcanic ejections, for we know that
it took place over tracts remote from centres of eruption. But I have
sometimes asked myself whether it was not possibly increased as a
sequel to vigorous volcanic action; whether, for instance, the great
depth of the Palæozoic sedimentary rocks in some regions, as compared
with their feeble development in others, may not have been due to an
acceleration of subsidence consequent upon volcanic action.
6. A review of the geological history of Britain cannot but impress the
geologist with a conviction of the essential uniformity of volcanism
in its manifestations since the early beginnings of geological time.
The composition and structure of the materials erupted from the
interior have remained with but little change. The manner in which
these materials have been discharged has likewise persisted from the
remotest periods. The three modern types of Vesuvian cones, puys and
fissure-eruptions can be seen to have played their parts in the past as
they do to-day.
Among the earliest igneous masses of which the relative geological
date can be fixed are the dykes which form so striking a system among
the Archæan rocks of the north-west, and show how far back the modern
type of volcanic fissures and dykes can be traced. No relic, indeed,
has survived of any lavas that may have flowed out from these ancient
fissures, but so far as regards underground structure, the type is
essentially the same as that of the Tertiary and modern Icelandic
lava-fields.
The early Palæozoic volcanoes formed cones of lava and tuff comparable
to those of such vents as Vesuvius and Etna. In the Lake District the
pile of material ejected during Lower Silurian time was at least 8000
or 9000 feet thick. In the Old Red Sandstone basins of Central Scotland
there were more than one mass of lavas and tuffs thicker than those of
Vesuvius.
The puys of the later half of Palæozoic time closely resembled their
Tertiary successors in Central France, the Eifel, and the Phlegræan
Fields.
Nor, as regards extent and vigour, did the eruptions of the geological
past differ in any important respect from those of the present time.
There is assuredly no evidence that volcanic energy has gradually
waned since the dawn of geological history. The latest eruptions
of North-Western Europe, forming the Tertiary basalt-plateaux, far
exceeded in area, and possibly also in bulk of material discharged, all
the eruptions that had preceded them in the geological record.
7. Nevertheless, while the Tertiary eruptions showed no diminution
of vigour, it is undoubtedly true that the volcanic energy has not
manifested itself in a uniform way since the beginning of geological
time. There have been periods of maximum activity followed by others of
lessened force. Thus if we take a broad view of the general features
of volcanic action during the Palæozoic ages in Britain, we see
clear evidence of a gradual diminution in its vigour. The widespread
outpourings of lava and tuff in the Silurian period in England, Wales,
Scotland and Ireland were succeeded by the somewhat diminished,
though still important, eruptions of the Lower Old Red Sandstone
basins. The latter were followed by the still lessened outflows of
the Carboniferous plateaux, which in turn were succeeded by the yet
feebler and more localized eruptions of the Carboniferous puys, the
whole prolonged volcanic succession ending in the small scattered vents
of the Permian period. There were of course oscillations of relative
energy during this history, some of the maxima and minima being of
considerable moment. But though progress towards extinction was not
regular and uniform, it was a dominant feature of the phenomena.
8. The Permian volcanoes were the last of the long Palæozoic series,
and, so far as we yet know, the whole of the Mesozoic periods within
the area of Britain were absolutely unbroken by a single volcanic
eruption. The chronological value of this enormous interval of
quiescence may, perhaps, never be ascertainable, but the interval
must assuredly cover a large part of geological time. It was an
era of geological calm, during which the Triassic, Jurassic and
Cretaceous formations were slowly accumulated over the larger part
of Europe. The stratigraphical quietude was not indeed unbroken. The
widespread subsidence of the sea-bottom was interrupted here and
there by important upheavals, and considerable geographical changes
were in process of time accomplished. But, save in one or two widely
separated areas of Europe, there were no active volcanoes over the
whole continent.[439] Here again the scarcity or absence of intercalated
volcanic rocks is in harmony with the general stratigraphy of the
formations.
[Footnote 439: The Triassic eruptions of Predazzo and Monzoni were
important, and traces of others are said to occur in the Cretaceous
system in Portugal and Silesia.]
9. After the prodigious interval represented by the whole of the
Mesozoic and the earlier part of the Tertiary formations, a time
of disturbance arose once more, and the great basalt-floods of the
north-west were poured forth. Evidence has been adduced in the
foregoing chapters that this latest volcanic period was one of vast
duration; that it was marked by long intervals of quiescence, and by
repeated renewals of volcanic energy. Yet over the area of Britain the
whole of its manifestations were probably comprised within the earlier
(Oligocene and perhaps early Miocene) part of older Tertiary time.
Since its eruptions ceased, another interval of profound quiescence has
succeeded, which still continues. But this interval is almost certainly
of less duration than that which elapsed between the Palæozoic and
Tertiary outbursts. In other words, remote as the date of these
Tertiary volcanoes appears to be from our own day, it comes much nearer
to us than did the era of the last Permian eruptions to the earliest of
the Tertiary series.
10. By the dissection which prolonged denudation has effected among the
old volcanic centres of Britain, materials are supplied for studying
the sequence of events from the beginning to the end of a volcanic
period. These events have generally followed the same tolerably
well-defined order.
In the case of fissure-eruptions, rents formed in the crust of the
earth and communicating with the surface have allowed lava to rise and
flow out above ground, either from the lips of the fissures or from
vents opened along the lines of chasm. The thousands of parallel dykes
in Britain remain as evidence of this mode of the ascent of the molten
magma. Lines of large cones of the Vesuvian type may be presumed to
have risen along guiding fissures in the terrestrial crust.
But it is evident from a study of the British examples that the
existence of a fissure in the visible part of the crust is not always
necessary for the production of a volcanic vent. In hundreds of
instances, communication from the internal magma to the surface was
effected by successive explosions, which finally blew out an orifice
at the surface with no visible relation to any fissures or dykes. Of
course, beneath the formations that now form the surface, and through
which the necks rise, there may be lines of fault or weakness in older
rocks which we cannot see. But, in what can be actually examined, vents
have commonly been drilled through rocks independently of faults.
The discharge of explosive vapours was sometimes the first and only
effort of volcanic energy. Generally, however, fragmentary volcanic
materials were ejected, or, if the eruption was more vigorous, lava was
poured out. In a vast number of cases, especially in the later ages
of Palæozoic time, only ashes were projected, and cones of tuff were
formed. In the earlier ages, on the other hand, there was a much larger
proportion of lava expelled. Towards the close of a volcanic period,
the vents were gradually choked up with the fragmentary materials
that were ejected from and fell back into them. Occasionally, during
the process of extinction, an explosion might still occur and clear
the chimney, so as to allow of the uprise of a column of molten rock
which solidified there; or the sides of the crater, as well as of the
cavernous funnel underneath, fell in and filled up the passage. Heated
vapours sometimes continued to ascend through the debris in the vent,
and to produce on it a marked metamorphism.
There seems to have been commonly a contraction and subsidence of the
materials in the vents, with a consequent dragging down or sagging of
the rocks immediately outside, which are thus made to plunge steeply
towards the necks.
When the vents were plugged up by the consolidation of fragmentary
matter or the uprise of lava in them, the final efforts of the
volcanoes led to the intrusion of sills and dykes, not only into the
rocks beneath the volcanic sheets, but also, in many instances, into
at least the older parts of the sheets themselves. These subterranean
manifestations of volcanic action may be recognized in almost every
district. They vary greatly in the degree to which they are developed.
Sometimes, as in the Cader Idris, Arenig and Snowdon regions, they
attain considerable importance, alike as regards the number and
thickness of the sheets. In other cases, they are exhibited on so
small a scale that they might be overlooked, as in the tract of
Carboniferous puy-eruptions in the north of Ayrshire. But they are so
generally present as to form a remarkably characteristic feature of
the volcanic activity of each geological period from the earliest time
to the latest. The basic sheets in the Dalradian series of Scotland
display early and colossal examples. All through the successive
eruptive periods of Palæozoic time, sills are found as accompaniments
of superficial ejections.
The Tertiary basalt-plateaux supply numerous and gigantic examples
of intruded sheets. Tertiary cones of Vesuvian type are not found in
Britain, but where on the continent they have been sufficiently laid
open by denudation, they present sometimes an astonishing series of
sills. As a striking illustration of this structure reference may be
made to the sheets of trachyte that have been injected between and
have marmorized the Cretaceous strata on which Monte Venda stands,
among the Euganean Hills.[440]
[Footnote 440: G. vom Rath, _Zeitsch. Deutsch. Geol. Gesellsch._, xvi.
(1864), p. 461. E. Suess, _Sitzungsber. k. Akad. Wien._, lxxi. (1875),
p. 7; _Antlitz der Erde_, vol. i. p. 193. E. Reyer, _Die Euganeen_,
1877. This volcano is further referred to, _postea_, p. 477.]
It is obvious that the time of intrusion of the sills cannot be
precisely determined. They were not likely to be injected at an epoch
when the volcanic magma could find ready egress to the surface. That
they did not arise before such egress was obtained may be inferred
from their petrographical characters, which are usually those of the
later and not of the earlier outflows of the magma; and from the fact
that they not only lie among the rocks below the volcanic series, but
intersect the lower parts of that series, sometimes even the higher
parts. We may therefore, with every probability, regard the sills as
among the closing phases of a volcanic period.
As the lavas and tuffs of each volcanic period are intercalated among
the successive geological formations, a definite beginning and end to
the period are stratigraphically fixed. We see exactly where in the
sedimentary series the first showers of ashes fell, and where the last
mingled with the ordinary sand and mud of the sea-door. The same record
shows that the volcanic accumulations were finally washed down, that
they subsided with the rest of the ground around them, and that usually
they were buried under overlying conformable sedimentary deposits. Thus
cones of ashes and lava which may have been several thousand feet high
completely disappeared.
10. A consideration of the distribution of the volcanic rocks in time
shows not only how singularly uniform the course of volcanic activity
has been, but that there is no evidence of the cessation of any of the
broader petrographical types during geological history. Quite as much
variety may be observed among the erupted materials of Tertiary time in
Britain as among those of the early ages, when the earth was younger
and its volcanic vigour might be supposed to have been greater and more
varied than it is now. The table on the following page will make these
features at once apparent. From this table it will be seen that while
some of the acid rocks have not always been extruded, the basic masses
have played their part in every volcanic period.
11. A study of the volcanic products of a long series of eruptions
within the same geographical region may be expected to throw light
on the changes that take place during the course of ages in the
character of the internal molten magma. In a former chapter (vol. i.
p. 27) reference was made to the subject of volcanic cycles and to the
sequence, observed in various widely separated parts of the world,
among the materials erupted from below. Allusion was likewise made
in a later chapter (vol. i. p. 90) to the remarkable differences in
texture and composition noticeable within some large bodies of eruptive
material, and to the evidence which these differences furnish of a
segregation or differentiation among the constituents of an eruptive
mass after it has been injected into its position within the crust of
the earth.
Table of the Periods of Volcanic Action in the British Isles and of the
Chronological Distribution of the Volcanic Products.
Key to Columns
====================================
Gr = Granites, Granophyres, etc.
Fe = Felsites, Rhyolites, etc.
Da = Dacite, "Pitchstone" of Eigg.
Tr = Trachytes.
An = Andesites (Porphyrites).
Ga = Gabbros.
Do = Dolerites, Basalts (Diabases).
Pi = Picrites and highly basic lavas.
Tu = Tuffs, acid or basic.
+--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| | Gr | Fe | Da | Tr | An | Ga | Do | Pi | Tu |
+--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Older Tertiary | | | | | | | | | |
| (Plateaux, dykes, | | | | | | | | | |
| necks, bosses, | | | | | | | | | |
| sills) | * | * | * | * | * | * | * | * | * |
| | | | | | | | | | |
| Mesozoic | | | | | | | | | |
| No volcanic rocks.| | | | | | | | | |
| | | | | | | | | | |
| Permian | ··· | * | ··· | ··· | * | ··· | * | * | * |
| | | | | | | | | | |
| Carboniferous | ? | | | | | | | | |
| Puy type | ··· | * | ··· | ··· | * | ··· | * | * | * |
| Plateau type | ··· | * | ··· | * | * | ··· | * | * | * |
| | | | | | | | | | |
|{Devonian | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| * |
|{ | | | | | | | | | |
|{Old Red Sandstone | | | | | | | | | |
|{ Upper | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| * |
|{ Lower | * | * | ··· | * | * | ··· | * | ···| * |
| | | | | | | | | | |
| Silurian | | | | | | | | | |
| Upper | ··· | * | ··· | ··· | ··· | ··· | ··· | ···| * |
| Lower, Bala | * | * | ··· | * | * | * | * | * | * |
| " Arenig | * | * | ··· | * | * | * | * | ···| * |
| | | | | | | | | | |
| Cambrian | ··· | * | ··· | ··· | * | ··· | * | ···| * |
| | | | | | | | | | |
| Uriconian | ··· | * | ··· | ··· | ··· | ··· | * | ···| * |
| | | | | | | | | | |
| Dalradian | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| ? |
| | | | | | | | | | |
| Torridonian | | | | | | | | | |
| | | | | | | | | | |
| Lewisian | * | ··· | ··· | ··· | ··· | ··· | * | * | ··· |
| | | | | | | | | | |
+--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+
From the history of volcanic action in the British Isles it is clear
that differentiation is effected under three distinct conditions.
In the first place, a notable difference may be occasionally observed
between two adjacent parts of the same mass of lava which has flowed
out at the surface. Thus, in the Carboniferous picrite of Blackburn,
there has been a separation of the heavy basic constituents, which have
in great part settled down into the lower part of the sheet, while the
lighter felspar has mainly come to the top. In this case the gradual
transition from top to bottom suggests that the separation occurred
after the lava had reached the surface and taken the form of a stream
or sheet.
