CHAPTER XXXIV

THE SYSTEM OF DYKES IN THE TERTIARY VOLCANIC SERIES

Geographical Distribution--Two Types of Protrusion--Nature

of Component Rocks--Hade--Breadth--Interruptions of Lateral

Continuity--Length--Persistence of Mineral Characters.

If a geologist were asked to select that feature in the volcanic

geology of the British Isles which, more than any other, marks this

region off from the rest of the European area, he would probably choose

the remarkable system of wall-like masses of erupted igneous rock, to

which the old Saxon word "dykes" has been affixed. From the moors of

eastern Yorkshire to the Perthshire Highlands, and from the basins of

the Forth and Tay to the west of Donegal and the far headlands of the

Hebrides, the country is ribbed across with these singular protrusions

to such an extent that it may be regarded as a typical region for the

study of the phenomena of dykes. That all the dykes in this wide tract

of country are of Tertiary age cannot be maintained. It has been shown

in previous Chapters that each of the great volcanic periods has had

its system of dykes, even as far back as the time of the Lewisian

Gneiss.

But when all the dykes which can reasonably be referred to older

geological periods are excluded, there remains a large series which

cannot be so referred, but which are connected together by various

kinds of evidence into one great system that must be of late geological

date, and can be assigned to no other than the Tertiary period in the

volcanic history of Britain. As far back as the year 1861, when I first

drew attention to this great system of dykes in connection with the

progress of volcanic action in the country, I pointed out the grounds

on which it seemed to me that these rocks belong to a comparatively

recent geological period.[159] My own subsequent experience and the full

details of structure collected by my colleagues of the Geological

Survey in all parts of the country, have amply confirmed this view.

The characters which link this great series of dykes together as one

connected system of late geological date are briefly enumerated in the

following list, and will be more fully discussed in later pages.

[Footnote 159: _Trans. Roy. Soc. Edin._ vol. xxii. (1861), p. 650.]

1. The prevalent tendency of the dykes to take a north-westerly

course. There are exceptions to this normal trend, especially where

the dykes are small and locally numerous; but it remains singularly

characteristic over the whole region.

2. The increasing abundance of the dykes as they are traced to the west

coast and the line of the great Tertiary volcanic plateaux of Antrim

and the Inner Hebrides.

3. The rectilinear direction so characteristic of them and so different

from the tortuous course of local groups of dykes. The exceptions to

this normal feature are as a rule confined to the same localities where

departures from the prevalent westerly trend occur.

[Illustration: Fig. 233.--Dyke on the south-east coast of the Island of

Mull.]

4. The great breadth of the larger dykes of the system and their

persistence for long distances. This is one of their most remarkable

and distinctive characters.

5. The posteriority of the dykes to the rest of the geological

structure of the regions which they traverse. They are not only younger

than the other rocks, but younger than nearly all the folds and faults

by which the rocks are affected.

6. The manner in which they cut the Jurassic, Cretaceous and older

Tertiary rocks in the districts through which they run. At the

south-eastern end of the region they rise through the Lias and Oolite

formations, in the west they intersect the Chalk and also the Tertiary

volcanic plateaux together with their later eruptive bosses.

7. Their petrographical characters, among which perhaps the most

distinctive is the frequent appearance of the original glass of the

plagioclase-pyroxene-magnetite (olivine) rock, of which they mostly

consist. This glass, or its more or less completely devitrified

representative, often still recognizable with the microscope among the

individualized microlites and crystals throughout the body of a dyke,

is also not infrequent as a black vitreous varnish-like coating on the

outer walls, and occasionally appears in strings and veins even in the

centre.

It is the assemblage of dykes presenting these features which I propose

to describe. Obviously, the age of each particular dyke can only be

fixed relatively for itself. But when this remarkable community of

characters is considered, and when the post-Mesozoic age of at least a

very large number of the dykes can be demonstrated, the inference is

reasonable that one great system of dykes was extravasated during a

time of marked volcanic disturbance, which could not have been earlier

than the beginning of the Tertiary period. And this inference may be

maintained even when we frankly admit that every dyke within the region

is by no means claimed as belonging to the Tertiary series.

[Illustration: Fig. 234.--Fissure left by the weathering out of a dyke.]

In spite of their number and the extraordinary volcanic activity to

which they bear witness, the dykes form a much less prominent feature

in the landscape than might have been anticipated. In the lowlands of

the interior, they have for the most part been concealed under a cover

of superficial accumulations, though in the water-courses they not

infrequently project as hard rocky barriers across the channels, and

occasionally form picturesque waterfalls. On the barer uplands, they

protrude in lines of broken crag and scattered boulders, which by their

decay give rise to a better soil covered by a greener vegetation than

that of the surrounding brown moorland. Among the Highland hills, they

are often traceable from a distance as long black ribs that project

from the naked faces of crag and corry. Along the sea-coast, their

peculiarities of scenery are effectively displayed. Where they consist

of a close-grained rock, they often rise from the beach as straight

walls which, with a strangely artificial look, mount into the face

of the cliffs on the one side, and project in long black reefs into

the sea on the other (Fig. 233). Every visitor to the islands of the

Clyde will remember how conspicuous such features are there. But it is

among the Inner Hebrides that this kind of scenery is to be found in

greatest perfection. The soft dark Lias shales of the island of Pabba,

for example, are ribbed across with scores of dykes which strike boldly

out to sea. Where, on the other hand, the material of the dykes is

coarse in grain, or is otherwise more susceptible to the disintegrating

influences of the weather, it has often rotted away and left yawning

clefts behind, the vertical walls of which are those of the fissures

up which the molten rock ascended (Fig. 234). Some good instances of

this kind are well known to summer visitors on the eastern shores of

Arran. Others, on a large scale, may be seen in the interior of the

same island along the crests of the granite ridges, and still more

conspicuously on the jagged summits of Blath Beinn and the Cuillin

Hills (Fig. 333), and intersecting the Jurassic strata along the cliffs

of Strathaird in Skye.

1. GEOGRAPHICAL DISTRIBUTION

The limits of the region within which the dykes occur cannot be very

precisely fixed. There can be no doubt, however, that on their southern

side they reach to the Cleveland Hills of Yorkshire and the southern

borders of Lancashire, perhaps even as far as North Staffordshire (p.

106), and on the northern side to the farther shores of the island of

Lewis--a direct distance of 360 miles. They stretch across the basin of

the Irish Sea, including the Isle of Man, and appear in Ireland north

of a line drawn from Dundalk Bay to the Bays of Sligo and Donegal.

Dykes are of frequent occurrence over the north of England and south

of Scotland, at least as far north as a line drawn from the coast of

Kincardineshire along the southern flank of the Grampian Hills, by

the head of Glen Shee and Loch Tay, to the north-western coast of

Argyleshire. They abound all along the line of the Inner Hebrides and

on parts of the adjacent coasts of the mainland, from the remoter

headlands of Skye to the shores of County Louth. They traverse also the

chain of the Long Island in the Outer Hebrides. So far as I am aware,

they are either absent or extremely rare in the Highlands north of the

line I have indicated. But a good many have been found by my colleagues

in the course of the Geological Survey of the northern lowlands of

Aberdeenshire and Banffshire. The longest of these has been traced by

Mr. L. Hinxman for rather more than two miles running in a nearly east

and west direction through the Old Red Sandstone of Strathbogie, with

an average width of about 35 feet. Another in the same district has a

width of from 45 to 90 feet, and has been followed for a third of a

mile. But far beyond these northern examples, I have found a number of

narrow basalt-veins traversing the Old Red flagstones of the Mainland

of Orkney, which I have little doubt are also a prolongation of the

same late series. Taking, however, only those western and southern

districts in which the younger dykes form a notable feature in the

geology, we find that the dyke-region embraces an area of upwards of

40,000 square miles--that is, a territory greater than either Scotland

or Ireland, and equal to more than a third of the total land-surface of

the British Isles (Map I.).

Of this extensive region the greater portion has now been mapped in

detail by the Geological Survey. Every known dyke has been traced,

and the appearances it presents at the surface have been recorded. We

are accordingly now in possession of a larger body of evidence than

has ever before been available for the discussion of this remarkable

feature in the geology of the British Isles. I have made use of this

detailed information, and besides the data accumulated in my own

note-books, I have availed myself of those of my colleagues in the

Survey, for which due acknowledgment is made where they are cited.

