The Ancient Volcanoes of Great Britain, Volume 1 (of 2)

by Archibald Geikie

i. GENERAL CHARACTER AND DISTRIBUTION

After the beginning of the Carboniferous Limestone period, when eruptions of the plateau-type had generally ceased, volcanic activity showed itself over the area of the British Isles in a different guise both as regards the nature of its products and the manner and scale of their discharge. Instead of widely extended lava-sheets and tuffs, piled above each other sometimes to a thickness of many hundred feet, and stretching over hundreds of square miles, we have now to study the records of another phase of volcanism, where scattered groups and rows of Puys, or small volcanic cones, threw out in most instances merely tuffs, and these often only in trifling quantity, though here and there their vents also poured forth lavas and gradually piled up volcanic ridges which, in a few cases, almost rivalled some of the plateaux. The evidence for these less vigorous manifestations of volcanic activity is furnished (1) by layers of tuff and sheets of basaltic-lavas intercalated among the strata that were being deposited at the time of the eruptions, (2) by necks of tuff, agglomerate, or different lava-form rocks that mark the positions of the orifices of discharge, and (3) by sills, bosses, and dykes that indicate the subterranean efforts of the volcanoes. The comparatively small thickness of the accumulations usually formed by these vents, their extremely local character, the numerous distinct horizons on which they appear, and the intimate way in which they mingle and alternate with the ordinary Carboniferous strata are features which at once arrest the attention of the geologist, presenting, as they do, so striking a contrast to those of the plateaux.

From the clear intercalation of these volcanic materials on successive platforms of the Carboniferous system, the limits of geological time within which they were erupted can be fixed with considerable precision. It may be said that, in a broad sense, they coincided with the period of the Carboniferous Limestone, and certainly it was during the deposition of that formation that the eruptions which produced them reached their greatest vigour and widest extent. Here and there in Scotland evidence may be found that the phase of the Puys began during that earlier section of Carboniferous time recorded by the Calciferous Sandstones. This is markedly the case in Liddesdale and the neighbouring territory. Over the western part of Midlothian also, the eastern portion of West Lothian, and the southern margin of Fife, abundant traces occur of puy-eruptions during the deposition of the Calciferous Sandstones. Elsewhere in Central Scotland there is no evidence of the vents having been opened until after the deposition of the Hurlet Limestone, which, as we have seen, may conveniently be taken as the base of the Scottish Carboniferous Limestone series. The volcanoes remained active in West Lothian until near the close of the time represented by that series; but in Ayrshire they continued in eruption until the beginning of the accumulation of the Coal-measures. These western examples of the puy-type are, so far as I am aware, the latest known in Britain.

Whether or not the earliest puy-eruptions began before the latest plateau-lavas and tuffs were accumulated is a question that cannot be readily answered. It will be remembered that in the basin of the Firth of Forth a thickness of more than 3000 feet of sedimentary strata, including the Burdiehouse Limestone and numerous oil-shales as well as thin coal-seams, lies above the red and green marls, shales, sandstones and cement-stones of the Calciferous Sandstone series. This remarkable assemblage of strata is absent in the western parts of the country, where the top of the Clyde volcanic plateau is almost immediately overlain by the Hurlet Limestone. If we were to judge of the sequence of events merely from the stratigraphy, as expressed in such sections as Figs. [137], [138], [139] and [140], we might naturally infer that as no trace of any break occurs at the top of the Clyde plateau, the tuffs shading upward there into the limestone series, no important pause in sedimentation took place, but that the last volcanic eruptions were soon succeeded by the conditions that led to the deposition of the widespread encrinite-limestones. If this inference were well founded it would follow that while the plateau-eruptions in the west lasted till the time of the Hurlet Limestone, those in the east ceased long before that time and were succeeded by the puys of Fife and the Lothians. There would thus be an overlap of the two phases of volcanic action.