In the second place, segregation has taken place in the magma within
the terrestrial crust after intrusion, for it is frequently observable
in large bosses and sometimes in sills, the basic elements having
tended to mass themselves towards the margins of the rock, leaving
more acid material in the centre. The cases of Garabol Hill among the
Dalradian schists of Scotland, of Carrock Fell among the Silurian
strata of the Lake District, and of the Cramond picrite among the
Carboniferous formations of Midlothian, with others that might be cited
from various other regions and geological formations in Britain, prove
to what a considerable extent a separation of ingredients may take
place in a boss, and even sometimes in a comparatively thin sill before
the molten mass consolidates.
In the third place, there is good evidence that already before the
magma is either intruded or extruded, and while it still lies within
the internal reservoir, it may not possess a general uniformity of
composition, but may have become more or less heterogeneous. In regard
to intrusive rocks, the extraordinarily banded gabbros of the Tertiary
series of Skye obviously proceeded from a magma in which the molten
material consisted in some parts mainly of felspar, and in others
mainly of the ferro-magnesian minerals and iron-ores. Streams from
these differently constituted parts of the magma were simultaneously or
successively injected as sills into the older portions of the volcanic
series, while, as the process of differentiation within the magma
proceeded, still more felspathic liquid was left behind, to be thrust
into cracks in the sills previously consolidated.
Moreover, the banded basalts of the Tertiary plateaux show that this
heterogeneity was not confined to internal intrusions, but maintained
its place even when the molten material was ejected to the surface.
The differentiation indeed is not so striking there as among the sills
of gabbro; but its presence, even in a less degree, proves that the
separation of constituent minerals was not due to any general cooling
of an erupted body of igneous rock, but was already developed in the
reservoir from which the molten material was propelled to the surface.
Attention has been called to the remarkable similarity of structure
between these banded intrusive rocks and some of the ancient gneisses.
The resemblance is so close that we may with every probability infer
that the gneisses acquired their characteristic banding as intrusive
masses of igneous rocks, discharged from heterogeneous magmas, like
that which supplied the gabbros of the Cuillin Hills. And as these
gneisses belong to pre-Cambrian formations, we are thus led to
the interesting result that the tendency to develop heterogeneity
was already as characteristic of the magma-basins of the earliest
geological time as it has been of those of later periods.
The evidence of differentiation presented by superficial lavas, and by
intrusive sills and bosses, acquires great interest when considered
in connection with the changes which are seen to have occurred in the
character of the materials erupted during the course of a definite
volcanic period. An attentive examination of the volcanic products of
the various ages, so fully recorded in the geological structure of the
British Isles, shows that a recognizable sequence in the nature of the
materials erupted during a single volcanic period can be traced from
the earliest to the latest times, and that, in spite of occasional
departures, the normal order remains broadly uniform.
With the important exception of the Snowdonian region and possibly
others, we find that the earlier eruptions of each period were
generally most basic, and that the later intrusions were most acid.
Thus the diabase-lavas and tuffs at the base of the Cambrian series of
St. David's are pierced by quartz-porphyry veins. The andesites of the
Lower Old Red Sandstone were succeeded by bosses, sills, and dykes of
granite, felsite, and lamprophyre. The eruptions of the Carboniferous
plateaux began with extremely basic lavas, and ended with trachytes,
felsites, and quartz-porphyries. The basalts of the great lava-fields
of the Tertiary period are pierced by masses of granophyre and even
granite.
There has evidently been, on the whole, a progressive diminution in
the quantity of bases and a corresponding increase in the proportion
of acid in the lavas erupted during the lapse of one volcanic period.
This sequence is so well marked and so common that it cannot be merely
accidental. The acid and basic rocks, occurring as they do at each
volcanic centre in the same relation to each other, are obviously parts
of one connected series of eruptions. We seem to see in this sequence
an indication of what was taking place within the subterranean magma.
There was first an extensive separation of the more basic constituents,
such as the ferro-magnesian minerals and ores, and the lavas which
came off at that time were heavy and basic basalts, and even picrites.
The removal of these elements left the magma more acid, and such rocks
as andesites were poured out, until at last the deeper intrusive
sills, dykes and bosses became thoroughly acid rocks, such as felsite,
quartz-porphyry and granite, while if any superficial outflow took
place it was such a rock as dacite.
In the case of the Tertiary volcanic series there is evidence that
after the acid protrusions a final uprise of basic material occurred.
No satisfactory proof of any similar return to basic eruptions has been
detected among the Palæozoic formations. But it is possible that some
of the basic sills and dykes, the precise age of which cannot be fixed,
may really mark such a reversion, even in the earlier volcanic periods.
Some illustrative examples of volcanic cycles from other countries
were cited in Chapter iii. To these I may add another instance which
presents a close analogy to some of the phenomena characteristic of
the British examples of Palæozoic as well as of Tertiary age. Monte
Venda in the Euganean Hills, already alluded to (p. 474), may be cited
as an interesting specimen of an older Tertiary volcano, which has
been so dissected by denudation as to show not only the succession
of its superficial discharges, but the position and order of its
subterranean intrusions. The volcanic eruptions of this neighbourhood,
judging from the area which they still cover and the height they
reach, may have piled up a mountain rivalling or surpassing Etna in
dimensions. In Monte Venda the lowest visible igneous rocks are sills
of oligoclase-trachyte that have been thrust between and have highly
altered Cretaceous (Tithonian) limestones. Other intrusive sheets
of trachyte follow in the overlying Cretaceous strata (Neocomian
and _Scaglia_). It is not until the older Tertiary formations are
reached that undoubted tuffs and lavas occur, indicative of truly
interstratified volcanic materials. These formations, consisting of
nummulitic limestones and other strata together with fossiliferous
tuffs, show that the volcano began as a submarine vent. It discharged
dark basic dolerites and tuffs. The highest lava, however, crowning
the summit of the mountain is a trachyte. There appears to have been
a rapid decrease of the bases in the magma, for the later lavas were
rhyolites, accompanied with rhyolitic tuffs of Oligocene age, and
followed in the end by the black vitreous trachyte of Monte Sieva.
12. From the evidence detailed in these volumes, it appears that the
sequence from basic to acid discharges was on the whole characteristic
of each eruptive period. It is obvious, however, that as the
protrusions of successive periods took place within the same limited
geographical area, the internal magma during the interval between
two such periods must in some way have been renewed as regards its
constitution, for when, after long quiescence, eruptions began once
more, basic lavas appeared first and were eventually followed by
acid kinds. This cycle of transformation is admirably exhibited in
Central Scotland, where the andesites of the Old Red Sandstone with
their felsite sills are followed by the limburgites, picrites and
other highly basic lavas at the bottom of the Carboniferous plateaux,
succeeded in turn by the andesites, trachytes and acid sills of that
series. When the puy eruptions ensued, the magma had once more become
decidedly basic.
That the true explanation of these alterations is of a complex order
may be inferred from the exceptions which occur to the general rule.
I have alluded to the Snowdon region, where the acid rhyolites are
followed by more basic andesites, and where the sills are also more
basic than the superficial lavas. In the Arenig and Cader Idris country
the sills are likewise more basic than the bedded lavas. Among the
Carboniferous puys of the basin of the Firth of Forth, the sills are
not sensibly more acid than many of the superficial basalts, and they
even include such rocks as picrite. Possibly in this last-named region
we see an arrested sequence, the volcanic protrusions having from some
cause ceased before the general uprise of the more acid magma.
INDEX
Aa form of lava in the Sandwich Islands, ii. 187
Abereiddy Bay, i. 206
Abich, H., i. 32
Acid igneous rocks, silica percentage of, i. 14;
devitrification of, 19;
flow-structure of, 21;
occur in thicker sheets than basic, 24;
alternations of, with basic, 28, 61, 152, 157, 165, 207, 213, 233,
284, 318; ii. 236, 266, 278;
metamorphic action of, i. 95, 96;
connection with mountains, ii. 98;
scenery of, 102.
Acids, mineral, at volcanoes, i. 72
Acland, Mr. H. D., i. 133
Aegean Sea, volcanoes of, i. 1
Agglomerates, i. 31, 57, 58;
in dykes, 70;
Archæan, 120, 130, 135;
Cambrian, 148, 149, 167;
Silurian, 178, 180, 181, 184, 185, 194, 199, 206, 214, 237, 241,
244, 247, 253, 255;
Old Red Sandstone, 279, 285, 289, 300, 313, 325, 338, 349, 352;
Carboniferous, 381, 399, 402, 404, 427, 429, 439, 440; ii. 13, 24,
28, 29;
Permian, 62, 64, 99;
Tertiary, 194, 277, 278, 281, 289, 292, 293, 384, 400, 423
Allan, T., i. 363
Allotriomorphic minerals, i. 21
Allport, Mr., i. 95, 130, 131, 260, 451; ii. 11, 42, 102, 103, 104,
106, 370
Amber in Tertiary volcanic series, ii. 198
America, Western North, volcanic rocks of, i. 10, 100; ii, 267
Amygdales, origin of, i. 15; ii. 189, 221, 285, 290
Amygdaloidal structure, i. 15, 16, 17, 59, 274, 385; ii. 3, 31, 57,
129, 188
Analyses of Cambrian tuffs, i. 148, 149;
of Cambrian diabases, 153;
of Old Red Sandstone diabases, 274;
of Old Red Sandstone andesites, 275;
of Old Red Sandstone trachytes, 276;