The Tertiary basalt-plateaux of Britain have their counterpart in

the Faroe Islands and in Iceland, and whether or not the lava-fields

stretched throughout North-western Europe from Antrim to the farthest

headlands of _Ultima Thule_, there can hardly be any doubt that,

if not continuous, these volcanic areas were at least geologically

contemporaneous in their activity. Their characteristic scenery and

structure are prolonged throughout the whole region, reappearing with

all their familiar aspects alike in Faroe and in Iceland. I have not

seen the latter island, but in the Faroe archipelago I have found the

dykes to be sufficiently common, and to cut the basalt-plateaux there

in the same way as they do those of the Inner Hebrides. On the whole,

however, dykes do not play, in these northern isles, the important part

which they take in the geology and scenery of the West of Scotland. I

have not had sufficient opportunity to ascertain whether there is a

general direction or system among the Faroe dykes. In the fjords north

of Thorshaven, and again along the west side of Stromö, many of them

show an E. and W. strike or one from E.N.E. to W.S.W.

2. TWO TYPES OF PROTRUSION

The dykes are far from being equally distributed over the wide region

within which they occur. In certain limited areas they are crowded

together, sometimes touching each other to the almost entire exclusion

of the rocks through which they ascend, while elsewhere they appear

only at intervals of several miles. Viewed in a broad way, they may

be conveniently grouped in two types, which, though no hard line can

be drawn between them, nevertheless probably point to two more or

less distinct phases of volcanic action and to more than one period

of intrusion. In the first, which for the sake of distinction we may

term the Solitary type, there is either a single dyke separated from

its nearest neighbours by miles of intervening and entirely dykeless

ground, or a group of two or more running parallel to each other, but

sometimes a mile or more apart. The rock of which they consist is,

on the whole, less basic than in the second type; it includes the

andesitic varieties. It is to this type that the great dykes of the

north of England and the south and centre of Scotland belong. The

Cleveland dyke, for example, at its eastern end has no known dyke near

it for many miles. The coal-field of Scotland is traversed by five

main dykes, which run in a general sense parallel to each other, with

intervals of from half a mile to nearly five miles between them. Dykes

of this type display most conspicuously the essential characters of

the dyke-structure, in particular the vertical marginal walls, the

parallelism of their sides, their great length, and their persistence

in the same line.

In the second, or what for brevity may be called the Gregarious type,

the dykes occur in great abundance within a particular district. They

are on the whole narrower, shorter, less strikingly rectilinear,

more frequently tortuous and vein-like, and generally more basic in

composition than those of the first type. They include the true basalts

and dolerites. Illustrative districts for dykes of this class are the

islands of Arran, Mull, Eigg and Skye.

The great single or solitary dykes may be observed to increase in

number, though very irregularly, from south to north, and also in

Central Scotland from east to west. They are specially abundant in the

tract stretching from the Firth of Clyde along a belt of country some

thirty miles broad on either side of the Highland line, as far at least

as the valley of the Tay. They form also a prominent feature in the

islands of Jura and Islay.

Dykes of the gregarious type are abundantly and characteristically

displayed in the basin of the Firth of Clyde. Their development in

Arran formed the subject of the interesting paper by Necker, already

mentioned, who catalogued and described 149 of them, and estimated

their total number in the whole island to be about 1500.[160] As the

area of Arran is 165 square miles, there would be, according to this

computation, about nine dykes to every square mile. But they are far

from being uniformly distributed. While appearing only rarely in many

inland tracts, they are crowded together along the shore, particularly

at the south end of the island, where the number in each square mile

must far exceed the average just given. The portion of Argyleshire,

between the hollow of Loch Long and the Firth of Clyde on the east

and Loch Fyne on the west, has been found by my colleague, Mr. C. T.

Clough, to contain an extraordinary number of dykes (see Fig. 257). The

coast line of Renfrewshire and Ayrshire shows that the same feature is

prolonged into the eastern side of the basin of the Clyde estuary. But

immediately to the westward of this area the crowded dykes disappear

from the basin of Loch Fyne. In Cantire their scarcity is as remarkable

as their abundance in Cowal.

[Footnote 160: _Trans. Roy. Soc. Edin._ xiv. (1840), p. 677.]

Both in the North of Ireland and through the Inner Hebrides, dykes

are singularly abundant in and around, but particularly beneath, the

great plateaux of basalt. Their profusion in Skye was described early

in this century by Macculloch, who called attention more especially to

their extraordinary development in the district of Strathaird. "They

nearly equal in some places," he says, "when collectively measured, the

stratified rock through which they pass. I have counted six or eight

in the space of fifty yards, of which the collective dimensions could

not be less than sixty or seventy feet." He supposed that it would not

be an excessive estimate to regard the igneous rock as amounting to

one-tenth of the breadth of the strata which it cuts.[161] This estimate,

however, falls much short of the truth in some parts of Strathaird,

where the dykes are almost or quite contiguous, and the Jurassic

strata, through which they rise, are hardly to be seen at all.

[Footnote 161: _Trans. Geol. Soc._ iii. (1815), p. 79. This locality is

further noticed on p. 164.]

Among the districts where dykes of the gregarious type abound at a

distance from any of the basalt-plateaux, reference should be made

to the curious isolated tract of the central granite core of Western

Donegal. In that area a considerable number of dykes rises through

the granite, to which they are almost wholly confined. Again, far to

the east another limited district, where dykes are crowded together,

lies among the Mourne Mountains. These granite hills are probably to

be classed with those of Arran, as portions of a series of granite

protrusions belonging to a late part of the Tertiary volcanic period

which will be treated of in Chapter xlvii.

Though the dykes may be conveniently grouped in two series or types,

which on the whole are tolerably well marked, it is not always

practicable to draw any line between them, or to say to which group

a particular dyke should be assigned. In some districts, however, in

which they are both developed, we can separate them without difficulty.

In the Argyleshire region above referred to, for example, which

Mr. Clough has mapped, he finds that the abundant dykes belonging

to the gregarious type run in a general N.W. or N.N.W. direction,

and distinctly intersect the much scarcer and less basic dykes of

the solitary type, which here run nearly E. and W. (Fig. 257).

Hence, besides their composition, distinction in number, breadth,

rectilinearity and persistence, the two series in that region

demonstrably belong to distinct periods of eruption.[162]

[Footnote 162: Mr. Clough is inclined to suspect that the E. and W. dykes

are older than the Tertiary series and may be later Palæozoic.]

The characteristic habit in gregarious dykes of occurring in crowded

groups which are separated from each other by intervals of variable

dimensions, marked by the presence of comparatively few dykes, is well

illustrated in the district of Strath in Skye, which indeed may be

taken as a typical area for this peculiarity of distribution. While the

dykes are there singularly abundant in the Cambrian Limestone and the

Liassic strata, they have been found by Mr. Clough and Mr. Harker to be

comparatively infrequent in the tracts of Torridon Sandstone. It is not

easy to understand this peculiar arrangement. As the Torridon Sandstone

is the most ancient rock of the district, it probably underlies all the

Cambrian and Jurassic formations, so that the dykes which penetrate

these younger strata must also rise through the Torridonian rocks.

Some formations appear to have been fissured more readily than others,

and thus to have provided more abundant openings for the uprise of the

basaltic magma from below. To the effect of such local differences in

the structure of the terrestrial crust we have to add the concentration

of the volcanic foci in certain areas, though there seems no means

of ascertaining what part each of these causes has played in the

distribution of the dykes of any particular district.

3. NATURE OF COMPONENT ROCKS

The Tertiary dykes of Britain include representatives of four distinct

groups of igneous rocks. 1st, The vast majority of them consist of

plagioclase-pyroxene-magnetite rocks with or without olivine. These are

the normal basalts and dolerites. 2nd, A number of large dykes have

a rather more acid composition and are classed as andesites. 3rd, A

few dykes of trachyte have been observed in Cowal and in Skye cutting

the dykes of basalt (p. 138). 4th, In some districts large numbers

of still more acid dykes occur. These are sometimes crystalline in

structure (granophyre), more frequently felsitic (felsite, spherulitic

quartz-porphyry), and often glassy (pitchstone). In some exceptional

cases the basic and acid materials are conjoined in the same dyke.

Such compound varieties are described at p. 161. The acid dykes,

connected as they so generally are with the large bodies of granophyre

or granite, are doubtless younger than the great majority of the basic

dykes. They will be treated in connection with the acid intrusions in

Chapter xlviii.

By far the greater number of the dykes of the Tertiary volcanic series

belong to the first group, and it is these more especially which will

be discussed in the present and the following Chapter. As, however, the

andesitic group is intimately linked with the basaltic it will be here

included with them.

1. Basalt, Dolerite and Andesite Dykes.--To the field-geologist, who

regards merely their external features, the Tertiary dykes present a

striking uniformity in general petrographical character. They vary

indeed in fineness or coarseness of texture, in the presence or absence

of porphyritic crystals, amygdales, glassy portions and other points

of structure. But there is seldom any difficulty in perceiving that

they generally belong to one or other of the types of the basalts,

dolerites, diabases or andesites. This sameness of composition,

traceable from Yorkshire to Skye and from Donegal to Perthshire, is

one of the strongest arguments for referring this system of dykes to

one geological period. At the same time, there are enough of minor

variations and local peculiarities to afford abundant exercise for the

observing faculties alike in the field and in the study, and to offer

materials for arriving at some positive conclusions regarding the

geological processes involved in the uprise of the dykes.

There appears to be reason to believe that, when the petrography of the

dykes is more minutely studied, marked differences of material will be

found to denote distinct periods of eruption. Already Mr. A. Harker of

the Geological Survey, who is engaged in mapping the interesting and

complicated district of Strath in Skye, has observed that the dykes

which are older than the great granophyre bosses of that tract may be

distinguished from those which are later than these protrusions. The

older basic dykes are not conspicuously porphyritic, are frequently

marked by a close-grained margin or even with a veneer of basalt-glass,

sometimes have an inclination of as much as 45°, are occasionally

discontinuous, and not infrequently branch or send out veins. The

younger dykes, on the other hand, as will be more particularly noticed

in the following chapter, are distinguished by the frequent and

remarkable character of their porphyritic inclusions, by the presence

of foreign fragments in them, by the greater perfection of their

jointing, and by their seldom departing much from the vertical.[163] They

are likewise often markedly acid in composition, including such rocks

as granophyre, felsite and pitchstone.

[Footnote 163: In the Blath Bheinn group of gabbro-hills, however, it is

the youngest dykes which have been found by Mr. Harker to possess the

lowest hade.]

(1) _External Characters._--As regards the grain of the rock, every

gradation may be found, from a coarsely crystalline mass, in which the

component minerals are distinctly traceable with the naked eye, to a

black lustrous basalt-glass. Each dyke generally preserves the same

character throughout its extent. As a rule, broad and long dykes are

coarser in grain than narrow and short ones. For the most part, there

runs along each side of a dyke a selvage of finer grain than the rest

of the mass. This marginal strip varies in breadth from an inch or less

up to a foot or more, and obviously owes its origin to the more rapid

chilling of the molten rock along the walls of the fissure. It usually

shades away inperceptibly into the larger-grained inner portion. Even

with the naked eye its component materials can be seen to be more

finely crystalline than the rest of the dyke, though where dispersed

porphyritic felspars occur they are as large in the marginal strip as

in any other part of a dyke, for they belong to an earlier period of

crystallization than the smaller felspars of the groundmass and were

already floating in the magma while it was still in a molten state.

This finer-grained external band, so distinctive of an eruptive and

injected rock, is of great service in enabling us to trace dykes

when they traverse other dykes or masses of igneous rock of similar

characters to their own. When one dyke crosses another, that which has

its marginal band of finer grain unbroken must obviously be the younger

of the two.

[Illustration:

Fig. 235.--Plan of basalt-veins with selvages of black

basalt-glass, east side of Beinn Tighe, Isle of Eigg.

]

But in many examples in the south of Scotland, Argyleshire and the

Inner Hebrides, the fineness of grain of the outer band culminates

in a perfect volcanic glass. Where this occurs, the glass is usually

jet black, more rarely greenish or bluish black in tint, and varies

in thickness from about a couple of inches to a mere varnish-like

film on the outer face of the dyke, the average width being probably

less than a quarter of an inch (Fig. 235). On their weathered surface

these external glassy layers generally present a pattern of rounded or

polygonal prominences, varying up to four or five lines or even more in

diameter, and separated by depressions or narrow ribs. The transition

from the glass to the crystalline part of the marginal fine-grained

strip is usually somewhat abrupt, insomuch that on weathered faces

it is often difficult to get good specimens, owing to the tendency

of the vitreous portion to fly off when struck with the hammer. The

glass doubtless represents the original condition of the rock of the

dyke. It was suddenly chilled and solidified by contact with the

cold walls of the fissure. Inside this external glassy coating, the

molten material could probably still move, and had time to assume a

more or less completely crystalline condition before solidification.

Not infrequently the glass shows spherulitic forms, visible to the

naked eye, and likewise a more or less distinctly developed perlitic

structure. These features, however, are best studied in thin sections

of the rock with the aid of the microscope, as will be subsequently

referred to.

In some dykes, the glass is not confined to the edges, but runs in

strings or broader bands along the central portions, or has been

squeezed into little cavities like steam-holes or into minute fissures.

One of the most remarkable examples of this peculiarity occurs in the

well-known dyke of Eskdale, which runs for so many miles across the

southern uplands of Scotland.[164] This dyke throughout most of its

course is a crystalline rock of the andesitic type. At Wat Carrick,

in Eskdale, it presents an arrangement into three parallel bands.

On either side, a zone about eight feet broad consists of the usual

crystalline material. Between these two marginal portions lies an

intercalated mass 16 to 18 feet broad, of a very compact and more or

less vitreous rock. The demarcation between this central band and the

more crystalline zones of the outside is quite sharp, and the two

kinds of rock show a totally distinct system of jointing. There can,

therefore, be little doubt that the glassy centre belongs to a later

uprise than the outer portions, though possibly it may still have been

included in the long process of solidification of one original injected

mass of molten material. If the marginal parts adhered firmly to the

walls, the centre, which with its band of vesicles seems often to have

been a line of weakness, might be ruptured and subsequent intrusions

would find their way along the rent. Examples of this splitting of

dykes with the intrusion of later eruptive Material will be cited in

later pages.

[Footnote 164: See _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 241.]

Mr. Clough, while mapping for the Geological Survey the extraordinarily

numerous dykes in the eastern part of Argyleshire between the Firth

of Clyde and Upper Loch Fyne, observed six or seven examples of dykes

showing glassy bands in their centres, with characters similar to those

of the Eskdale dyke. He found an absence of definite and regular joints

in the central glassy band, and on the other hand, an irregular set of

divisional planes by which the rock is traversed, and which he compared

to those seen in true perlitic structure.

While, as a general rule, the external portions of a dyke are

closer-grained than the centre, rare cases occur where the middle is

the most finely crystalline part. I am disposed to regard these cases

and the glassy centres as forming in reality no true exceptions to the

rule, that the outer portions of a dyke consolidated first, and are

therefore finest in texture. For the most part, each dyke appears to be

due to a single uprise of molten matter, though considerable movements

may have taken place within its mass before the whole stiffened into

stone. Some particulars regarding these movements will be given in

section 12 of the next Chapter. It has already been mentioned that in

large dykes which have served as volcanic pipes, it is conceivable

that while the material next the outside consolidated and adhered to

the walls, the central portion may have remained liquid, and may even

have been propelled upward and have been succeeded by a different kind

of magma, as has been suggested by Mr. Iddings. In such cases, which,

if they occur, are probably excessively rare, we may expect that the

earlier and later material will not be sharply marked off from each

other, unless we suppose that the whole of the earlier liquid magma was

so entirely ejected that only its congealed marginal selvage was left

as bounding walls for the newer injection.

Where, after more or less complete consolidation had taken place,

the fissure opened again, or from any other cause the dyke was split

along its centre, any lava which rose up the rent would tend to take a

finer grain than the material of the rest of the dyke, and might even

solidify as glass.

Large scattered crystals of felspar, of an earlier consolidation than

that of the minuter forms of the same mineral in the general groundmass

of the rock, give a porphyritic structure and andesitic character to

many dykes. Occasionally such crystals attain a considerable size. Mr.

Clough has observed them in some of the Argyleshire dykes reaching

a length of between three and four inches, with a thickness of two

inches. Sometimes they are distributed with tolerable uniformity

through the substance of the dyke. But not infrequently they may be

observed in more or less definite bands parallel with the boundary

walls. Unlike the younger lath-shaped and much smaller felspars of the

groundmass, they show no diminution either in size or abundance towards

the edge of the dyke. On the contrary, as already mentioned, they are

often conspicuous in the close-grained marginal strip, and may be found

even in the glassy selvage, or touching the very wall of the fissure.

Indeed, they are sometimes more abundant in the outer than in the

inner portions of a dyke, having travelled outwards to the surfaces of

earliest cooling and crystallization.