of Old Red Sandstone felsites, 278;
of Carboniferous limburgite, 377;
of Carboniferous basalts, 379;
of Carboniferous trachytes, 380;
of Carboniferous phonolite, 381;
of Tertiary trachyte, ii. 139;
of Tertiary dacite, 244
Anderson, Dr. Tempest, ii 261, 262, 263
Andesite, i. 24, 131, 136, 164, 165, 167, 178, 180, 184, 189, 190, 204,
212, 213, 214, 215, 229, 230, 245, 246, 247, 252, 274, 275,
(analyses), 277, 292, 300, 306, 309, 315, 318, 325, 330, 333, 345,
377, 379, 386, 403, 421; ii. 45, 57, 96, 125, 137, 184, 236, 424
Anglesey, gneisses and schists of, i. 126;
volcanic rocks of, 189, 219
Anhydrite deposits, ii. 54
Annandale, Permian volcanic rocks of, ii. 56, 58, 60, 61, 66
Antrim, Old Red Sandstone volcanic rocks of, i. 314;
Tertiary volcanic rocks of, 47, 52; ii. 109, 110, 113, 139, 140, 199;
basalts of, 192, 193, 199, 202, 206;
clays and iron-ore of, 204;
rhyolites of, 185, 364, 370, 371, 426, 445;
deceptive agglomerate of, 188;
rhyolitic conglomerate of, 195, 206;
plateau of, 199;
tuffs of, 202, 204;
vents of, 271, 277;
sills of, 298;
central subsidence of basalt-plateau of, 448
Apatite, ii. 135
Apjohn, J., ii. 42
Applecross, volcanic vents in, ii. 292
Arans, the, i. 175, 176, 179, 184, 186, 207
Archæan period, i. 110, 111;
volcanic rocks of, 120
Ardnamurchan, dykes and veins of, ii. 154, 320;
basalt-plateau of, 208;
vents of, 287;
sills of, 318;
gabbro of, 355
Arenig group, i. 175;
lower limit of, 177, 185;
top of, 178, 228, 246
---- volcano of, i. 42, 175, 176, 179, 186, 207
---- rocks in Scottish Highlands, i. 123, 126;
in Merionethshire, 176, 179;
of Shropshire, 189;
of Ayrshire, 196;
of Scottish Highlands, 201;
of Anglesey, 221;
of Lake district, 229;
of Ireland, 239
Argyll, Duke of, ii. 113, 114, 198
Argyllshire, dykes of, ii. 127, 128, 138, 142, 146, 171, 172;
vents of, 278
Arizona, explosion crater in, i. 58;
laccolites in, 86
Arran, Old Red Sandstone volcanic rocks of, i. 298, 311;
Carboniferous volcanic rocks of, 386, 392;
possible Permian volcanic rocks of, ii. 58;
granite of, i. 93; ii. 366, 367, 418;
pitchstone of, i. 19; ii. 445;
dykes of, 123, 139, 140, 142, 146, 154, 161
Arthur Seat, i. 364, 373, 378, 385, 386; ii. 67
"Arvonian," i. 145, 156
Asbestos in volcanic breccia, ii. 51
Ascension Island, cellular lava of, i. 15
Ashes, volcanic (_see_ Tuffs)
Ashprington volcanic series, i. 262
Asphalt, ii. 79
Atherstone, i. 170
Augite, loose crystals of, in volcanic vents, i. 62, 178, 181; ii. 58, 79;
lumps of, in volcanic vents, i. 352
Augite-aphanites, i. 178
Auvergne, old volcanoes of, i. 29, 32, 66, 70, 100; ii. 373
Aveline, Mr. W. T., i. 227, 230; ii. 32
Ayrshire, example of volcanic neck in, i. 56;
Silurian volcanic rocks of, 192;
Old Red Sandstone volcanic rocks of, 275, 282, 283, 285, 291, 331;
Carboniferous volcanic plateau of, 102, 368, 388, 393, 398, 410;
Carboniferous Puys of, 415, 416, 434, 440, 474;
Permian volcanic rocks of, ii. 55, 58, 62
Azoic period, i. 109
Bäckström, Mr., ii. 266
Baily, W. H., i. 251, 252; ii. 198, 449
Bala group, i. 175, 190, 196, 201, 206, 207, 223, 242;
limestone of, 47, 175, 229, 245, 251;
volcanic rocks of, 186, 190, 207, 213, 221, 241, 248
Balbriggan, igneous rocks of, i. 244
Ballagan beds (Lower Carboniferous), i. 384, 387, 392, 393, 412, 447
Ballantrae, volcanic rocks at, i. 192, 199
Ballypallidy, tuffs and leaf-beds of, ii. 204, 429
Bamborough, Whin Sill at, ii. 2, 3, 5
Banding of igneous rocks, i. 84, 207; ii. 189, 294, 329, 354, 357, 476
---- of gneiss, i. 116
Bangor group, i. 166
Banks, Sir Joseph, ii. 109
Barnavave, eruptive rocks of, ii. 421
Barrow, Mr. G., i. 201, 226, 272, 279, 380; ii. 147, 148
Basalt, columnar structure of, i. 24, 25;
relation to gabbro, 78;
altered by carbonaceous strata, 95;
shells supposed to occur in, ii. 110;
banded, 189;
thickness of sheets of, 192;
meaning of red layer between sheets of, 197, 203, 206, 254;
metamorphism of, 272, 276, 337, 339, 340, 347, 355, 356, 357, 358,
362, 378, 383, 386, 397, 399, 400, 404, 413
---- pre-Cambrian, i. 119, 131;
Silurian, 206, 207, 230, 245;
Carboniferous, 378, 403, 407, 417; ii. 11, 45, 46;
Permian, 57, 96;
Tertiary, 125, 136, 183, 199, 208, 291
Basalt-conglomerate, ii. 195
Basic volcanic rocks, silica-percentage of, i. 14;
devitrification of, 20;
flow-structure of, 21;
occur in thinner sheets than the acid, 24;
metamorphic action of, 94;
erupted at low levels, 98;
scenery of, 102;
converted into schists by deformation, 75, 114, 118, 119, 124, 129;
alternation with acid, 28, 61, 131, 157, 165, 207, 213, 233, 284, 318;
ii. 236, 266, 278
Bass Rock, i. 372, 373, 403
Bassenthwaite Lake, i. 335
Bathgate, puy eruptions of, i. 440, 442, 445, 456, 461
Bauer, Dr. M., i. 62
Bauxite, ii. 197, 204
Bayley, Mr. W. S., ii. 330
Bedding in lavas, i. 24
Bell, Sir I. Lowthian, ii. 1, 113, 137, 165
Bemrose, Mr. H. A., ii. 10, 11, 13, 16, 17, 18, 20, 21
Ben Cruachan, alteration of granite at, i. 343
---- Hiant, basic sills of, ii. 318
Benaun More, felsite of, i. 347
Berger, J. F., ii. 22, 95, 110, 113, 139, 140, 141, 145, 199, 364, 426
Bertrand, Prof. M., i. 28
Berwickshire, i. 272, 290, 338, 375, 385, 401, 413
Berwyn Hills, i. 176, 186, 208, 218
Biggar, volcanic area, i. 287, 325
Binney, E., ii. 56
Binny Craig type of basalt, i. 419, 421 (444)
Biotite (_see_ Mica)
Bitumen in intrusive rocks, i. 421
Blackstone (Derbyshire), ii. 18, 21
Blair-Atholl Limestone, i. 122
Blake, Rev. J. F., i. 126, 130, 144, 160, 161, 162, 163, 165, 166, 168,
220, 221, 222
Blocks, ejected, i. 36, 423, 438; ii. 197, 221
Bole between lavas, i. 442; ii. 197, 203, 206, 254
Bombay, volcanic plateau of, ii. 180
Bombs, volcanic, i. 60; ii. 39
Bonney, Prof., i. 95, 126, 130, 136, 144, 160, 162, 163, 164, 165, 166,
167, 168, 192, 210, 227
Borrowdale Volcanic Series, i. 227
Bosses, volcanic, i. 56, 78, 88;
petrography of, 89;
differentiation in, 90; ii. 476;
granitic, i. 93;
metamorphism around, 94, 95;
conditions of their intrusion, 97, 98;
weathering of, 102
---- Silurian, i. 215, 235;
Old Red Sandstone, 277, 288;
Carboniferous, 403, 458
---- Tertiary, ii. 271, 284, 327, 366, 378, 395, 403;
boundaries of, 382;
relation to older eruptive vents, 280, 384, 399;
relation to plateau basalts, 386, 396, 402, 404;
relation to gabbro intrusions, 391, 402, 404;
relation to the basic dykes, 395
Bostonite, ii. 47
Boué, Ami, i. 268, 363; ii. 112, 372
Boule, M., i. 27, 29, 44, 45, 46, 61; ii. 375
Boutan, M., i. 62
Bowden Hill, type of doleritic basalt, i. 418, 421
Braid Hills, great vent of, i. 289, 293, 311, 318, 323
Branco, Prof. W., i. 46, 417
Breccias, volcanic, i. 31, 32, 120, 131, 135, 147, 165, 189, 190, 197,
213, 224, 225, 233, 234, 246, 252, 255, 289, 347; ii. 39, 41, 49, 195
---- of non-volcanic materials, ii. 196, 423
Brecciated structure, i. 162, 211
Breidden Hills, i. 176, 190, 208
Brent Tor, ii. 33, 35, 36
Bréon, M. R., ii. 191
Britain, advantageous position of, for the study of ancient volcanic
action, i. 6;
completeness of the Geological Record in, 6;
direction of folds and fractures in, 11;
chief lavas found in, 31;
Vesuvian cones of, 42;
volcanic plateaux of, 43;
puys of, 46;
lacustrine volcanoes of, 49;
fissure eruptions of, 52;
scenery of volcanic rocks of, 100, 101;
pre-Cambrian rocks of, 111;
in Cambrian time, 141;
in Silurian time, 173;
in Devonian time, 258;
in Old Red Sandstone time, 263;
in Carboniferous time, 355;
in Permian time, ii. 53;
in older Tertiary time, 108
Brögger, Prof., i. 28, 88, 90, 91, 92
Bryce, J., i. 314, 369
Buch, L. von, i. 27; ii. 381
Buckland, W., ii. 95, 110, 113
Buddle, J., ii. 113
Builth, i. 176, 203
Burdiehouse Limestone, i. 361, 374, 388, 415, 463
Burnt Country of Asia Minor, i. 2
Burntisland, Binn of, i. 428, 429, 433, 435, 457, 459
Burntisland Sill type of dolerite, i. 418, 421
Busz, Mr. K., i. 261
Bute, Isle of, i. 369, 378, 407
Cadell, Mr. H. M., i. 114, 423; ii. 334
Cader Idris, volcanic rocks of, i. 42, 175, 176, 177, 178, 179, 180,
181, 182, 188, 207
Caer Caradoc, i. 131, 132, 170
Caerfai group (Cambrian), i. 155
Caernarvonshire, volcanic rocks of, i. 159, 207
Caithness Flags, i. 343, 352
---- volcanic vents in, i. 352
Calciferous Sandstones, i. 361, 366, 415
Calcite as a matrix of tuffs, ii. 27, 39, 41
Caldecote volcanic rocks, i. 170
Callaway, Dr. C., i. 126, 130, 132, 134, 220, 221
Calton Hill, lavas and tuffs of, i. 373, 378, 385, 386, 389
Cambrian system, i. 112, 123, 133, 139, 143, 144;
volcanoes of, 145, 159
Campbeltown, volcanic rocks of, i. 312, 386
Campsie Fells, i. 102, 368, 369, 384, 386, 389, 393, 397, 398, 400, 403,
410, 412, 447
Canary Islands, i. 27
Canna, basalts of, ii. 184, 187, 190, 215, 216;
vent in, 288
Cantyre, volcanic rocks of, i. 311, 369, 370, 386
Caradoc group, i. 175, 196
Carbonaceous rocks, influence of, on igneous masses, i. 95, 426, 449, 456;
ii. 65, 87, 104, 165
Carboniferous Limestone, origin of, i. 357
---- system, subdivisions of, in Britain, i. 358, 360, 366;
ancient geography of, 355, 361, 362, 432, 462;
flora and fauna of, 356
---- volcanic plateaux, distribution of, i. 364, 367;
nature of materials constituting, 377;
structure of, 383;
bedded lavas and tuffs of, 383;
vents of, 54, 394, 399
---- Puys, i. 46, 47, 308, 364;
of Scotland, 414;
nature of the materials erupted by, 416;
necks of, 424;
bedded lavas and tuffs of, 417, 436, 440;
sills of, 446, 472;
bosses of, 458, 465;
dykes of, 460;
of Derbyshire, ii. 8;
Isle of Man, 22;
of Somerset, 32;
of Devonshire, 32;
of King's County, 37;
of Limerick, 41
Carlingford, igneous rocks of, i. 96; ii. 175, 371, 420
Carnedd Dafydd, i. 209
Carnmony Hill, ii. 272
Carrock Fell, differentiation in rocks of, i. 91;
metamorphism at, 94, 96;
as a volcanic boss, 235, 236
Cement-stone group, i. 362, 366, 387, 418, 462
Cellular structure of volcanic rocks, i. 15, 33
Chalk, metamorphism of, by a dyke, ii. 164
Champernowne, A., i. 260, 262
Charnwood Forest, i. 134; ii. 53
Cherts associated with volcanic rocks, i. 123, 167, 169, 173, 174, 184,
196, 197, 240, 254; ii. 25, 36
Cheviot Hills, i. 102, 271, 272, 274, 275, 277, 278, 290, 293, 336
Chilled margin in intrusive rocks, i. 81, 83; ii. 126, 158, 160, 172,
299, 303, 310, 317, 321, 392, 402
Christiania, eruptive rocks of, i. 28
Chronology, volcanic, how determined, i. 46
Clark, Mr. G. T., ii. 180
Claystone, i. 277, 279, 318, 324, 327; ii. 403
Cleavage, effects of, on igneous rocks, i. 162, 165, 224, 231, 232, 234,
237, 260, 261; ii. 36
Clee Hills, ii. 101, 102
Cleveland Dyke, ii. 1, 122, 139, 140, 142, 144, 146, 147, 150, 153, 167,
168, 169
Clough, Mr. C. T., i. 114, 201, 236, 274, 290, 337; ii. 123, 124, 127,
128, 132, 137, 138, 142, 145, 146, 152, 162, 171, 172, 316, 384, 437