Mr. Clough has given me the details of an interesting case of this kind

observed by him in Glen Tarsan, Eastern Argyleshire:--"For an inch

or so from the edge of this dyke," he remarks, "porphyritic felspars

giving squarish sections, and ranging up to one-third of an inch in

length, are so abundant as nearly to equal in bulk the surrounding

groundmass. For the next inch and a half, they are decidedly fewer,

occupying perhaps hardly an eighth of the area exposed. Then for a

breadth of three inches they come in again nearly as abundantly as at

the sides; after which they diminish through a band 27 inches broad,

where they may form from 1/8 to 1/12 of the rock." He found another

case where, in a dyke several yards wide, porphyritic felspars,

sometimes an inch long, are common along the eastern margin of the

dyke in a band about two inches broad, but nearly absent from the rest

of the rock. Elsewhere the crystals are grouped rather in patches than

in bands. Among the dykes south of Oban some similar instances of

coarsely porphyritic felspars may be observed.

Not only are these porphyritic felspars apt to occur in bands parallel

with the outer margins of the dykes, but they tend to range themselves

with their longer axis in the same direction, thus even on a large

scale, visible at some distance, showing the flow-structure, which is

so often erroneously regarded as essentially a microscopic arrangement,

and as specially characteristic of superficial lava-streams.

Mr. Harker in his survey of Strath, Skye, has met with some remarkable

examples of the enclosure and incorporation of foreign materials

in the younger group of dykes which in that district traverse the

granophyres and gabbros. He remarks that the great majority of these

dykes are basic, and he has found them to be capable of convenient

division into two groups. 1st, Non-porphyritic basic dykes with a

specific gravity between 2·87 and 2·97, and an amygdaloidal structure

affording clear indication of flowing movement, either at the sides or

along a central band. These dykes do not greatly differ from those of

pre-granophyre eruption. 2nd, Porphyritic basic dykes which present

features of peculiar interest. The porphyritic (or pseudo-porphyritic)

elements, according to Mr. Harker's observations, are constantly

felspar, frequently subordinate augite, and exceptionally quartz. The

felspars have for the most part rounded outlines with a bordering

zone of glass cavities apparently of secondary origin. The augite, in

rounded composite crystal-grains, differs from that of the groundmass

and resembles the augite of the gabbros. The quartz-grains are likewise

rounded, and show sometimes a distinct corroded border.

These characters, Mr. Harker observes, are those of crystals derived

from some foreign source, and it can scarcely be doubted that this

is the explanation of their presence. He noticed that the dykes in

question frequently enclose fragments, varying up to several inches in

diameter, of gabbro, granite or granophyre, bedded lava, quartzite,

etc., which show clear evidence of having been rounded and corroded

by an enveloping magma, and recognizable crystals from some of the

fragments may be observed in the surrounding parts of the matrix

of the dykes. Most of the felspar and augite crystals disseminated

through these porphyritic basic dykes may be referred to the partial

reabsorption of enclosed fragments of gabbro. The same observer has

found that many of the dykes which rise through the basalt-plateau of

Strathaird are crowded with gabbro fragments.

Another megascopic character of the material composing the dykes is the

frequent presence of amygdales. It has sometimes been supposed that

amygdaloidal structure may be relied upon as a test to distinguish

a mass of molten rock which has reached the surface from one which

has consolidated under considerable pressure below ground. That this

supposition, however, is erroneous is demonstrated by hundreds of dykes

in the great system which I am now describing. But the amygdales of a

dyke offer certain peculiarities which serve in a general way to mark

them off from those of an outflowing lava. They are usually smaller

and more uniform in size than in the latter rock. They are also more

regularly spherical and less frequently elongated in the direction of

flow. Moreover, they are not usually distributed through the whole

breadth of a dyke, but tend to arrange themselves in lines especially

towards its centre (Fig. 236). In these central bands the cavities are

largest and depart farthest from the regular spherical form, so that

for short spaces they may equal in bulk the mass of enclosing rock.

In some rare instances, a whole dyke is composed of cellular basalt,

like one of the lava-sheets in the plateaux, as may be seen on the

north flank of Beinn Suardal, Skye. Mr. Harker has observed that an

amygdaloidal structure is more common among the earlier than among the

later dykes of that district.

[Illustration: Fig. 236.--Arrangement of lines of amygdales in a dyke,

Strathmore, Skye.]

Besides the common arrangement of fine-grained edges and a more

coarsely crystalline centre, instances are found where one of the

contrasted portions of a dyke traverses the other in the form of veins.

Of these, I think, there are two distinct kinds, probably originating

in entirely different conditions. In the first place, they may be of

coarser grain than the rest of the rock; but such a structure appears

to be of extremely rare occurrence. I have noticed some examples on the

coast of Renfrewshire, where strings of a more coarsely crystalline

texture traverse the finer-grained body of the rock. Veins of this kind

are probably of the same nature as the so-called "segregation-veins,"

to be afterwards referred to as of frequent occurrence among the

thicker Tertiary sills. They consist of the same minerals as the

rest of the rock, but in a different and more developed crystalline

arrangement, and they contain no glassy or devitrified material, except

such portions of that of the surrounding groundmass as may have been

caught between their crystalline constituents.

The second kind of veins, which, though not common, is of much more

frequent occurrence than the first, is more particularly to be met with

among the broader dykes, and is distinguished by a remarkable fineness

of grain, sometimes approaching the texture of felsite or jasper, and

occasionally taking the form of actual glass. Such veins vary from

half an inch or less, up to four or five inches in breadth. They run

sometimes parallel with the walls of the dyke, but often irregularly

in all directions, and for the most part avoid the marginal portions,

though now and then coming up to the edge. They never extend beyond

the body of the dyke itself into the surrounding rock. Though they

have obviously been injected after the solidification of the rock

which they traverse, they may quite possibly be extrusions of a deeper

unconsolidated portion of the same rock into rents of the already

stiffened overlying parts. The field-geologist cannot fail to be struck

with the much greater hardness of these fine-grained veins and strings

that ramify through the coarsely crystalline dolerite, andesite or

other variety of the broader dykes. He can readily perceive in many

cases their more siliceous composition, and the inferences he deduces

from the rough observations he can make in the field are confirmed by

the results of chemical analysis (see p. 137).

In connection with veins of finer material, that may belong to a late

stage of the consolidation of the general body of a dyke, reference may

be made here to the occasional occurrence of patches of an exceedingly

compact or homogeneous texture immersed in the usual finely crystalline

marginal material. They look like angular and subangular portions of

the more rapidly cooled outer edge, which have been broken off and

carried upward by the still moving mass in the fissure.[165]

[Footnote 165: See Mr. J. J. H. Teall, _Quart. Journ. Geol. Soc._ xl.

(1884), p. 214.]

In general, each dyke is composed of one kind of rock, and retains

its chemical and mineralogical characters with singular persistence.

The difference of texture between the fine-grained chilled margin,

with its occasional glassy coating, and the more coarsely crystalline

centre is due to cooling and crystalline segregation in what was no

doubt originally one tolerably uniform molten mass. The glassy central

bands, too, though they indicate a rupture of the dyke up the middle,

may at the same time quite conceivably be, as I have said, extrusions

from a lower portion of the dyke before the final solidification of the

whole. The ramifying veins of finer grain that now and then traverse

one of the large dykes are likewise explicable as parts of a stage

towards entire consolidation. All these vitreous portions, whether

still remaining as glass or having undergone devitrification, are

more acid than the surrounding crystalline parts of the rock. They

represent the siliceous "mother-liquor," so to speak, which was left

after the separation from it of the crystallized minerals, and which,

perhaps, entangled here and there in vesicles of the slowly cooling

and consolidating rock, was ready to be forced up into cracks of the

overlying mass during any renewal of terrestrial disturbance.

But examples occur where a dyke, instead of consisting of one rock, is

made up of two or more bands of rock which, even if they resemble each

other closely, can be shown to be the results of separate eruptions.

These, which are obviously not exceptions to the general rule of the

homogeneity of dykes, I will consider in the next Chapter.

Among the petrographical varieties observable in the field is the

occasional envelopment of portions of the surrounding rocks in the

body of a dyke. Angular fragments torn off from the fissure-walls

have been carried upwards in the ascending lava, and now appear more

or less metamorphosed, the amount of alteration seeming to depend

chiefly upon the susceptibility of the enclosed rock to change from

the effects of heat. Cases of such entanglement, however, are of less

common occurrence than those already referred to, where pieces of some

deep-seated rock, such as the gabbros of Skye, have been carried up in

the ascending magma. Occasionally, where the enclosed fragments are

oblong, they are arranged with their longer axes parallel to the walls

of the dyke, showing flow-structure on a large scale. Mr. Clough has

found some dykes near Dunoon which enclose fragments of schist nearly

three feet in length.

One of the most interesting of the megascopic features of the dykes is

the joints by which they are traversed. These divisional planes are no

doubt to be regarded as consequences of the contraction of the original

molten rock during cooling and consolidation between its fissure-walls.

They are of considerable interest and importance, inasmuch as they

furnish a ready means of tracing a dyke when it runs through rock of

the same nature as itself, and also help to throw some light on the

stages in the consolidation of the material of the dyke.

[Illustration: Fig. 237.--Systems of joints in the dykes.

_a_, parallel; _b_, transverse.]

Two distinct systems of joints are recognizable (Fig 237). Though

sometimes combined in the same dyke, they are most conspicuously

displayed when each occurs, as it generally does, by itself. The first

and less frequent system of joints (_a_) has been determined by lines

of retreat, which are parallel to the walls of the dyke. The joints

are then closest together at the margin, and may be few or altogether

absent in the centre. They are sometimes so numerous, parallel and

defined towards the borders of the dyke, as to split the rock up into

thin flags. Where transverse joints are also present these flags are

divided into irregular _tesseræ_.

In the second or transverse system of joints (_b_), which is the more

usual, the divisional lines pass across the breadth of the dyke,

either completely from side to side, or from one wall for a longer or

shorter distance towards the other. Where this series of joints is

most completely developed the dyke appears to be built up of prisms

piled horizontally, or nearly so, one above another. These prisms, in

rare instances, are as regular as the columns of a basalt-sheet (see

Fig. 166). Usually, however, they have irregularly defined faces, and

merge into each other. Where the prismatic structure is not displayed,

the joints, starting sharply at the wall of the dyke, strike inwards

in irregular curving lines. It is such transverse joints that enable

the eye, even from a distance, to distinguish readily the course of

a dyke up the face of a cliff of basalt-beds, for they belong to the

dyke itself, are often at right angles to those of the adjacent basalt,

and by their alternate projecting and re-entering angles seam the dyke

with parallel bars of light and shade (see the double dyke in Fig.

333). Where they traverse not only the general mass of a dyke, but

also the "contemporaneous veins" which cross it, it may be inferred

that these veins were injected before the final solidification and

contraction of the whole dyke.

[Illustration:

Fig. 238.--Section of cylindrical vein or dyke, cutting the bedded

lavas, east side of Fuglö, Faroe Islands.

]

An interesting modification of the transverse joints may sometimes be

observed, where, as in the case of the Palæozoic "Rock and Spindle,"

at St. Andrews (Fig. 222), the molten material has solidified in a

tubular or spherical cavity. The joints then radiate inwards from the

outer curved surface. The most remarkable instance of this structure

which I have found among the Tertiary volcanic plateaux occurs on the

east side of the island Fuglö, the most north-easterly of the group of

the Faroes. It is cut in section by the face of the precipice, where

it appears as a round mass about 40 or 50 feet in diameter piercing

the plateau-basalts. A selvage of finer material round its outer edge

shows the effect of rapid chilling, while the joints diverge from the

periphery and extend in fan-shape towards the centre (Fig. 238).

[Illustration: Fig. 239.--Joint-structures in the central vitreous

portion of the Eskdale Dyke (B. N. Peach).

A, View of a square yard of the outer wall of the vitreous central

band, showing the polygonal arrangement of the prisms and their

investing sheath of ribs.

B, View of a smaller portion of the same wall to show the detailed

structure of the ribs (_a_ _a_) and their vitreous cores (_b_ _b_).

C, Profile of a part of the weathered face of the wall, showing the

way in which the hard ribs or sheaths project at the surface.

]

One of the most remarkable exhibitions of joint-structure hitherto

noticed among the Tertiary dykes is that which occurs in the central

vitreous band of the Eskdale dyke already referred to. The rock is

divided into nearly horizontal prisms, each of which consists of an

inner more vitreous core and an outer more lithoid sheath. By the

coherence of their polygonal and irregular faces, and the greater

durability of their material, these sheaths project on the weathered

wall of the vitreous centre of the dyke in a curiously reticulated

grouping of prominent ribs each about two inches broad (Fig. 239, A),

while the vitreous cores, being more readily acted on by the weather,

are hollowed out into little cup-shaped depressions. Each rib is thus

composed of the sheaths or outer lithoid portions of two prisms, the

line of separation being marked by a suture along the centre (B).

Between this median suture and the inner glassy core the rib is further

cut into small segments by a set of close joints, which are placed

generally at right angles to the course of the rib (C). Examined with a

lens, the lithoid substance of these sheaths has a dull finely granular

aspect, like that of felsitic rocks, with scattered felspars. It is

obviously a more devitrified condition of the material which forms

the core of each prism. This material presents on a fresh fracture a

deep iron-black colour, dull resinous lustre and vitreous texture.

It at once recalls the aspect of many acid pitchstones, and in the

early days of petrography was naturally mistaken for one of these

rocks. Through its substance numerous kernels of more glassy lustre

are dispersed, each of which usually contains one or more amygdales

of dull white chalcedony, but sometimes only an empty black cavity.

These black glistening kernels of glass, of all sizes up to that of a

small bean, scattered through the dull resinous matrix, form with the

white amygdales the most prominent feature in the cores; but crystals

of felspars may also be observed. Some details of the microscopic

characters of this remarkable structure will be given in a subsequent

page. The relation of the cores and sheaths to the prismatic jointing

of the rock seems to show that devitrification had not been completed

when these joints were established, and that it proceeded from the

faces of each prism inwards.

(2) _Microscopic Characters._--Much information has now been obtained

regarding the microscopic structure of the basaltic, doleritic and

andesitic dykes. The crystalline characters of those in the North of

England have been studied by Mr. Teall,[166] and some of those from

the West of Scotland have been investigated by Professors Judd and

Cole.[167] Taken as a whole, the rocks composing the dykes are found,

when examined microscopically, to consist essentially of mixtures of a

plagioclase felspar, pyroxene and iron oxide, with or without olivine,

and usually with more or less interstitial matter.

[Footnote 166: _Quart. Journ. Geol. Soc._ vol. xl. (1884).]

[Footnote 167: _Op. cit._ vol. xxxix. (1883) p. 444 (basalt-glass); xlii.

(1886) p. 49, where Professor Judd discusses the gabbros, dolerites and

basalts as a whole.]

The felspar appears to be in some cases labradorite, in others

anorthite, but there may be a mingling of several species in many

of the dykes, as in the augite-andesite of the Santorin eruption in

1866, wherein Professor Fouqué found that the larger porphyritic

felspars were mainly labradorite, but partly anorthite, while those

of the groundmass were microlites of albite and oligoclase.[168] The

large felspars scattered porphyritically through the groundmass

are evidently the result of an early consolidation, unless where

they are survivals from fragments of older porphyritic rocks which

have been enveloped and partially dissolved in the dykes. They are

often cracked, penetrated by the groundmass, or even broken into

fragments, and have corroded borders. They sometimes include portions

of the groundmass, and present the zonal growth structure in great

perfection. The small felspars of the groundmass, on the other hand,

are as obviously the result of a later crystallization, for they vary

in size and crystallographic development according to their position

in the dyke. Those from the centre are often in well-formed crystals,

which sometimes pass round their borders into acicular microlites.

Those in the marginal parts of the dyke occur chiefly in the form

of these microlites, forming the felted aggregate so characteristic

of the andesites. Curious skeleton forms, composed of aggregates of

microlites, connect the latter with the more completely developed

crystals, and illustrate the mode of crystallization of the felspathic

constituents of the dykes.[169]

[Footnote 168: _Santorin et ses Éruptions_, 1879, p. 203.]

[Footnote 169: See Mr. Teall's excellent description of the Cleveland

dyke, in the paper above cited.]

The pyroxene is probably in most cases monoclinic (black or common

augite), but is sometimes rhombic (usually enstatite, less frequently

perhaps hypersthene). It occurs in (_a_) well-developed crystals,

(_b_) crystalline masses with some of the faces of the crystals

developed, (_c_) granular aggregates which polarise in one plane, (_d_)

separate granules and microscopic microlites, which may be spherical

(globulites) or oblong (longulites).

The black iron-oxide is sometimes magnetite, sometimes ilmenite, or

other titaniferous ore. Apatite not infrequently occurs among the

original constituents. Olivine is entirely absent from most of the

large solitary dykes, especially at a distance from the great volcanic

centres, and no serpentinous matter remains to indicate that it was

ever present in them. But it is to be met with in numerous basalt-dykes

in the volcanic areas, either in sparsely scattered or in tolerably

abundant crystals. Biotite occasionally appears. Among the secondary

products, calcite and pyrites are doubtless the most common. To these

must be added quartz, chalcedony and various zeolitic substances,

besides the aggregates which result from the decomposition of the

ferro-magnesian constituents and the oxidation of the ferrous oxides.