Clyde, Carboniferous volcanic plateau of, i. 368, 384, 385, 393, 400,
407, 411
Coal interbedded among volcanic rocks, i. 392, 423; ii. 198, 213, 251, 287
---- alteration of, at volcanic vents, i. 72; ii. 64
---- alteration of intrusive rocks by, i. 95, 451
---- alteration of by sills, dykes, etc., ii. 67, 164, 166
Coal-measures, i. 358, 360, 366
Coalbrookdale Coal-field, ii. 103
Cole, Prof. G. A., i. 176, 177, 178, 179, 180, 181, 184, 187, 188, 210,
211; ii. 134, 205, 212, 245, 370, 371, 378, 426
Colorado, Grand Cañon of, i. 30;
laccolites of, 86
Columnar structure, i. 25, 27, 343, 378, 385, 459; ii. 164, 186, 206, 301
Comley Sandstone, i. 144
Cones, volcanic, connection of, with necks, i. 70;
contemporaneous denudation of, 73; ii. 202, 218, 230;
entombment of, i. 433, 463; ii. 66
Conglomerates, volcanic, i. 31, 37, 183, 190, 286, 300, 307, 309, 310,
314, 315, 330, 341; ii. 195, 198, 218, 284
Coniston Limestone, i. 229, 231
Contemporaneity in Geology, 201
Continents, origin of, i. 11
Contraction, effects of terrestrial, i. 12, 97, 98
Conybeare, J. J., ii. 95
---- W., i. 171; ii. 9, 95, 110, 113, 199
Cooling, effects of, in inducing varieties of texture in igneous rocks,
i. 78, 79, 81; ii. 274, 275, 299, 303, 310, 317, 392, 402
Coon Butte, Arizona, i. 58
Cork, County, volcanic breccias of, ii. 49
Corndon, sill of, i. 176, 189, 190
Corston Hill, i. 373, 386, 387
Craiglockhart type of dolerite and basalt, i. 418
Crater, consolidation of tuff within a, i. 429
Crater-lakes, i. 58; ii. 266, 275
Cretaceous period, geography of the, ii. 108, 182
Cross Fell, i. 228, 229, 238
Cross, Mr. Whitman, i. 86
Crush-conglomerates or breccias, i. 32, 220, 223, 225, 244; ii. 281,
347, 352
Crushing, mechanical effects of, i. 315 (_see_ Schist)
Crust, contraction of the terrestrial, i. 12, 97, 98;
oldest rocks of, 110;
deformation of, 117, 121, 264, 295, 297
Cryptocrystalline type of basalt, i. 419
Crystallites of volcanic rocks, i. 18
Crystals, different periods of formation of, in volcanic rocks, i. 19, 20,
21, 421; ii. 128, 131, 134, 135;
ejected by volcanic vents, i. 62, 178, 180, 181, 195, 213, 234, 245;
ii. 27, 49, 58, 79
Cuillin Hills, scenery of, i. 106;
gabbro of, ii. 329, 361;
acid rocks of, 391
Culots, i. 78, 88
Culm-measures, ii. 33
Cumbrae Islands, i. 368, 369, 378, 407
Cumming, J. G., ii. 22
Cycles, volcanic, i. 27, 92; ii. 116
Dacite, i. 230; ii. 185
Dakyns, Mr. J. R., i. 90, 229, 272; ii. 10
Dalmellington, volcanic rocks at, i. 333; ii. 62
Dalmeny type of dolerite and basalt, i. 418, 420
Dalradian rocks, probable crushed necks of, i. 75, 125;
lavas and sills of, 121;
green schists of, 124
Dalry, Ayrshire, buried volcanoes of, i. 434
Dana, J. D., ii. 189
Darwin, C., i. 27
Daubrée, A., i. 72, 404
Davies, J., i. 156, 157
Dechen, H. von, i. 46; ii. 112, 280, 333, 340, 367, 372, 381
Deformation, effects of, on volcanic rocks, i. 75, 115, 117, 119, 121,
127, 129, 162
De la Beche, H., i. 142, 143, 170, 175, 204, 205, 207, 259; ii. 9, 10,
19, 33, 95, 96, 97
Delessite, ii. 79
Denudation, influence of, on volcanoes, i. 3, 4, 8, 40, 43, 45, 46, 54,
58, 71, 73, 75, 79, 87, 100-107, 370, 433, 434, 436, 476; ii. 55, 61,
62, 179, 181, 182, 241, 245, 248, 249, 255, 257, 282, 283, 292, 316,
317, 363, 373, 407, 455
Derbyshire, toadstones of, i. 359; ii. 8
Desmarest, ii. 373
Devitrification of volcanic rocks, i. 18, 19, 78; ii. 437, 446
Devonian system, i. 257;
volcanoes of, 259
Devonshire, volcanic scenery of, i. 103;
Devonian volcanic rocks of, 259;
Carboniferous volcanic rocks of, ii. 32;
Permian volcanic rocks of, 94
Diabase, i. 151, 153 (analyses), 156, 192, 194, 204, 206, 214, 217, 235,
240, 247, 249, 273 (analyses), 278, 292, 318, 320, 330, 335, 344, 345,
351, 403; ii. 5, 136, 415
Diabase-porphyrite, i. 192, 204
Diamond found in volcanic vents, i. 62
Dick, Mr. A., jun., i. 380
Dickson, Mr. E., ii. 23
Differentiation in igneous rocks, i. 22, 27, 84, 85, 90, 91, 449;
ii. 300, 475
"Dimetian," i. 145
Dingle-beds, i. 346
Dingle, Upper Silurian nodular lavas of, i. 20, 254
Diorite, i. 78, 247, 249, 277, 278, 288; ii. 36
Dirrington Law, i. 290
Dittmar, Prof., ii. 137
Dolerite, i. 119, 134, 178, 190, 206, 230, 247, 261, 378, 403, 407, 417,
448; ii. 5, 11, 35, 49, 102, 103, 104, 125, 136, 157, 183, 271, 299,
303, 307, 319, 328
Dolgelli, i. 169, 178, 188
Donegal, Dalradian rocks of, i. 122;
dykes in, ii. 124
Drogheda, volcanic rocks near, i. 244
Duffin, W. le S., ii. 426
Dumbarton, rocks near, i. 402, 404
Dumfoyn, a volcanic neck, i. 395, 398, 400
Dumgoyn, a volcanic neck, i. 395, 397, 398, 400
Dundee, sills and bosses near, i. 292, 306
Duneaton Water, volcanic rocks of, i. 329
Dunite, ii. 309
Du Noyer, G. V., i. 245, 250, 254; ii. 272, 426
Durham, Mr., i. 275
Durness Limestone, i. 112, 121, 123, 141
Dust, volcanic, i. 13
Dutton, Capt. C. E., i. 68; ii. 267
Dykes, vitreous margins of, i. 18;
formation of, 54, 98;
in necks, 66;
filled with agglomerate, 70;
grouping of, among intrusive rocks, 77;
character of, 79;
extent of, in Britain, 80;
age of, 81;
compound, 81; ii. 59;
expulsion of lava from, i. 82; ii. 128;
connected with the surface, i. 82;
pre-Cambrian, 118;
flow-structure in, 161;
Cambrian, 156;
Silurian, 187, 216, 235, 237, 248, 249;
Old Red Sandstone, 277, 291, 338, 345;
Carboniferous, 406, 429, 460; ii. 1, 30;
Permian, 83;
amygdaloidal structure of, 85
---- Tertiary, ii. 114;
arguments for their geological age, 118, 125, 171;
geographical distribution, 121;
two types of protrusion of, 122;
nature of component rocks of, 125;
external character of, 126;
classification of basic, 129;
enclosed fragments in, 129, 131, 144;
porphyritic and amygdaloidal structures of, 128, 129, 130;
veins in, 130;
joints in, 132, 166;
microscopic characters of, 134;
chemical characters of, 137, 139;
hade of, 139;
breadth of, 139;
interruptions of, 142;
length of, 142;
persistence of mineral characters of, 144;
direction of, 145, 159;
upward termination of, 147;
known vertical extension of, 150;
evidence of movement of molten rock of, 151;
branches and veins from, 152;
connection with sills, 155;
intersecting, 158;
compound or of more than one infilling, 159;
double, treble, and multiple, 160, 318, 417, 439;
compound, with basic and acid bands, 161, 435;
contact metamorphism of, 163;
relation of, to geological structure, 166;
origin and history of, 175;
Icelandic example of, 261;
example communicating with cinder cone in Utah, 268;
connection of, with surface, 179, 269, 280;
latest protrusions of, 381, 408, 416;
of granophyre, 435, 436, 437, 439
Earth, condition of the interior of the, i. 10;
fractures in crust of the, 11
Earthquakes, influence of, on early man, i. 1;
transient effects of, 3, 8
East Lothian, trachyte lavas of, i. 24;
Carboniferous volcanic plateau of, 370, 389, 403, 409
Edinburgh, volcanic rocks near, i. 24, 102, 104, 269, 273, 276, 279,
281, 285, 287, 289, 291, 293, 311, 317, 318, 323, 364, 370, 373, 385,
386, 387, 389, 410, 420, 436, 449
Egan, Mr. F. W., i. 242; ii. 201, 423
Eifel, i. 4, 46, 58, 100
Eigg, Isle of, ii. 115;
pitchstone of, 185, 217, 242, 445;
brecciated basalt in, 189, 192;
basalt plateau of, 215, 234;
Scuir of, 217, 234, 447;
sills of, 318;
acid bosses of, 403;
acid sills of, 431;
proofs of subsidence at, 447;
enormous denudation of, 239, 447, 458
Eildon Hills, i. 375
Electric Peak, i. 79, 82, 84
Elvans, i. 249, 281
Engulphment craters, i. 58
Ennerdale, granite of, i. 236
Enniscorthy, volcanic rocks near, i. 245
Environment, influence of, on early man, i. 1
Eozoic period, i. 110
Epidiorite, i. 118, 124, 129, 184, 247, 249
Erosion, laws of, i. 101
Eruptions, transient effects of, i. 3;
old submarine, how ascertained, 48;
lacustrine, 49;
fluviatile, 49;
terrestrial, 50;
evidence of intervals between, 283, 287, 300, 442; ii. 42, 59, 203,
205, 221, 251, 254, 287
Erzeroum, old volcanoes near, i. 32
Eskdale Dyke, ii. 127, 133, 136, 137, 140, 143, 145, 146, 153
---- (Lake District), granite of, i. 236
Etheridge, Mr. R, jun., ii. 24
Etna, i. 2, 4, 10, 55; ii. 261
Eurite, i. 188
Europe, basalt plateaux of north-western, i. 51, 52; ii. 181;
pre-Cambrian disturbances of north-western, i. 117
Explosion-craters, i. 58; ii. 266
Explosions, volcanic, i. 246; ii. 196, 266, 425, 472
Extrusive rocks, defined, i. 14;
textures of, 78
Fair Head, sills of, ii. 301
Farey, J., ii. 9
Farne Islands, ii. 2
Faroe Isles, basalt plateaux of, i. 52, 102; ii. 191, 192, 194, 256;
vents in, i. 63; ii. 293;
dykes of, 122, 133;
tuffs and lignites of, 258;
sills of, 322;
absence of gabbro bosses in, 355;
subsidence of, 447;
dip of basalts in, 448;
proofs of denudation in, 458
Faujas St. Fond, ii. 109, 112
Faults, connexion of volcanic vents with, i. 69; ii. 65;
boundary, i. 294, 303, 305, 369; ii. 169;
effects of, i. 446; ii. 200;
connection with, dykes, 168
---- of Tertiary basalt-plateau, ii. 452
Felsite (Felstone), Torridonian, i. 120;
Uriconian, 130, 133;
of Malverns, 134;
Cambrian, 151, 160, 161, 164, 165, 167, 168;
Silurian, 184, 199, 205, 206, 207, 210, 212, 218, 231, 232, 246, 247,
252, 255;
Old Red Sandstone, 276, 277, 291, 293, 321, 327, 335, 346;
Carboniferous, ii. 36, 49;
Permian, 85;
Tertiary, 174, 369, 424, 446
Felsitic breccia, ii. 195
---- type of devitrification, i. 19
Felspar, ejected crystals of, i. 181; ii. 58, 79;
large porphyritic crystals of, in dykes, 129, 135
Fife, Old Red Sandstone volcanic rocks of, i. 307;
Carboniferous volcanic rocks of, 428, 429, 430, 433, 437, 448;
Permian volcanic rocks of, ii. 56, 69
Fingal's Cave, i. 25; ii. 210
Fisher, Rev. O., i. 98
Fishguard, volcanic rocks at, i. 205
Fissure type of volcanoes, i. 42, 52; ii. 108, 115, 267
Fissures, volcanic, i. 42, 52, 53, 54, 425; ii. 141, 145, 159, 176;
filled with agglomerate, i. 70;
filled with dykes, 81, 118;
compound, 82, ii. 159;
pre-Cambrian, i. 118, 119;
Carboniferous, 425;
Tertiary, ii. 141, 159, 176, 425;
modern of Iceland, 262;
cause of, 177
Fleming, John, i. 268
Flow-structure, i. 21, 157, 160, 161, 162, 184, 210, 232, 246, 248, 255,
315, 321, 327, 346; ii. 129, 152, 190, 191, 332, 369, 392, 402, 424,
437, 441
Foot, F. J., i. 316
Forbes, D., ii. 370
---- Edward, ii. 66, 113, 114, 198
---- J. D., ii. 112, 333, 372, 381
Forellenstein, ii. 332
Forest of Wyre coal-field, ii. 102
Forfarshire, volcanic rocks of, i. 285, 299;
flagstones of, 299
Forster, M., ii. 113
Forth-basin, Carboniferous system of, i. 361;
Carboniferous plateaux of, 370;
Carboniferous puys of, 416, 427, 429, 430, 432, 434, 437, 440, 446, 462;
Permian volcanoes of, ii. 55, 67
Foster, Mr. C. le Neve, ii. 10
Fouqué, Prof., i. 18, 21; ii. 134
Fox, Mr. Howard, ii. 36
Fox Strangways, Mr. C., i, 135
Fragmental volcanic rocks, i. 14;
only arise from explosions which reach the surface, 57
(_see_ Agglomerates, Conglomerates, Tuffs)
France, Tertiary volcanoes of Central, i. 4, 10, 29, 41, 45, 49, 58, 60,
70; ii. 31, 271, 281, 373;
Carboniferous volcanic action in, i. 357
Frankland, Prof. E., i. 273, 278
Fundamental complex of oldest gneiss, i. 114, 115
---- gneiss, i. 115
Gabbro, granulitic, ii. 329;
banded structure of, i. 116; ii. 329, 354, 357, 476;
coarse-grained massive, 330;
pale varieties in veins, 330;
gneiss-like aspect of, 342, 254, 358
---- of Carrock Fell, i. 91;
Silurian, 195, 206, 247;
Devonian, 262;
Tertiary, 84, 90, 116; ii. 307, 308, 309, 319, 327, 334, 349, 355, 358,
391, 406, 407
Gairloch, peculiar pre-Cambrian rocks of, i. 115, 117
Galapagos Islands, i. 27
Gallaston type of dolerite and basalt, i. 418
Galloway, granites of, i. 93, 95, 272, 277, 290, 331
Garabol Hill, differentiation at, i. 90
Gardiner, Miss, i. 95
Gardiner, Mr. C. J., i. 256
Gardner, Mr. Starkie, ii. 196, 198, 212
Garlton Hills, i. 102, 370, 377, 378, 379, 380, 386, 390, 405, 412
Garnet found in volcanic vents, i. 62
Garth Grit, i. 177, 185, 208
Gases dissolved in the volcanic magma, i. 13, 15, 72, 97, 99
Geikie, Prof. J., i. 277, 306, 308, 331, 336, 339, 340, 369, 375, 426;
ii. 57, 191, 259, 322
Genèvre, Mont, i. 194
Geological action, supposed former greater intensity of, i, 139
---- contrasts, i. 103
---- history, i. 109, 113
---- Survey of Great Britain, i. 113, 115, 118, 119, 121, 122, 123, 124,
125, 126, 129, 130, 133, 135, 142, 143, 144, 145, 159, 160, 166, 170,
171, 175, 176, 179, 181, 182, 183, 186, 187, 188, 190, 196, 198, 201,
204, 205, 207, 208, 212, 214, 215, 216, 217, 218, 219, 220, 221, 225,
227, 228, 232, 233, 238, 239, 240, 242, 243, 244, 245, 250, 251, 254,
259, 270, 275, 278, 294, 299, 306, 307, 308, 314, 315, 317, 318, 325,
329, 331, 336, 339, 340, 344, 346, 349, 350, 352, 364, 369, 372, 373,
375, 397, 403, 404, 406, 407, 411, 423, 425, 434, 449, 462, 475, 476;
ii. 3, 4, 9, 10, 12, 13, 16, 17, 20, 23, 33, 36, 37, 42, 43, 46, 48, 49,
56, 58, 65, 66, 68, 94, 95, 102, 103, 118, 121, 125, 127, 144, 145, 148,
162, 164, 170, 174, 175, 190, 192, 199, 201, 203, 253, 272, 277, 292,
347, 384, 391, 420, 422, 423, 426, 428, 433, 435, 446, 449
Giant's Causeway, ii. 80, 109, 186, 188, 192, 206
Gilbert, Mr. G. K., i. 87; ii. 362, 363
Girvan, i. 192, 200
Glaciation, absence of, in Devonshire, i. 261
Glass in volcanic rocks, i. 18, 33, 60, 78, 180, 211, 216, 230, 232, 235,
316; ii. 85, 120, 126, 133, 135, 137, 184, 204, 247, 272, 285, 316, 317
Globulites, ii. 135
Gloucestershire, Silurian volcanoes of, i. 238
Gneiss, analogies of, with igneous rocks, i. 93; ii. 476;
oldest, i. 110, 115
Godwin-Austen, A. C., i. 259, 262
Goodchild, Mr. J. G., i. 229, 449; ii. 150
Grainger, Rev. Dr., ii. 198
Grand Sarcoui, ii. 373, 374, 381
Granite, bosses of, i. 88, 90, 93;
plutonic and volcanic, 89;
metamorphism by, 95;
altered by dykes, ii. 164;
pre-Cambrian, i. 119;
post-Arenig in Highlands, 126, 310;
in Cambrian rocks, 155;
in Silurian rocks, 200, 229, 236, 238, 249;
of probably Old Red Sandstone age, 272, 277, 290, 331, 337;
Tertiary, ii. 366, 418, 420
Granitite, i. 188, 277, 290, 337; ii. 367
Granophyre, alteration of rocks by, i. 95, 96;
scenery of, 105;
solvent action of, 82, 84, 85, 96, 99; ii. 163, 392, 415, 422, 433;
brecciated, 382;
spherulitic, 381;
bedded structure of, 381, 403, 404;
apt to be intruded at the base of a volcanic series, 403;
shattering of rocks invaded by, 405, 411, 413, 416, 439;
veins of, 409, 410, 432, 437;
Silurian, i. 214, 215
---- Tertiary, i. 339; ii. 368, 395, 408, 430;
boundaries of, 382, 409;
relation to older vents, 280, 384, 399;
relation to plateau-basalts, 386, 396, 402, 404;
relation to gabbro, 391, 402, 404, 410;
relation to basic dykes, 395;
proof of liquidity of, 413;
sills of, 430, 436, 437;
dykes of, 435
Granophyric structure, i. 20; ii. 366
Graphite in Tertiary volcanic series, ii. 198
Graptolites, i. 174, 196, 197
Grauwacke or Devonian rocks, De la Beche on, i. 259, ii. 33
Graves, Lieut. T., ii. 451
Great Glen of Scotland, i. 121
Greeks, influence of volcanoes on, i. 1
Green, A. H., i. 133, 134, 163; ii. 10, 12
Greenland, Tertiary basalts of, ii. 182
Greenly, Mr. E., i. 129, 214
Greenock, Lord, i. 363
Green schists of the Scottish Highlands, i. 122;
of Anglesey, 129
Greenstone, i. 183, 187, 206, 217, 219, 249, 259, 261; ii. 34, 35, 37,
103, 104, 355
Greenstone-ash, i. 219
Griffith, Sir R., ii. 299, 422
Gunn, Mr. W., i. 114, 298, 311, 336, 369, 407, 410; ii. 58, 172, 420
Gypsum deposits, ii. 54
Hade of dykes, ii. 139
Hæmatitic iron-ore, ii. 197
Hall, Sir James, i. 72, 363
Hälleflinta, i. 131, 167
Hardman, E. T., ii. 365, 449
Harker, Mr. A., i. 90, 91, 93, 95, 96, 99, 188, 209, 210, 211, 212, 213,
214, 217, 218, 222, 227, 228, 230, 231, 232, 235, 236, 237, 238, 290;
ii. 124, 125, 126, 129, 130, 139, 144, 146, 160, 162, 163, 164, 174,
185, 189, 190, 223, 224, 247, 269, 281, 284, 285, 309, 310, 318, 320,
334, 339, 347, 348, 368, 382, 384, 385, 387, 389, 392, 407, 408, 409,
413, 415, 433, 434, 437, 441, 446
Harkness, R., i. 228; ii. 56
Harlech anticline, i. 159, 179, 187;
group, 176
Hatch, Dr. F. H., i. 183, 184, 187, 188, 229, 230, 246, 247, 248, 249,
261, 277, 278, 306, 377, 380, 381, 417, 419, 420; ii. 57, 96, 184, 274,
276, 299, 319, 332, 367, 368, 369, 370, 388, 398
Haughton, Prof. S., i. 346; ii. 422
Hawaii, lava-fountains of, i. 12;
differentiation in lavas of, 27;
lava-cauldron of, 58
Haworth, Mr. E., ii. 96
Hay Cunningham, R. I., i. 269, 317, 363, 372, 373, 449, 451; ii. 237,
238, 244
Heaphy, Mr. C., i. 432
Hebrides, basalt-sheets of, i. 24, 47, 52, 102;
acid rocks of, 95, 102;
gabbros of, 84, 90, 102;
scenery of, 105;
pre-Cambrian rocks of, 112, 114, 117, 121;
Cambrian land of, 141;
early observations on the Tertiary volcanic rocks of, ii. 109, 110, 111;
dykes of, 118, 146, 158, 174;
basalts of, 181, 186, 215;
pitchstone lava of, 238, 246;
plateau-scenery of, 249;
Tertiary rivers and lakes of, 217, 228, 231, 234, 252;
vents of, 274;
basic sills of, 304;
gabbro intrusions of, 327;
acid intrusions of, 364, 379, 430, 437;
dislocations of, 452;
denudation of, 455
Heddle, Dr., i. 274, 302; ii. 78, 79, 246, 307, 406
Helland, Prof. A., ii. 191, 261, 263, 264
Henderson, Mr. J., i. 449
Henry Mountains, laccolites of, i. 86; ii. 362
Henslow, J. S., ii. 22, 224
Heterogeneity in volcanic magmas, i. 85, 90; ii, 190, 334
Hett Dyke, ii. 1, 7, 147
Hibbert, S., i. 46
Hicks, Dr. H., i. 126, 145, 154, 158, 159, 166, 206
Hill, Mr. J. B., ii. 140
Hill, Rev. E., i. 135
Hinde, Dr. G. J., i. 198; ii. 35
Hinxman, Mr. L., i. 114, 344; ii. 121
Hobson, Mr. B., i. 260; ii. 23, 27, 96, 99
Holden, Mr. J. S., ii. 204
Holl, H. B., i. 133, 134, 170
Holland, Mr. P., i. 177, 178, 179; ii. 23
Hollybush Sandstone, i. 133, 170
Holocrystalline structure, i. 78; ii. 136, 184
Hopkins, W., ii. 177, 179, 268
Hornblende, ejected crystals of, i. 178, 181; ii. 49, 51, 58, 79
Hornblende-schists formed from basic igneous rocks, i. 75, 114, 118, 119,
124, 129
Horne, Mr. John, i. 114, 196, 199, 200, 344, 345, 375; ii. 23, 144, 292
Hornito of a lava-stream, i. 55; ii. 264
Hornstone, i. 131, 136, 277, 278 (analyses), 324
Houston Marls, i. 423, 436, 440, 444, 466
Howard, Mr. H. T., i. 207
Howell, Mr. H. H., i. 294, 307, 364
Hoy, Island of, i. 350
Hughes, Prof. T. M'K., i. 126, 144, 160, 161, 166, 168, 220, 222, 223, 227
Hull, Mr. E., ii. 42, 95, 103, 272, 421, 426, 449
Hurlet Limestone, i. 360, 366, 394, 410, 415, 444, 452, 456, 467, 470, 474
Huronian rocks, i. 111
Hutchings, Mr. W. M., i. 227, 230, 233
Hutton, James, i. 363; ii. 9, 110
Hutton, W., ii. 3
Hyperite, i. 279
Hysgeir, pitchstone of, ii. 246
Iceland, Tertiary basalts of, ii. 182, 260;
Tertiary gabbros and liparites or granophyres of, 261;
continuity of volcanic phenomenon of, 261;
lava-fields of, i. 24, 42, 53, 100; ii. 260;
lava-domes of, i. 10; ii. 265;
fissures of, i. 70; ii. 262, 271, 454;
dykes of, 122, 261;
cinder cones of, 264, 271;
subsidence of, 447
Idaho, lava-fields of, ii. 267
Iddings, Prof., i. 28, 29, 30, 78, 79, 82, 84, 90; ii. 128, 178
Idiomorphic crystals, i. 21, 417, 420; ii. 40
Index Limestone of the Scottish coal-fields, i. 360, 444, 452
India, fissure-eruptions of, i. 10;
volcanic plateau of, ii. 180
Intermediate volcanic rocks, silica-percentage of, i. 14
Intersertal structure, i. 417; ii. 136
Intrusive rooks, defined, i. 14;
occasional cellular character of, 16;
flow-structure in, 22, 161;
varieties of, 77;
textures of, 78, 449; ii. 274, 360, 392;
in sheets, sills, and laccolites, i. 83;
melting of rocks by, 82, 84; ii. 129, 163, 392;
consolidation of, i. 84;
banding of, 84, 450; ii. 329, 342;
heterogeneity of, i. 85; ii. 344;
metamorphism by, i. 94, 451;
influence of surrounding rocks on, 95;
conditions of their intrusion, 97;
columnar structure in, ii. 187, 291, 301
---- Pre-Cambrian, i. 116;
Cambrian, 156;
Silurian, 187, 195, 206, 216, 235, 237, 248, 249;
Devonian, 261;
Old Red Sandstone, 277, 291, 321, 335, 338, 343, 345;
Carboniferous, 406, 408, 420, 446; ii. 1, 21, 30, 33, 40, 48;
Permian, 58, 64;
Tertiary, 270, 298
Ireland, submarine eruptions of, i. 48;
Dalradian rocks of, 122, 123, 126;
Arenig rocks in, 123;
Silurian volcanic rocks of, 239, 251;
granites of, 290;
Old Red Sandstone volcanic rocks of, 346, 348;
Carboniferous volcanic rocks of, 359; ii. 37;
early observers among the Tertiary volcanic rooks of, 109;
Tertiary basalt plateau of, 364, 370, 371;
gabbros of, 359;
acid rocks of, 420
Iron-ore, pisolitic (Arenig), i. 181, 208;
Tertiary, of Antrim, ii. 204
Irvine, Mr. D. R., i. 294, 299
Irving, Rev. A., ii. 95
Isogeotherms, shifting of, i. 98
Italy, old volcanoes of, i. 4; ii. 474, 477
Jack, Mr. R. L., i. 294, 308, 369, 375, 396, 404; ii. 57, 145
Jameson, Robert, i. 268, 269, 317, 363; ii. 109, 161, 244, 333, 355, 364
Jan Mayen, ii. 182
Jedburgh type of dolerite and basalt, i. 418
Jennings, Mr. C. V., i. 176, 177, 179, 180, 181, 184, 185, 186, 187, 188
Johnston-Lavis, Dr., ii. 261
Joints in dykes, ii. 132
Judd, Prof. J. W., i. 157, 275; ii. 115, 116, 134, 137, 162, 185, 209,
211, 245, 247, 267, 274, 278, 280, 303, 307, 309, 315, 316, 319, 322,
328, 329, 332, 333, 349, 356, 360, 372, 388, 410, 439
Jukes, J. B., i. 143, 171, 175, 208, 218, 219, 245, 246, 250, 254, 316;
ii. 10, 20, 42, 47, 49, 101, 103, 105
Jurassic period, physical conditions of the, ii. 108, 182
Kelly, J., i. 314
Kenmare, Old Red Sandstone volcanic rocks of, i. 350
Keratophyre, i. 247
Kerrera, Isle of, i. 342
Kersantite, i. 261