In many dykes there is little or no interstitial matter between the

crystalline constituents of the groundmass. In others this matter

amounts to a half or more of the whole composition, and from such cases

a series of gradations may be traced into a complete glass containing

only the rudimentary forms of crystals (globulites, longulites, etc.),

with scattered porphyritic crystals of an earlier consolidation. The

process of the disappearance of this original glass may be admirably

studied in many dykes. At the outer wall, the glass remains nearly

as it was when contact with the cold walls of the fissure solidified

it. From that external vitreous layer the successive devitrification

products and crystalline growths may be followed inwards until in the

central parts of a broad dyke little or no trace of the interstitial

matter may be left.

[Illustration: Fig. 240.--Microscopic structure of the vitreous part of

the Eskdale Dyke.

This section shows a crystal of augite, enclosing magnetite and

surrounded with microlites, each of which consists of a central

pale yellow rod crusted with pale yellow isotropic globulites. The

glass around this aggregation is clear, but at a little distance

globulites (many of them elongated and dichotomous) abound, with

here and there scattered microlites, some of which are curved and

spiral. (800 diameters.)[170]

]

[Footnote 170: _Proc. Roy. Phys. Soc. Edin._ v. (1880), p. 255.]

The most instructive example of the process of devitrification which

has come under my observation occurs in the Eskdale dyke. The central

"cores" already referred to present a true glass, which in thin

sections is perfectly transparent and almost colourless, but by streaks

and curving lines of darker tint shows beautiful flow-structure. The

devitrification of this glass has been accomplished by the development

of crystallites and crystals, which increase in number until all

the vitreous part of the rock disappears. What seems under a low

power to be a structureless or slightly dusty glass can be resolved

with a higher objective into an aggregate of minute globules or

granules (globulites), which average perhaps 1/20,000 of an inch in

diameter. Some of these bodies are elongated and even dichotomous at

the ends. These granules are especially crowded upon clear yellow

dart-shaped rods, which in turn are especially prominent upon crystals

and crystalline grains of augite that bristle with them, while the

immediately surrounding glass has become clear. There can be little

doubt that these rudimentary bodies are stages in the arrested

development of augite crystals. There occur also opaque grains, rods

and trichites, which no doubt consist in whole of magnetite (or other

iron oxide), or are crusted over with that mineral.

At least two broad types of microscopic structure may be recognized

among the basic and intermediate dykes. (1) Holocrystalline, or with

only a trifling proportion of interstitial matter. This type includes

the dolerites and basalts, as well as rocks which German petrographers

would class as diabases or diabase-porphyrites. The rocks are very

generally characterized by ophitic structure, where the lath-shaped

felspars penetrate the augite, and are therefore of an earlier

consolidation. In such cases there is a general absence of any true

interstitial matter. The rocks of this type are often rich in olivine,

and appear to be on the whole considerably more basic than those of

the second group. It is observable that they increase in numbers from

the centre of Scotland westwards, and throughout the region of the

basalt-plateaux they form the prevailing type. (2) In this type there

is a marked proportion of interstitial substance, which is inserted in

wedge-shaped portions among the crystallised constituents ("intersertal

structure" of Rosenbusch). The ophitic structure appears to be absent,

and olivine is either extremely rare or does not occur at all. The

rocks of this group are obviously less basic than those of the other.

They form the large dykes that rise so conspicuously through the South

of Scotland and North of England, and their general characters are well

described by Mr. Teall in the paper already cited. In some instances

they enclose abundant porphyritic felspars of earlier consolidation,

and then present most of the characters of andesites. Professor

Rosenbusch has extended the name of "Tholeiites" to rocks of this group

in the North of England.[171] The vitreous condition is found in both

types, but is perhaps more frequent in the second. The glass of the

basalts, however, even in thin slices, is characteristically opaque

from its crowded inclusions; while that of the andesitic forms, though

black in hand specimens, appears perfectly transparent and sometimes

even colourless in thin slices.

[Footnote 171: _Mikroskopische Physiographie_, 3rd edit. 1071 _et seq._]

(3) _Chemical Characters._--The only one of these to which reference

will be made here is the varying proportion of silica. While the dykes

as a whole are either intermediate or basic, some of them contain so

high a percentage of silica as to link them with the acid rocks. The

average proportions of this ingredient range from less than 50 to

nearly 60 per cent. The rocks with the lower percentage of acid are

richer in the heavy bases, and have a specific gravity which sometimes

rises above 3·0. They include the true dolerites and basalts. Those,

on the other hand, with the higher ratio of silica, are poorer in

the heavy bases, and have a specific gravity from 2·76 to 2·96. They

comprise the tholeiites, andesites and other more coarsely crystalline

rocks of the great eastern and south-eastern dykes.[172]

[Footnote 172: For analyses of dykes, see Sir I. L. Bell, _Proc. Roy.

Soc._ xxiii. p. 546; Mr. J. S. Grant Wilson, _Proc. Roy. Phys. Soc.

Edin._ v. p. 253; Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 209;

Professors Judd and Cole, _Quart. Jour. Geol. Soc._ xxxix. p. 444.]

Not only do the dykes differ considerably from each other in their

relative proportions of silica, but even the same dyke may sometimes be

found to present a similar diversity in different parts of its mass. It

has long been a familiar fact that the glassy parts of such rocks are

more acid than the surrounding crystalline portions. The original magma

may be regarded as a natural glass or fused silicate, in which all the

elements of the rock were dissolved, and which necessarily became more

acid as the various basic minerals crystallised out of it.[173] In the

Eskdale dyke the silica percentage of this glassy portion is 58·67,

that of the little kernels of black glass dispersed through the rock

as much as 65·49.[174] In the Dunoon dyke observed by Mr. Clough the

siliceous finer-grained veins contain no less than 68·05 per cent of

silica, while the mass of the dyke itself shows on analysis only 47·36

per cent.[175] Similar red strings have been noticed by the same careful

observer in an east and west dyke near Lochgoilhead. From Mr. Teall's

examination a large part of the felspar in these veins is probably

orthoclase. It forms a much larger percentage of the entire rock than

the felspar does in normal dolerites.

[Footnote 173: On this subject see a paper by Dr. A. Lagorio, "Über die

Natur der Glasbasis sowie der Krystallisationsvorgänge im eruptiven

Magma," Tschermak's _Mineralog. Mittheil._ viii. (1887), p. 421.]

[Footnote 174: Mr. J. S. Grant Wilson, _Proc. Roy. Soc. Phys. Edin._ v.

(1880) p. 253.]

[Footnote 175: Unpublished analyses made by the late Professor Dittmar of

Glasgow, and communicated to me by Mr. Clough.]

2. Trachyte Dykes.--In the Cowal District of Argyleshire, and in

the south of Skye, Mr. Clough has encountered a limited number

of dykes of trachyte. On a hasty inspection these are not always

readily distinguished from the basalt-dykes with which they agree in

general external aspect and in direction. Where their relation to

these dykes, however, can be determined they are found to traverse

them, and thus to be on the whole later, though one case has been

observed where a trachytic dyke is in turn traversed by one of the

basic series. Mr. Clough has supplied me with the following notes

of his observations regarding the trachytic dykes. They are all

characterized by the possession of spherulitic structures near their

margins. These features, easily perceptible to the naked eye, afford

the readiest means of distinguishing the dykes of this group. So

abundant are the spherulites that they not infrequently impinge on

each other in long parallel rows forming rod-like aggregates. Thus

in a dyke near Craigendavie, at the head of Loch Striven, numerous

planes about a quarter of an inch apart, and composed of such close-set

rods, may be observed running parallel to the marginal wall for a

distance of several inches from the edge. Most of these planes show

on their surfaces that the rods are always parallel to each other,

but may run in different directions in the different layers, being

sometimes horizontal, sometimes vertical, or at any angle between. On

examination, each rod is found to be made up of polygonal bodies, the

angles of which are quite sharp, but with their sides often slightly

curved, as if they had assumed their forms from the mutual pressure of

original spherical bulbs. Further scrutiny shows that the polygonal

bodies often exhibit an internal radiate structure.

In the central parts of the dyke the spherulitic arrangement is not

traceable. About a foot from the margin it begins to be recognizable.

At a distance of three or four inches the spherulites are about the

size of peas, and gradually diminish towards the edge until they can no

longer be seen.