Keswick, i. 229
Kildare, Chair of, Bala volcanic rocks at, i. 245, 256
Killarney, nodular lavas of, i. 20, 272, 346
Kilpatrick Hills, i. 385, 388, 403, 410
Kilroe, Mr. J. R., i. 251, 253, 315
Kilsyth type of dolerite and basalt, i. 418
Kinahan, Mr. G. H., i. 349; ii. 45, 49, 426
Kincardineshire, volcanic necks of, i. 281, 286, 293, 299;
Old Red Sandstone of, 301
King, Mr. Clarence, i. 27
King's County, volcanic necks of, ii. 37
Kippie Law type of basalt, i. 418
Kirkby, Mr. J., ii. 106
Kirwan, R., ii. 110
Knockfeerina, Old Red Sandstone volcanic rocks of, i. 349
Knocklayd, ii. 200
Kynaston, Mr. H., i. 343
Labyrinthodonts, i. 356
Laccolites, i. 77, 83, 86, 88, 98, 99, 190; ii. 363
Lacroix, Prof., i. 96
Lacustrine volcanic eruptions, i. 49
Lagorio, Dr. A., ii. 137
Lake, Mr. P., i. 177, 179
Lake-District, i. 227, 290;
Vesuvian cone of, i. 42, 45
"Lake Caledonia," i. 272, 294, 296
"Lake of Lorne," i. 341
"Lake Orcadie," i. 266, 271, 343, 350
Lakes, eruptions in, i. 49;
crater, 58;
of Old Red Sandstone, 264
Lambay Island, conglomerates of, i. 244
Lammermuir, granites of, i. 290, 340
Lamplugh, Mr. G. W., i. 32, 220; ii. 23, 28
Lamprophyre, i. 291, 293
Lanarkshire, i. 291, 368, 416
Land, sculpture of the, i. 101, 102
Landslips, ii. 200, 287
Lankester, Prof. E. Ray, i. 310
Lapilli, volcanic, i. 33, 34, 61, 151
Lapworth, Prof. C., i. 130, 132, 137, 171, 172, 189, 190, 196
Largs, volcanic vent near, i. 56, 396, 397, 401
Lasaulx, Prof. von, ii. 365, 371, 426
Laurentian gneiss, i. 110
Lavas, classification of, i. 14;
flow-structure of, 16, 21;
vesicular structure of, 17;
glass in, 18;
devitrification of, 19;
bedding of, 24;
effect of water on molten, 25, 334;
sack-like or pillow-structure of, 26, 184, 193, 201, 240, 244, 252;
seldom occur in solitary sheets, 26;
variations in structure in, 27;
sequence of, in eruptions, 28, 92, 377, 386;
crusts of, disrupted in volcanic explosions, 58, 59, 60; ii. 189;
alternations of acid and basic, i. 28, 61, 152, 157, 165, 207, 213,
284, 318; ii. 236, 266;
contrasted with intrusive rocks, i. 78;
sandstone veins in, 283, 300, 303, 320, 327, 333, 337; ii. 59, 98;
shattered or agglomerate structure of, 99;
metamorphism of, i. 231, 240, 338; ii. 272, 276, 337, 339, 340, 347,
355, 379, 386, 397, 399, 400, 404, 413
---- Cambrian, i. 152, 168
---- Silurian of Merionethshire, i. 183;
Scotland, 191;
Builth, 203;
Pembrokeshire, 205;
Caernarvonshire, 207;
Berwyn Hills, 218;
Anglesey, 219;
Lake District, 227;
Gloucestershire, 238;
Ireland, 239, 254
---- Lower Old Red Sandstone, i. 273, 281, 294, 317
---- Carboniferous, i. 377, 384, 417, 436, 440, 443; ii. 8, 18, 34, 45
---- Permian, ii. 68, 96
---- Tertiary, ii. 183;
types of, 186;
banding of, 189;
thickness of, 192;
lenticular character of, 193;
of Antrim, 199;
irregular bedding of the vitreous, 243
---- modern Icelandic eruptions of, ii. 261
Lava-domes, ii. 265
Lava-plug of volcanic funnels, permanence of, i. 40, 41, 55, 73, 76, 430
Lawson, Prof. A. C., i. 82
Leaf-beds, ii. 198
Lebour, Prof., i. 336; ii. 2, 3, 5, 7
Leckstone, i. 419, 442, 443
Lecoq, H., i. 45; ii. 373
Leinster granite, ii. 245, 249, 290
Lewisian Gneiss, i. 81, 110, 111, 113, 118
Liddesdale, Carboniferous volcanic vents of, i. 55, 416, 425, 440, 475
Life, earliest traces of, i. 140
Lignite in Tertiary volcanic series, ii. 198
Limburgite, i. 377, 408, 417, 420, 448; ii. 40, 46
---- type, i. 418
Limerick, Carboniferous volcanic rocks of, i. 421, 430; ii. 41;
Old Red Sandstone volcanic rocks of, i. 348
Limestone, metamorphism of, i. 72, 451; ii. 14, 22, 164, 280, 383
Lindley and Hutton on Eigg conifer, ii. 238
Lingula Flags, i. 144, 177
Linlithgowshire (_see_ West Lothian)
Lintrathen, porphyry of, i. 277, 292, 311
Lion's haunch type of dolerite and basalt, i. 418
Lithomarge, ii. 197, 204
Lizard, rocks at the, i. 194
Llanberis, Pass of, i. 159, 163
---- group, i. 144
Llandeilo group, i. 175, 196, 242;
volcanic rocks of, 186, 202, 221, 227, 241
Llandeiniolen, i. 160
Llandovery group, i. 175, 196;
possible volcanic rocks of, 238
Llangadock, i. 205
Llangefni, i. 220
Llanllyfni, i. 161
Llanwrtyd, i. 204
Lleyn Peninsula, i. 208, 209, 213, 215
Lloyd Morgan, Prof., i. 145, 147, 154
Llyn Padarn, i. 157, 159, 160
Loch Carron, pre-Cambrian rocks of, i. 115, 117
Loch Lomond, dykes at, ii. 180
Loch Tay Limestone, i. 122, 124, 125
Lomas, Mr. J., ii. 191, 322
Longulites, ii. 135
Lonsdale, W., i. 257
Longmyndian rocks, i. 111, 129, 132
Lorne, volcanic rocks of, i. 102, 271, 281, 341
Lough Mask, Silurian volcanic rocks of, i. 251
---- Nafooey, Silurian volcanic rocks of, i. 251
---- Neagh, subsidence of site of, ii. 201, 205;
history of, 448
Ludlow group, i. 175
Lycopods, fossil, i. 174
Maare, volcanic, i. 58; ii. 287, 288, 296
Macconochie, Mr. A., ii. 58
Macculloch, John, i. 95, 269, 270; ii. 22, 111, 113, 123, 140, 154, 156,
159, 172, 174, 175, 213, 217, 231, 237, 244, 251, 280, 293, 304-307,
310, 314, 315, 327, 349, 364, 371, 403, 406, 408, 409, 418
Macknight, Dr., i. 268, 317
Maclaren, Charles, i. 269, 317, 325, 363, 372, 373, 451, 462; ii. 67
M'Henry, Mr. A., i. 32, 240, 242, 244, 314, 347; ii. 201, 204, 272, 293,
426, 427, 428, 429
M'Mahon, General, i. 260; ii. 35, 36
Magma, volcanic, explosive energy of, i. 13;
gases and vapours dissolved in, 13, 72, 97, 99;
differentiation of, 22, 84, 91;
solvent action of, 82, 84, 85, 99; ii. 392, 415, 422, 433;
heterogeneity of, i. 85, 90, 91; ii. 344, 360, 476;
metamorphic action of, i. 94;
alteration of, by incorporation of foreign material, 96;
ii. 386, 390, 392;
conditions for the injection of, i. 97, 99
Magnesian Limestone, ii. 54
Malvern, pre-Cambrian volcanic rocks of, i. 133;
Cambrian volcanic rocks of, 169
Man, Isle of, Carboniferous volcanic rocks of, ii. 22
Manod, i. 184
Marl Slate, ii. 54
Marl, volcanic, i. 423, 436, 440, 444, 466
Marr, Mr. J. E., i. 227, 228, 230, 231, 232, 236, 237, 238, 290; ii. 189
Matlock Bath, ii. 13, 22
Mediterranean, earthquakes and volcanoes of, i. 1
Melaphyre, i. 131
Mello, Mr. J. M., ii. 22
Melrose, rocks near, i. 397, 398, 400, 425
Melting of rocks by igneous intrusions, i. 82, 84, 85, 96, 99; ii. 163,
392, 415, 422, 433
Menai Strait, i. 159
Menevian group, i. 114
Merse, volcanic plateau of the, i. 375
Metamorphism of tuffs, i. 157;
of lavas (_see_ under Lavas)
---- by lavas, i. 27;
by sills and bosses, 87, 94, 95, 216, 236, 331, 338, 431; ii. 7, 22,
36, 131, 148, 163, 299, 300, 310, 337, 339, 340, 347, 355, 356, 357,
358, 362, 378, 383, 386, 397, 399, 400, 404, 413
---- in vents, i. 67, 71, 82, 93, 399, 404; ii. 39, 78, 292
---- around vents, i. 72, 349, 350, 352, 399, 404, 432; ii. 76, 272, 273,
276, 280, 292
---- regional, i. 121, 123; ii. 35
Mica, ejected crystals of, in volcanic breccias, ii. 49, 58, 79, 80
Mica-porphyrite, i. 277, 338
Michel Lévy, M., i. 18, 21, 46, 88; ii. 373, 374
Microgranite, i. 131, 215, 235, 249; ii. 367, 437
Microlites of igneous rocks, i. 18, 21, 33; ii. 135, 275
Micropegmatitic structure, i. 20, 418, 449; ii. 5, 368, 437
Microscopic examination of rocks, i. 18, 21
Midlands, eruptive rocks of English, ii. 100
Midlothian, Carboniferous volcanic plateau of, i. 373, 385, 387;
sills of, 446
Miller, Hugh, ii. 237
---- Mr. H., i. 336
Mills, Abraham, ii. 109
Millstone grit, i. 358, 360, 366
Minette, i. 277, 278, 291, 293
Minto Crags, i. 375, 397
Mitchell, Rev. Hugh, i. 301
Moel Siabod, i. 175
---- Wyn, i. 175, 176, 184, 185
Monckton, Mr. H. W., i. 449; ii. 224
Montana, lava-fields of, ii. 115, 267
Montrose, volcanoes of, i. 299
Moray Firth, basin of, i. 271, 343
Morton, Mr. G. H., i. 189
Morven, basalt-plateau of, ii. 208
Mountain-chains, origin of, i. 11, 12, 98
Mourne mountains, granite of, i. 93; ii. 124, 366, 367, 420
Muck, Isle of, ii. 215
Mud-lava, ii. 85
Mudstone, volcanic, i. 423, 436, 440, 444, 466; ii. 86, 222, 258
Mull, branching amygdales of, i. 17;
perlitic glass from, 19;
pale lavas of, ii. 184, 213;
basalt of, 188, 192, 193;
breccias of, 196;
plateau of, i. 24, ii. 208;
non-volcanic breccias in, 196, 211;
flint gravel in, 211;
leaf-beds of, 212;
vents in, 274, 278;
gabbro of, 355;
acid bosses of, i. 20; ii. 395;
acid sills of, 430;
acid dykes and veins of, 443;
enormous denudation of, 457, 461
Murchison, R. I., i. 113, 121, 129, 142, 173, 175, 189, 204, 205, 207,
238, 257; ii. 56, 95
Mynydd-mawr, i. 209, 211, 216
Mythology, influence of earthquakes and volcanoes on, i. 2
Nant Francon, i. 161
Naples, puys of, i. 100, 429
Napoleonite, i. 22
Necker, L. A., ii. 112, 123, 139, 140, 146
Necks, volcanic, i. 56;
of fragmentary materials, 56;
of non-volcanic detritus, 57, 289, 343, 426;
of agglomerate, 58; ii. 276;
internal stratification in, i. 63; ii. 80, 294;
with central lava-plug, i. 64, 430;
with dykes and veins, 66, 430; ii, 291;
of lava-form material, i. 67, 430; ii. 271;
parasitic, i. 69;
connection of, with cones, 70, 435; ii, 70, 89, 277, 281, 290;
metamorphism of, i. 67, 71, 82, 93, 399; ii. 39, 78;
metamorphism of rocks around, i. 72, 349, 350, 352, 399, 404, 432;
ii. 76, 272, 273, 274, 276, 280, 292;
inward dip of rocks towards, i. 73, 352; ii. 80;
connection of, with bosses, i. 93; ii. 276, 284;
entombment and exposure of, i. 434;
connection of with valleys, 272, 366, 375; ii. 61, 65;
relation between their size and the character of the agglomerate, 76;
connection of, with sheets of tuff or lava, 70, 89, 277, 284
---- Silurian, i. 215, 235;
Old Red Sandstone, 277, 288, 293, 311, 318, 323, 328;
Carboniferous, 394, 399, 400, 404, 406, 424, 465; ii. 13, 28, 47;
Permian, ii. 62, 67;
Tertiary, ii. 202, 270, 276
Neptunist and Plutonist controversy, i. 363; ii. 67, 95, 110, 112
New Mexico, necks in, i. 68
Newry granite, i. 290
Nicholson, Prof. Alleyne, i. 228, 229
Nicol, James, i. 311, 369
---- W., ii. 238
Nigrine, ii. 79
Nithsdale, Permian volcanic rocks of, ii. 58, 60, 62, 65
Nodular structure of lavas, i. 20, 162, 204, 206, 207, 211, 232, 247,
255, 274, 346
Nolan, Mr. J., i. 240, 251, 315; ii. 423, 424, 425
Non-volcanic debris, among volcanic rocks, i. 31, 57, 289, 313, 345, 381,
399, 402, 422, 426, 437; ii. 18, 27, 28, 58, 64, 76, 78, 99, 195, 196,
281, 423;
indicates comparatively feeble eruptions, i. 57, 289, 345, 426, 438;
ii. 293;
points to earliest eruptions of a vent, ii. 76
Nordenskjöld, Mr. O., i. 120
North Berwick Law, i. 371, 373, 403
North, Mr. Barker, ii. 244
Norway, eruptive rocks of, i. 28
Nuneaton, i. 171
Obsidian, i. 18, 19; ii. 370
Ochil Hills, i. 274, 276, 277, 279, 281, 286, 287, 288, 293, 303, 308, 311
Oil-shales of the Lothians, i. 361, 362, 462
O'Kelly, J., i. 349; ii. 49
Oldham, T., ii. 299
Old Red Sandstone, lines of vents in, i. 69;
granite protrusions of, 236, 272, 277, 290, 337;
of County Waterford, 251;
distribution in Britain, 257;
an exceptional stratigraphical type, 258;
conditions of its deposit, 259, 263, 297;
original scenery of, 265;
vegetation of, 265;
isolation of the water-basins of, shown by fossil evidence, 265;
classification of, 260;
history of the investigation of, 268;
volcanic centres in, 271;
nature of volcanic products in, 273;
structure of lavas and tuffs of, 281;
volcanoes of, 259, 263, 294, 303, 323, 325, 337, 341, 343, 346, 348, 352;
subdivisions of, 297;
thickest conglomerates of, 301;
composition of conglomerates of, 302, 315, 341;
unconformabilities in, 267, 328, 333;
Upper division of, 348, 375, 383; ii. 42
Olenellus-zone, i. 112, 130, 132, 140, 144
Olenus-zone, i. 144
Olivine, i. 154, 418, 420; ii. 58, 135
Omagh, i. 315
Ophitic structure, i. 21, 417; ii. 136, 184, 274
---- type of dolerite, i. 418, 421
Oregon, crater lake in, i. 58
Orkney Isles, i. 271, 344, 350; ii. 121
Orthoclase, ejected crystals of, ii. 79
Orthophyre, i. 273, 276, 277, 308
Osann, A., ii. 191
Oyenhausen, C. von, ii. 112, 280, 333, 340, 367, 372, 381
Palæopicrite, i. 261
Palæozoic systems, i. 139;
volcanic rocks resemble modern, i. 30
Palagonite, i. 33, 61, 151, 180, 246, 422, 423; ii. 44, 46, 57, 223
Paradoxides-zone, i. 144
Paramorphism, i. 249
Peach, Mr. B. N., i. 114, 128, 147, 168, 191, 192, 195, 196, 197, 198,
199, 200, 216, 240, 277-294, 307, 308, 329, 331, 344, 345, 369, 375,
425, 426, 476; ii. 133, 145
Pebidian, i. 145
Pegmatite, i. 20, 119, 127, 418, 449; ii. 5, 368, 437
Pembrokeshire, volcanic rocks of, i. 145, 159, 205
Penmaen-mawr, i. 209, 215
Pennant, T., ii. 109
Pennine chain, ii. 8
Pentland Hills, volcanic series of the, i. 102, 269, 273, 276, 279, 281,
285, 287, 289, 291, 311, 317
Perlite, i. 130
Perlitic structure, i. 19, 196, 199, 206, 211, 216, 232, 274
Permian system, geographical conditions accompanying the deposition of,
ii. 53, 97;
subdivisions of, in S. W. England, 94;
volcanic phenomena of, i. 46; ii. 55;
lavas and tuffs of, 57, 58;
vents of, 62, 67, 70, 96;
sills of, 91, 100
Permo-carboniferous strata, ii. 54
Petersen, Dr., i. 275, 336
Phillips, J., i. 133, 170, 205, 238; ii. 3
---- J. A., i. 260, 261
---- W., i. 171; ii. 95
Phonolite, i. 380 (analysis); ii. 375
Phyllite, i. 162, 222
Picrite, i. 377, 417, 420, 448, 450; ii. 57
---- type, i. 418
Pillow-structure in lavas, i. 26, 184, 193, 201, 240, 244, 252;
ii. 189, 259
Pitchstone, i. 18, 19, 130; ii. 134, 174, 204, 238, 242, 246, 370,
437, 444
Plagioclase, ejected crystals of, ii. 79
Plants fossil, in tuffs, i. 392; ii. 113, 198, 212, 222
Platania, G., i. 26
Plateau-type of volcanoes, i. 42, 100, 308, 341, 364
Plateaux, Carboniferous, of Scotland, i. 364;
distribution of, 367;
composition of, 377;
structure of, 383;
lavas and tuffs of, 383;
vents of, 394;
dykes and sills of, 406;
close of eruption of, 410;
Tertiary, ii. 181;
formation of modern Icelandic, 265
Player, Mr. J. H., analyses by, i. 377, 381; ii. 138, 330
Playfair, John, i. 363; ii. 110
Plinthite, ii. 197
Pliocene (supposed) of Lough Neagh, ii. 449
Plutonic operations of volcanoes, i. 77;
granite, 88
Plutonists and Neptunists, i. 363; ii. 67, 95, 110, 112
Pomeroy, volcanic series near, i. 315
Porphyrite, i. 190, 193, 207, 229, 240, 252, 273, 274, 377, 379
Porphyritic structure, i. 19, 274; ii. 128
Portlock, J. E., ii. 110, 111, 113, 199, 201, 299, 364
Portraine, conglomerates at, i. 244
Portrush, shells in supposed basalt at, ii. 110, 299
Potstone, i. 125
Pre-Cambrian rocks, i. 111, 121, 126
Pressure, experimental proof of effects of, i. 24
Prestwich, Sir J., ii. 103
Propylites, ii. 185, 388
Proterobase, i. 247
_Pterygotus_, i. 265
Pumice in tuffs, i. 244, 422; ii. 17, 27, 28, 32, 44, 46, 286, 288;
in volcanic necks, i. 60, 180; ii. 17, 39, 195
Pumiceous structure, i. 15, 33, 34, 60
Puys, as a type of volcano, i. 10, 44, 100, 414;
probable subærial nature of some, 432;
Carboniferous, 308, 364, 414, 424, 463; ii. 13, 28, 34, 47;
Permian, ii. 62;
Tertiary, 271, 276
Puy de Chopine, i. 32; ii. 374
---- Dôme, ii. 373
---- Montchar, i. 32
---- Pariou, i. 66, 70; ii. 31, 281
Pyroclastic detritus, i. 31, 58, 61
Pyromeride, i. 211
Pyrope, ii. 58, 79
Pyroxene, ii. 135
Quartz-porphyry, i. 19, 156, 159, 160, 161, 165, 277, 291, 314; ii. 96,
369, 420, 423, 430, 431
Quartz-trachyte, ii. 371
Quartzite, i. 112, 170
Raasay, basalt of, ii. 192;
neck-like breccias in, 293;
acid sill of, 430
Raddling or red-staining of rocks, i. 250, 261
Radiolarian cherts, i. 123, 167, 169, 173, 174, 184, 196, 201, 244
Rain-pittings in strata, i. 342
Raisin, Miss, i. 161, 163, 164, 165, 210
Ramsay, A. C., i. 126, 142, 143, 144, 145, 158, 159, 168, 175, 176, 177,
178, 179, 180, 182, 183, 204, 205, 208, 210, 212, 214, 223, 237, 364
Ratho type of dolerite, i. 418, 421
Red Head, section at, i. 300
Red Hills, Skye, scenery of, i. 105; ii. 379
Reed, Mr. Cowper, i. 205
Reid, Mr. Clement, ii. 449, 450
Renard, Prof. A., i. 148, 149
Renfrewshire, Carboniferous volcanic rocks of, i. 368, 385, 397, 400, 404,
408, 416, 430, 447
Reyer, Prof. E., ii. 474
Reynolds, Mr. S. H., i. 177, 179, 256
Rhobel Fawr, i. 177, 178, 186
Rhyolite, i. 19, 22, 24, 131, 161, 165, 167, 168, 178, 204, 210, 231, 232,
255, 276, 278; ii. 185, 205, 371, 424, 437
Rhyolitic conglomerate, ii. 195, 206, 429
Richardson, Rev. W., ii. 110
Richthofen, F. von, i. 28; ii. 115, 116
Rivers made to shift their channels by volcanic eruptions, i. 42, 49;
of the Tertiary volcanic period, ii. 217, 228, 231, 234, 456
Rocks, oldest known, i. 110
Roscommon, volcanic rocks of, i. 316
Rosenbusch, Prof. H., ii. 136, 137
Ross, Mr. Alexander, ii. 406, 409
Rothliegende, ii. 95
Roxburghshire, Carboniferous vents of, i. 55, 403, 404
Rubers Law, i. 375, 380, 404
Rum, i. 112;
basalt-plateau of, ii. 215;
gabbros of, 332, 349;
acid bosses of, 403;
acid sills of, 431;
pitchstone of, 445
Rutley, Mr. F., i. 131, 133, 170, 207, 210, 227, 231, 232, 238, 260;
ii. 23, 35
Sahlite found in volcanic vents, i. 62
Saline Hill, volcanic vents of, i. 433, 435, 440
Sanday, basalts of, ii. 215;
conglomerates of, 226
Sandstone altered into quartzite, i. 72, 349, 350, 404, 432, 451;
ii. 76, 164;
veinings of, in lava, i. 283, 300, 303, 320, 327, 333, 337;
ii. 59, 98
Sandwich Islands, lava-cones of, i. 10
Sanidine ejected from volcanic vents, ii. 58, 79
Sanquhar, Silurian volcanic rocks at, i. 192, 195, 199;
Permian volcanic rocks at, ii. 62
Santorin, ii. 134
Saponite, ii. 79
Scenery, origin of, i. 8, 100
Schalstein, i. 262; ii. 36
Schists, primeval, i. 110, 114, 118, 119;
produced by deformation of igneous rocks, 75, 114, 118, 119, 121, 162,
240, 249, 252, 261
Schmidt, Dr. C. W., ii. 266
Scoriaceous structure, i. 15, 16, 282, 327, 339
Scorpions, fossil, i. 174, 356, 466
Scotland, lines of fault in, i. 11;
Vesuvian cones of, 42;
plateaux of, 43;
puys of, 46;
submarine lavas of, 48;
Carboniferous vents of, 55;
volcanic scenery of, 104;
pre-Cambrian rocks of, 111;
Cambrian rocks of, 112;
Lewisian gneiss of, 114;
Dalradian rocks of, 121;
Arenig rocks of, 123, 191;
Old Red Sandstone of, 266, 273, 281, 291;
Carboniferous geography of, 356;
Carboniferous volcanoes of, 359;
Carboniferous plateaux of, 367;
Carboniferous puys of, 414;
Permian volcanoes of, ii. 55;
Tertiary dykes of, 122;
Tertiary basalt-plateaux of, 208, 274, 304;
Tertiary gabbros of, 327;
Tertiary acid rocks of, 379
Scrope, G. P., i. 27, 82, 45, 116; ii. 373, 374, 381
Sedgwick, A., i. 142, 166, 175, 218, 227, 257; ii. 1, 2, 3, 5, 113,
139, 153, 157
Segregation (_see_ Differentiation)
Segregation-veins, i. 84, 92; ii. 66, 130, 300, 303
Selwyn, Mr. A. C. R., i. 143, 175, 208, 221
Semi-opal, ii. 79
Sepulchre Mountain, i. 79
Serpentine, i. 195, 293
Shale, alteration of, i. 72, 451; ii. 164
Shap, granite of, i. 236, 238, 271, 290
Sheets, intrusive (_see_ Sills)
Shelve, i. 176, 190
Shetland, i. 271, 289, 292, 293, 345
Shiant Isles, ii. 307
Shineton Shales, i. 144
Shore-lines, traces of ancient, i. 295, 305
Shropshire, ancient volcanic rocks of, i. 129, 189;
latest eruptive rocks of, ii. 101
Sicily, i. 26
Sidlaw Hills, i. 286, 294, 303
Sills, vitreous margins of, i. 18;
tectonic relations of, 77, 83, 451;
origin of name, 83;
differentiation (segregation) in, 81, 450; ii. 476;
ordinary stratigraphical position of, i. 85;
considered as parts of incompleted volcanoes, 86;
metamorphism by, 87, 94, 451; ii. 299, 303, 310;
conditions for injection of, i. 97, 98, 99, 458;
columnar structure of, ii. 187, 291, 301, 306, 308, 319;
amygdaloidal structure in, 299, 312;
banding of, 309;
split by later sills, 310, 316;
extreme subdivision of, 311;
slaggy surface in some, 312;
give off veins, 313;
double and multiple, 318, 434;
connection with vents, 322
---- pre-Cambrian, i. 118, 124;
Cambrian, 155, 170, 171;
Silurian, 187, 195, 206, 216, 237, 249;
Devonian, 261;
Old Red Sandstone, 277, 291, 321, 335, 343, 345;
Carboniferous, 408, 446, 472; ii. 2, 21, 30, 48;
Permian, 64, 66;
of Midlands, 102, 103;
Tertiary, (1) Basic, 298;
(2) Acid, 366, 430
Silurian system, i. 173;
vegetation of, 174;
geography of, 263;
volcanoes of, 175;
classification of, 175;
two volcanic series of, 177
---- volcanoes in Shropshire, i. 189;
in Scotland, 191;
at Builth, 203;
in Pembrokeshire, 205;
in Caernarvonshire, 207;
in the Berwyn Hills, 218;
in Anglesey, 219;
in the Lake District, 227;
in Gloucestershire, 238;
in Ireland, 239, 254
Skae, H. M., i. 294, 299, 306, 375; ii. 57
Skiddaw, i. 228;
granite of, 236
---- Slate, i. 229
Skomer Island, i. 207
Skye, spherulitic dykes and sills of, i. 20;
ophitic structure from, 20;
basalt-terraces of, 24;
metamorphism by granophyre of, 95, 96;
volcanic scenery of, 103, 105;
dykes of, ii. 123, 124, 129, 139, 140, 146, 150, 152, 154, 160, 162,
164, 165, 173, 269;
bedded basalts of, 192, 249, 269;
tuffs of, 251;
connection of dykes and superficial lavas in, 269;
vents in, 280;
sills of, 304;
gabbro bosses of, i. 116; ii. 334;
acid bosses of, 379;
acid sills of, 431;
acid dykes of, 437;
pitchstone veins of, 445;
subsidence of, 447
Slaggy structure, i. 16, 33, 59, 282, 327, 339; ii. 98, 187
Slane, volcanic rocks near, i. 244
Slate-tuffs, i. 180, 213, 234
Slemish a volcanic neck, ii. 271
Slieve Foye, ii. 421
---- Gallion, ii. 200
---- Gullion, ii. 422
Small Isles, basalt-plateau of, ii. 215;
vents of, 288;
sills of, 318;
acid bosses of, 403;
acid sills of, 431
Small, Mr. E. W., i. 207
Smaragdite found in volcanic vents, i. 62
Snowdon, volcanic rocks of, i. 20, 42, 47, 102, 175, 208, 209, 210, 211,
212, 213, 218, 226
Soda-felsites, i. 183, 196, 247
Solfataric action, i. 71; ii. 185, 205, 388