Another characteristic of the trachyte dykes has been found by

Mr. Clough to be a useful guide in discriminating them from the

basalt-group. While the amygdales in the latter are generally rudely

spherical, those in the trachytes are commonly elongated in the

direction of the length of the dyke, and are frequently three quarters

of an inch, sometimes even an inch and a half, in length, though less

than a quarter of an inch in breadth.

A good example of these trachytic dykes, which occurs at Dunans,

about the head of Glendaruel, has been examined microscopically and

chemically. The central better crystallised portion was found by

Mr. Teall to be composed mainly of small lath-shaped crystals of

orthoclase, together with scales of brown biotite, a few prismatic

crystals of pale somewhat altered pyroxene and scattered granules of

magnetite. The chemical analysis of this rock by Mr. J. H. Player gave

the following composition:--

Silica 56·4

Alumina 19·0

Ferric oxide 3·5

Ferrous oxide 4·8

Lime 2·6

Magnesia 1·5

Soda 4·5

Potash 5·0

Loss on ignition 2·6

----

99·9

====

4. HADE

In the majority of cases, especially among the great single dykes,

the intrusive rock has assumed a position nearly or quite vertical.

But occasionally, where one of these solitary examples crosses a deep

valley, a slight hade is perceptible by the deviation of the line of

the dyke from its normal course. Sedgwick long ago noticed that the

Cleveland dyke has, in places, an inclination of at least 80° to its

N.E. side.[176] In the coal-workings, also, a trifling deviation from

the vertical is sometimes perceptible, especially where a dyke has

found its way along a previously existing line of fault, as in several

examples in Stirlingshire. But in those districts where the dykes are

gregarious, departures from the vertical position are not infrequent,

more particularly near the great basalt-plateaux. It was noticed by

Necker, that even in such a dyke-filled region as Arran, almost all of

the dykes are vertical, though sometimes deviating from that position

to the extent of 20°.[177] Berger found that the angle of deviation

among those of the north of Ireland ranges from 9° to 20°, with a mean

of 13°.[178] The most oblique examples are probably those which occur

in the basalt-plateaux of the Inner Hebrides, where the same dyke in

some parts of its course runs horizontally between two beds, across

which it also descends vertically (see Figs. 251, 252, 374). But with

these minor exceptions, the verticality of the great system of dykes,

pointing to the perpendicular fissure-walls between which the molten

rock ascended, is one of the most notable features in their geological

structure. In the Strath district of Skye Mr. Harker has noticed that

while the earlier dykes have sometimes a hade of 45°, those younger

than the granophyre are generally vertical or nearly so. In the Blath

Bheinn group of hills, however, as already alluded to, he has observed

that it is the youngest dykes which are inclined in a north-westerly

direction, with a hade of as much as 40° from the horizon.

[Footnote 176: _Cambridge Phil. Trans._ ii. p. 28.]

[Footnote 177: _Trans. Roy. Soc. Edin._ xiv. p. 677.]

[Footnote 178: _Trans. Geol. Soc._ iii. p. 227.]

5. BREADTH

An obvious characteristic of most dykes is the apparent uniformity of

their breadth. Many of them, as exposed along shore-sections, vary as

little in dimensions as well-built walls of masonry do. Departures

from such uniformity may often indeed be noted, whether a dyke is

followed laterally or vertically. The largest amount of variation is,

of course, to be found among the dykes of the gregarious type, the

thinner examples of which may diminish to a width of only one inch or

less, while their average breadth is much smaller than in the case

of the great solitary dykes. In the district of Strathaird, in Skye,

Macculloch estimated that the remarkably abundant dykes there developed

vary from 5 to 20 feet in breadth, but with an average breadth of not

more than 10 feet.[179] In the isle of Arran, according to Necker's

careful measurements, most of the dykes range from 2 or 3 to 10 or 15

feet, but some diminish to a few inches, while others reach a width of

20, 30, or even 50 feet.[180] In the North of Ireland, Berger observed

that the average breadth of thirty-eight dykes traversing primitive

rocks (schist, granites, etc.) was 9 feet; and of twenty-four in

Secondary rocks, 24 feet.[181]

[Footnote 179: _Trans. Geol. Soc._ iii. p. 80.]

[Footnote 180: _Trans. Roy. Soc. Edin._ xiv. p. 690 et seq.]

[Footnote 181: _Trans. Geol. Soc._ iii. p. 226. He believed that dykes

in Secondary rocks reach a much greater thickness than in other

formations. My own observations do not confirm this generalisation.]

But when we pass to the great solitary dykes, that run so far and so

continuously across the country, we encounter much thicker masses of

igneous rock. Most of the measurements of these dykes have been made

at the surface, and the variations noted in their breadth occur along

their horizontal extension. The Cleveland dyke, which is the longest

in Britain, varies from 15 feet to more than 100 feet, with perhaps an

average width of between 70 and 90 feet.[182] Some of the great dykes

that cross Scotland are of larger dimensions. Most of them, however,

like that of Cleveland, are liable to considerable variations in

breadth when followed along their length. The dyke which runs from

the eastern coast across the Cheviot Hills and Teviotdale to the head

of the Ale Water, is in some places only 10 feet broad, but at its

widest parts is probably about 100 feet. The Eskdale and Moffat dyke

is in parts of its course 180 feet wide, but elsewhere it diminishes

to not more than 40 feet. These variations are repeated at irregular

intervals, so that the dyke alternately widens and contracts as its

course is traced across the hills. Some of the dykes further to the

north and west attain yet more gigantic proportions. That which crosses

Cantyre opposite Ardlamont Point has been measured by Mr. J. B. Hill,

of the Geological Survey, who finds it to be from 150 to 180 feet broad

on the shore of Loch Fyne, and to swell out beyond the west side of

Loch Tarbert to a breadth of 240 to 270 feet. A dyke near Strathmiglo,

in Fife, is about 400 feet wide. The broadest dyke known to me is one

which I traced near Beith, in Ayrshire, traversing the Carboniferous

Limestone. Its maximum width is 640 feet.

[Footnote 182: At Cockfield, where it has long been quarried, it varies

from 15 to 66 feet; at Armathwatie, in the vale of the Eden, it is

about 54 feet (Mr. Teall, _Quart. Journ. Geol. Soc._ xl. p. 211).]

Unfortunately, it is much less easy to get evidence of the width of

dykes at different levels in their vertical extension. Yet this is

obviously an important point in the theoretical discussion of their

origin. Two means are available of obtaining information on the

subject--(_a_) from mining operations, and (_b_) from observations at

precipices and between hill-crests and valley-bottoms.

(_a_) In the Central Scottish coal-field and in that of Ayrshire,

some large dykes have been cut through at depths of two or three

hundred feet beneath the surface. But there does not appear to be any

well-ascertained variation between their width so far below ground and

at the surface. In not a few cases, indeed, dykes are met with in the

lower workings of the coal-pits which do not reach the surface or even

the workings in the higher coals. Such upward terminations of dykes

will be afterwards considered, and it will be shown that towards its

upper limit a dyke may rapidly diminish in width.

(_b_) More definite information, and often from a wider vertical range,

is to be gathered on coast-cliffs and in hilly districts, where the

same dyke can be followed through a vertical range of many hundred

feet. But so far as my own observations go, no general rule can be

established that dykes sensibly vary in width as they are traced

upward. Every one who has visited the basalt-precipices of Antrim or

the Inner Hebrides, where dykes are so numerous, will remember how

uniform is their breadth as they run like ribbons up the faces of the

escarpments.[183] Now and then one of them may be observed to die out,

but in such cases (which are far from common) the normal width is

usually maintained up to within a few feet of the termination.

[Footnote 183: This point did not escape the attention of that excellent

observer, Berger, in his examination of the dykes in the North of

Ireland. We find him expressing himself thus:--"The depth to which the

dykes descend is unknown; and after having observed the sections of a

great many along the coast in cliffs from 50 to 400 feet in height, I

have not been able to ascertain (except in one or two cases) that their

sides converge or have a wedgeform tendency" (_Trans. Geol. Soc._ iii.

p. 227).]