Sollas, Prof., i. 96; ii. 175, 293, 415, 421, 422
Solway, Carboniferous volcanic plateau of, i. 375, 385, 413
Somerset, volcanic rocks of, ii. 32
Somma, denudation of, i. 3, 100
Spheroidal structure of dolerite, i. 456
Spherulitic structure, i. 19, 20, 95, 120, 130, 155, 162, 184, 211, 232,
235, 346; ii. 369, 381, 392, 432, 435, 437, 441, 446
Spilosite, i. 262
Springs, mineral, connected with volcanic action, i. 390, 445
St. Abb's Head, i. 338
Staffa, i. 25;
first notice of, ii. 109;
columnar basalts of, 186, 188, 210;
basalt conglomerate of, 195
Staffordshire, latest eruptive rocks of, ii. 101, 103
St. Andrews, old volcanoes near, ii. 71, 73, 87
St. David's, Cambrian volcanic rocks of, i. 145
---- Head, i. 205
Steam in volcanic action, i. 13, 15, 16, 71
Stecher, Dr., i. 421, 451; ii. 165
St. Kilda, dykes of, ii. 173, 416;
gabbro of, 358;
general account of geology of, 405;
granophyre of, 408
Stocks, or bosses, i. 78, 88
Strahan, Mr. A., i. 171; ii. 10, 12, 23, 28, 32
Strathaird, ii. 123, 140, 164, 269
Strathbogie, i. 344; ii. 121
Strathmore, i. 304
Stromboli, i. 4
Sublimations, traces of ancient, i. 445
Submarine eruptions, i. 48
Sub-ophitic structure, i. 417
Subsidence and volcanic action, i. 295, 297, 444, 463; ii. 42, 205,
447, 470
Subterranean igneous injections, i. 77 (_see_ Bosses, Dykes, Sills)
Suess, Prof. E., ii. 474
Sun-cracks, i. 342
Sweden, Archæan volcanic rocks of, i. 120
Syenite, ii. 366
Symes, Mr. R. G., i. 311, 343, 369; ii. 201, 428
Tate, G., ii. 3, 113
---- R., ii. 204
Tatlock, Mr. R. R., i. 273, 278
Tawney, E. B., i. 157
Teall, Mr. J. J. H., i. 90, 114, 116, 117, 118, 119, 120, 156, 192, 194,
200, 207, 210, 275, 277, 290, 311, 336, 338, 346, 407, 449; ii. 2, 3,
5, 7, 11, 32, 44, 113, 131, 134, 135, 137, 138, 140, 144, 149, 292, 293,
329, 367, 368, 369
Teesdale, i. 228
Termier, M. P., ii. 375
Terrestrial volcanic eruptions, i. 50
Tertiary Volcanic Series, ii. 181
Subaerial character of eruptions, ii. 103, 198
Scenery of, ii. 255, 256, 349, 391, 405, 408
The Plateaux, ii. 183, 249;
lavas of, 183, 218, 236, 256;
thickness of individual sheets, 192, 206, 254, 257;
lenticular character of lavas of, 193, 257;
greatest depth of, 210, 211, 213, 260;
tuffs and clays of, 194, 202, 204, 211, 222, 225, 251, 258, 277,
284, 287;
non-volcanic fragments in, 196, 211, 213, 219;
lignites of, 198, 203, 208, 213, 251;
gravels and conglomerates of, 198, 212, 238, 256;
coal of, 213, 251, 256, 287;
leaf-beds of, 204, 212, 222, 225, 288;
carbonaceous nature of the upper parts of intercalated sediments in,
223, 226, 227, 229, 232, 251, 288;
evidence for intervals between the eruptions in, 203, 205, 208, 221,
228, 240, 245, 251, 254, 288;
no evidence of great central vents in, 208, 214, 255, 258, 260, 267;
faulted condition of, 200, 208, 209, 452;
subsidences of, 205, 208, 209, 214, 447;
ancient river channels of, 217, 228, 231, 234, 456;
volcanic cones of, 202, 218, 230, 277, 281, 285;
paralleled by the modern Icelandic eruptions, 260;
vents of, 202, 218, 230, 270, 276
The Basic sills, ii. 298, 304
The Gabbro bosses, ii. 327, 349, 355, 358;
history of the gabbro intrusions, 359
The Acid rocks, ii. 364;
petrography of, 366;
history of their investigation, 371;
analogies with trachytes of Central France, 373;
intruded at base of the gabbros or of the bedded basalts, 337, 353,
357, 431, 432, 444;
bosses of Skye, 378;
of Mull, 395;
of Small Isles, 405;
of St. Kilda, 405;
of Arran, 418;
of Carlingford, 420;
of Slieve Foye, and Barnavave, 421;
of Slieve Gullion, 422;
of Antrim, 426;
acid sills, 430;
acid dykes and veins, 437
Metamorphism of the basalts, ii. 272, 276, 337, 339, 340, 347, 355, 356,
357, 358, 362, 378, 383, 386, 397, 399, 400, 404, 413
Texture, varieties of, in igneous rocks, i. 78, 449; ii. 5, 299, 360
Tholeiites, ii. 137, 158
Tholeiite type of basalt, i. 419, 421
Thornhill, volcanic rocks of, ii. 60
Thoroddsen, Th., ii. 261, 262, 263, 264, 265, 266, 278
Thrust-planes, i. 229
Time in geological history, ii. 107, 461, 465
Timmins, J. H., i. 133
Tinto, i. 278, 288, 329
Titterstone Clee Hill, ii. 101
Toadstones of Derbyshire, i. 359; ii. 8
Topley, W., i. 147; ii. 3, 5, 7
Torridonian rocks, i. 111, 112, 113, 120; ii. 350
Tortworth, volcanic rocks at, i. 238
Tourmakeady, volcanic rocks of Bala age at, i. 251
Townson, R., i. 363
Trachyte, i. 183, 230, 246, 273, 276 (analysis), 379 (analysis), 386, 403,
407, 421; ii. 36, 47, 96, 138, 152, 184, 236
Traill, Mr. W., ii. 175, 421, 422
Traprain Law, i. 372, 380, 403, 405
Traquair, Dr. R. H., i. 266
Tremadoc group, i. 144, 177
Trevelyan, W. C., ii. 3
Triassic eruptive rocks, i. 29;
geography, ii. 108
Trichites, i. 19; ii. 136
Troctolite, ii. 332
Tuffs, i. 31;
association of, 33;
composition of, 34;
alternations of, 34, 61;
blending of, with non-volcanic sediment, 35, 437;
fossiliferous, 36;
without lava, 36;
necks of, 58;
relation of, to lavas, 61;
metamorphism of, ii. 224
---- pre-Cambrian, i, 125, 135;
Cambrian, 147, 151, 155, 163, 165, 167;
Silurian, 178, 189, 190, 195, 205, 209, 212, 213, 222, 224, 229, 232,
241, 245, 246, 254, 255;
Devonian, 262;
Old Red Sandstone, 279, 281, 289, 337, 339, 351;
Carboniferous, 381, 384, 387, 399, 422, 427, 429, 432, 436, 466;
ii. 11, 18, 24, 36;
Permian, 57, 58;
Tertiary, 194, 197, 202, 204, 211, 222
Tyrol, Triassic eruptive rocks of, i. 29
Tyrone, Old Red Sandstone of, i. 314
Ulster, Old Red Sandstone volcanic rocks of, i. 314
Ultra-basic rocks, i. 14, 118, 377, 417
Unconformability, deceptive case of, i. 163
Urgneiss, i. 110
Uriconian volcanic rocks, i. 129
Ussher, Mr. W. A. E., i. 260, 262; ii. 35, 95
Utah, laccolites of, i. 86;
volcanic regions of, ii. 115, 267
Valleys, tendency of vents to appear in, i. 272, 368, 376; ii. 61, 65, 96
Vapours, action of volcanic, i. 13, 15, 16, 17, 31, 57, 72, 78, 97, 99,
180, 289, 426
Variolitic structure, i. 21, 206, 235
Veins, intrusive, i. 66, 77, 79, 98, 426, 429; ii. 311, 313, 400, 410,
432, 437
Velay, volcanic rocks of, i. 26, 27, 29, 60; ii. 271, 373, 375
Vents, volcanic, i. 53;
ground-plans of, 54;
size of, 55;
filled with non-volcanic detritus, 57;
ejected crystals found in, 62;
agglomerates of, 62; ii. 13, 28, 47, 61, 69, 276, 280, 284, 288, 289;
stratification in, i. 63; ii. 80;
metamorphism in, i. 67, 71;
metamorphism of rocks around, 72, 349, 350, 352, 399, 404, 432; ii. 76,
272, 280;
connection of, with geological structure-lines, i. 68;
occurrence of, in lines and in groups, 69;
double and multiple, 69;
possible indications of length of activity of, 72;
inward dip of strata around, 73, 352; ii. 76, 295;
stages in history of, i. 74;
tendency of, to rise in lines of valley, 272, 368, 376; ii. 61, 65,
96, 468;
criteria for the relative ages of, 270;
connection of, with later eruptive bosses, 280, 384, 399, 400
---- Silurian, i. 209, 214, 234;
Old Red Sandstone, 272, 287, 298, 305, 323, 328, 337;
Carboniferous, 394, 399, 400, 404, 406, 424, 465; ii. 13, 28, 47;
Permian, 61, 69;
Tertiary, 202, 270, 294, 400
Vesicular structure of lavas, i. 15; ii. 187
Vesuvius, denudation of, i. 3;
as an active volcano, 4;
as a type of volcano, 10, 39, 53, 100; ii. 108, 115, 261, 266
Vicary, Mr. W., ii. 95
Vogesite, i. 277, 293
Volcanello Island, i. 70
Volcanic action, permanent traces of, i. 4;
of present time elucidates that of the past, 5;
submarine, 5;
transient effects of, 8;
chief factors in, 10;
explosive energy of, 13, 99;
uniformity of, in geological time, 13, ii. 470;
metamorphism by, i. 67, 71;
underground phases of, 77;
proofs of gradual quiescence of, 155, 157, 166;
connected with subsidence, 295, 297; ii. 205, 444, 463, 470;
repetition of, in the same region, i. 368, 375, 377; ii. 42, 69,
94, 467;
developed along continental borders, 466;
persistence of, in Britain, i. 7; ii. 466;
connection of, with lines of geological structure, 468;
connection of, with terrestrial disturbance, 469;
gradual decline of, during Palæozoic time, 471;
quiescence of, during Mesozoic time, 472
Volcanic cycles, i. 27, 92
---- products, general characters of, i. 14;
persistent uniformity of, 30, 46;
thickest mass of, in Britain, 229 (_see_ Agglomerate, Lava, Tuff)
Volcano, Island of, i. 4, 24
Volcanoes, their influence on mythology, i. 1, 2;
denudation of, 3;
number of extinct, 4;
ancient, of Britain, 6;
influence on scenery, 8, 100, 102;
defined, 10;
types of, 10, 39; ii. 471;
determination of relative dates of, i. 46;
their geographical condition in old times, how ascertained, 48;
parasitic, 69;
contemporaneous denudation of, 73, 100;
connected with granite, 89;
incompleted, 86, 93, 99
Vom Rath, G., ii. 474
Wacke, i. 157
Walcott, Mr., i. 30
Wales, pre-Cambrian rocks of, i. 126, 142;
early geological work in, 142;
Cambrian volcanoes of, 145, 159;
volcanic scenery of, 176;
Silurian volcanoes of, 176, 202, 205, 207, 218, 219;
Old Red Sandstone of, 257, 259
Waller, Mr. T. H., i. 171, 278
Ward, J. C., i. 227, 228, 229, 230, 231, 233, 234, 235, 236, 237; ii. 23
Warwickshire, Cambrian rocks of, i. 137, 171
Waterford, volcanic region of, i. 247
Watts, Mr. W. W., i. 131, 132, 135, 137, 189, 190, 191, 243, 276, 278,
336, 347, 417, 421, 423; ii. 40, 42, 43, 45, 57, 96, 184, 204, 224,
272, 424, 425
Weaver, T., i. 238
Wenlock group, i. 175;
volcanic rocks of, 552
Wernerian School, ii. 109
West Lothian, volcanic rocks of, i. 47, 55, 415, 433, 437
Whin Sill of England, i. 83, 85, 97, 449; ii. 2
Whitehurst, J., ii. 9, 109
White trap, i. 96, 426, 449, 456; ii. 65, 87, 103, 165, 252
Williams, Mr. G. J., i. 179, 185, 186, 188
Williamson, W. C., i. 392
Wilson, Mr. J. S. Grant, i. 148, 149, 153, 276, 344, 375, 379, 380;
ii. 137, 164
Wilson, Mr. A., ii. 49
Winch, N. T., ii. 113, 147
Witham, H. T. M., ii. 113, 238
Wood, N., ii. 113
Woods, Mr. H., i. 204
Woodward, Dr. Henry, ii. 449
Woodward, Mr. H. B., ii. 32, 435, 453
Worcestershire, latest eruptive rocks of, ii. 101
Worth, Mr. R. N., ii. 99
Wrekin, i. 130
Wright, J. R., ii. 102
Wunsch, E., i. 369, 392
Würtemberg, puys of, i. 46
Wyoming, lava-fields of, ii. 115
Yates, J., i. 171
Yellowstone Park, volcanic phenomena of, i. 29, 31
Y-foel-frâs, i. 209, 214
Y Glyder-Fach, i. 209
Yoredale group, ii. 9, 13, 17
Young, Mr. John, i. 369, 392
Young, Prof. John, i. 294, 308
Zircon, found in volcanic vents, i. 62
Zirkel, Prof., ii. 327, 329, 334, 356, 364, 370, 372, 379, 430
THE END
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Transcriber Note
Minor typos were corrected. A paragraph break was inserted in the
last paragraph on pages 271 and 334 to accommodate placement of
illustrations. To prevent images from splitting paragraphs, some text
was moved.