All over the southern half of Scotland, where the dykes run along

the crests of the hills and also cross the valleys, a difference of

level amounting to several hundred feet may often be obtained between

adjacent parts of the same dyke. But the breadth of igneous rock

is not perceptibly greater in the valleys than on the ridges. The

depth of boulder clay and other superficial deposits on the valley

bottoms, however, too frequently conceals the dykes at their lowest

levels. Perhaps the best sections in the country for the study of this

interesting part of dyke-structure are to be found among the higher

hills of the Inner Hebrides, such as the quartzites of Jura and the

granophyres and gabbros of Skye. On these bare rocky declivities,

numerous dykes may be followed from almost the sea-level up to the

rugged and splintered crests, a vertical distance of between 2000 and

3000 feet. The dykes are certainly not as a rule sensibly less in width

on the hill-tops than in the glens. So far, therefore, as I have been

able to gather the evidence, there does not appear to me to be, as

a general rule, any appreciable variation in the width of dykes for

at least 2000 or 3000 feet of their descent. The fissures which they

filled must obviously have had nearly parallel walls for a long way

down.

6. INTERRUPTIONS OF LATERAL CONTINUITY

In tracing the great solitary dykes across the country, the geologist

is often surprised to meet with gaps, varying in extent from a few

hundred feet to several miles, in which no trace whatever of the

igneous rock can be detected at the surface. This disappearance

is not always explicable by the depth of the cover of superficial

accumulations; for it may be observed over ground where the naked

rocks come almost everywhere to the surface, and where, therefore, if

the conspicuous material of the dykes existed, it could not fail to

be found. No dyke supplies better illustrations of this discontinuity

than that of Cleveland. Traced north-westward across the Carboniferous

tracts that lie between the mouth of the Tees and the Yale of the Eden,

this dyke disappears sometimes for a distance of six or eight miles.

In the mining ground round the head of the South Tyne the rocks are

bare, so that the absence of the dyke among them can only be accounted

for by its not reaching the surface. Yet there can be no doubt that

the various separated exposures, which have the same distinctive

lithological characters and occur on the same persistent line, are all

portions of one dyke which is continuous at some depth below ground. We

have thus an indication of the exceedingly irregular upward limit of

the dykes, as will be more particularly discussed further on.

But there are also instances where the continuity is interrupted

and then resumed on a different line. One of the best illustrations

of this character is supplied by the large dyke which rises through

the hills about a mile south of Linlithgow and runs westward across

the coal-field. At Blackbraes it ends off in a point, and is not

found again to the westward in any of the coal-workings. But little

more than a quarter of a mile to the south a precisely similar dyke

begins, and strikes westward parallel to the line of the first one.

The two separated strips of igneous rock overlap each other for

about three-quarters of a mile. But that they are merely interrupted

portions of what is really a single dyke can hardly be questioned. A

second example is furnished by another of the great dykes of the same

district, which after running for about twelve miles in a nearly east

and west direction suddenly stops at Chryston, and begins again in the

same direction, but on a line about a third of a mile further north.

Such examples serve to mark out irregularities in the great fissures up

which the materials of the dykes rose.

7. LENGTH

In those districts where the small and crowded dykes of the gregarious

type are developed, one cannot usually trace them for more than a

short distance. The longest examples known to me are those which have

been mapped with much patience and skill by Mr. Clough in Eastern

Argyleshire. Some of them he has been able to track over hill and

valley for four or five miles, though the great majority are much

shorter. In Arran and in the Inner Hebrides, it is seldom possible to

follow what we can be sure is the same dyke for more than a few hundred

yards. This difficulty arises partly, no doubt, from the frequent

spread of peat or other superficial accumulation which conceals the

rocks, and partly also from the great number of dykes and the want of

sufficiently distinct lithological characters for the identification of

any particular one. But making every allowance for these obstacles, we

are compelled, I think, to regard the gregarious dykes as essentially

short as well as relatively irregular.

In striking contrast to these, come the great solitary dykes. In

estimating their length, as I have already remarked, we must bear

in mind the fact that they occasionally undergo interruptions of

continuity owing to the local failure of the igneous material to

rise to the level of what is now the surface of the ground. A narrow

wall-like mass of andesite or dolerite, which sinks beneath the surface

for a few hundred yards, or for several miles, and reappears on the

same line with the same petrographical characters, while there may

be no similar rock for miles to right and left, can only be one dyke

prolonged underneath in the same great line of fissure. But even if we

restrict our measurements of length to those dykes or parts of dykes

where no serious interruption of continuity takes place, we cannot

fail to be astonished at the persistence of these strips of igneous

rock through the most diverse kinds of geological structure. A few

illustrative examples of this feature may be selected. It will be

observed that the longest and broadest dykes are found furthest from

the basalt-plateaux, while the shortest and narrowest are most abundant

near these plateaux.

Not far from what I have taken provisionally as the northern boundary

of the dyke region, two dykes occur which have been mapped from the

head of Loch Goil by Arrochar across Lochs Lomond and Katrine by Ben

Ledi to Glen Artney, whence they strike into the Old Red Sandstone of

Strathmore, and run on to the Tay near Perth--a total distance of about

60 miles. If the dyke which continues in the same line on the other

side of the estuary of the Tay beyond Newburgh, is a prolongation of

one of these, then its entire length exceeds 70 miles. A few miles

further south, one of a group of dykes can be followed from the

heart of Dumbartonshire by Callander across the Braes of Doune to

Auchterarder--a distance of 47 miles, with an average breadth of more

than 100 feet. In the district between the Forth and Clyde a number

of long parallel dykes can be traced for many miles across hill and

plain, and through the coal-fields. One of these is continuous for

25 miles from the heart of Linlithgowshire into Lanarkshire. Still

longer is the dyke which runs from the Firth of Forth at Grangemouth

westward to the Clyde, opposite Greenock--a distance of about 36 miles.

Coming southward, we encounter a striking series of single dykes on

the uplands between the counties of Lanark and Ayr, whence they strike

into the Silurian hills of the southern counties. One of these runs

across the crest of the Haughshaw Hills, and can be followed for some

30 miles. But if, as is probable, it is prolonged in one of the dykes

that traverse the moorlands of the north of Ayrshire and south of

Renfrewshire to the Clyde, its actual length must be at least twice

that distance. The great Moffat and Eskdale dyke strikes for more than

50 miles across the South of Scotland and North of England. The Hawick

and Cheviot dyke runs for 26 miles in Scotland and for 32 miles in

Northumberland.

But the most remarkable instance of persistence is furnished by the

Cleveland dyke. From where it is first seen near the coast-cliffs of

Yorkshire the strip of igneous rock can be followed, with frequent

interruptions, during which for sometimes several miles no trace of it

appears at the surface, across the North of England as far as Dalston

Hall south of Carlisle, beyond which the ground onwards to the Solway

Firth is deeply covered with superficial deposits. The total distance

through which this dyke can be recognized is thus about 110 miles. But

it probably goes further still. On the opposite side of the Solway,

a dyke which runs in the same line, rises through the Permian strata

a little to the east of the mouth of the Nith. Some miles further

to the north-west, near Moniaive, Mr. J. Horne, in the progress of

the Geological Survey, traced a dark compact dyke with kernels of

basalt-glass near its margin, running in the same north-westerly

direction. Still further on in the same line, another similar rock is

found high on the flanks of the lofty hill known as Windy Standard.

And lastly, in the Ayrshire coal-field, a dyke still continuing the

same trend, runs for several miles, and strikes out to sea near

Prestwick. It cannot, of course, be proved that these detached Scottish

protrusions belong to one great dyke, or that if such a continuous dyke

exists, it is a prolongation of that from Cleveland. At the same time,

I am on the whole inclined to connect the various outcrops together as

those of one prolonged subterranean wall of igneous rock. The distance

from the last visible portion of the Cleveland dyke near Carlisle to

the dyke that runs out into the Firth of Clyde near Prestwick, is about

80 miles. If we consider this extension as a part of the great North

of England dyke, then the total length of this remarkable geological

feature will be about 190 miles.

8. PERSISTENCE OF MINERAL CHARACTERS

Not less remarkable than their length is the preservation of their

normal petrographical characters by some dykes for long distances.

In this respect the Cleveland dyke may again be cited as a typical

example. The megascopic and microscopic structures of the rock of this

dyke distinguish it among the other eruptive rocks of the North of

England. And these peculiarities it maintains throughout its course.[184]

Similar though less prominent uniformity may be traced among the long

solitary dykes of the South of Scotland, the chief variations in

these arising from the greater or less extent to which the original

glassy magma has been retained. The same dyke will at one part of its

course show abundant glassy matter even to the naked eye, while at a

short distance the vitreous groundmass has been devitrified, and its

former presence can only be detected with the aid of the microscope.

Where a dyke has caught up and absorbed abundant foreign materials its

composition naturally varies considerably from point to point. Mr.

Harker has observed some good examples of this variation in Skye.

[Footnote 184: See the careful examination of this dyke by Mr. Teall,

_Quart. Journ. Geol. Soc._ xl. p. 209.]