I am inclined to believe, however, that in spite of the superposition of the Hurlet Limestone almost immediately upon the volcanic rocks of the Clyde plateau, and the absence of any trace of a break in the process of sedimentation, a long interval nevertheless elapsed between the last eruptions and the deposit of that limestone. The Campsie section ([Fig. 140]) shows us how rapidly a thick mass of strata can come in along that horizon. The volcanic ridges may have remained partly unsubmerged for such time as was required for the subsidence of the Forth basin and the deposit of the thick Calciferous Sandstone series there, and their summits may only have finally sunk under the sea not long before the Hurlet Limestone grew as a continuous floor of calcareous material over the whole area of central Scotland. In these circumstances, the puy-eruptions of that basin would be long subsequent to the eruptions of the Clyde plateau, as they certainly were to those of the plateaux of Midlothian and the Garleton Hills.

In tracing the geographical distribution of the puy-eruptions we are first impressed with the force of the evidence for their extremely local and restricted character ([Map IV.]). Thus in the area of the basin of the Firth of Forth, which may be regarded as the typical region in Britain for the study of this form of Carboniferous volcano, traces of them are abundant to the west of the line of the Pentland Hills. To the east of that line, however, not a vestige of puy-eruptions, save a few sills of uncertain relationship, can be detected, though the same series of stratigraphical horizons is well developed on both sides of the Lothian coal-field. Again, to the westward of the Forth basin over the area of Stirlingshire, Lanarkshire and Renfrewshire lying to the north of the great volcanic plateau, no record of puy-eruptions has been noticed. Immediately to the south of that plateau, however, these eruptions were numerous in the north of Ayrshire. Yet the rest of the Carboniferous area in that large county has supplied no relics of these eruptions save at one locality—the Heads of Ayr. Lastly, while no trace of any younger display of volcanic activity occurs in the Merse of Berwickshire, east of the plateau series of that district, the ground immediately to the west abounds in puys, and contains likewise extensive sheets of tuff and beds of basic lavas connected with these vents.

Another fact which at once attracts notice in Scotland is the way in which the puy-vents have generally avoided the areas of the plateaux, though they sometimes approach them closely. As a rule, it is possible to distinguish the tuffs and agglomerates which have filled up these vents from those that mark the sites of the eruptive orifices of the plateaux. There are, no doubt, some instances, as in Liddesdale, where puys have appeared on the sites of the older lavas, but these are exceptional collocations.[442] On the other hand, many examples may be found where puys have risen in the interspace between the limits of the eruptions of two plateau-areas. Thus the tract between the Clyde plateau-eruptions on the west and those of the Garleton Hills on the east was dotted over with puys. Again, the southern margin of the Clyde plateau in Ayrshire, from Dalry to Galston is flanked with puys and long sheets of their lavas and tuffs.[443]

[442] A means of definitely placing some of these vents in the series of puy-eruptions is stated further on, at [p. 476].

[443] Reference may again be made here to the remarkable similarity between the Scottish Carboniferous puy-vents and those of older Tertiary time in the Swabian Alps so fully described by Professor Branco in the work already cited [p. 46]. Denudation in that region has bared the cones and exposed the structure of the necks which, down to even minute details, repeat the phenomena of Carboniferous and Permian time in Scotland.

ii. NATURE OF THE MATERIALS ERUPTED

A. The Lava-form Rocks

We have now to consider the nature of the materials erupted by the volcanic activity of the puys. The geologist who passes from the study of the plateau lavas to those of the puys at once remarks the prevalent more basic character of the latter. The great majority of them are basalts, generally olivine-bearing, in the various types embraced in the table on the following page. The olivine-free dolerites are generally found as intrusive bosses, sills and dykes. Such more acid rocks as andesites occur only rarely, and still more seldom are quartziferous masses met with in some of the bosses.

Dolerites and Basalts.—The great majority of the lava-form rocks connected with the puys are basic in composition, and belong to the family of the Dolerites and Basalts. They graduate, on the one hand, into ultra-basic rocks such as limburgite and picrite, and on the other, into compounds that approach andesites or trachytes in composition. A large series of specimens from Central Scotland was studied a few years ago by Dr. Hatch, who proposed a petrographical classification of the rocks, and arranged them in a number of types which he named after localities where they are well developed.[444] More recently the rocks have again been subjected to microscopic investigation by my colleague Mr. Watts, who, confirming generally Dr. Hatch's discriminations, has made some modifications of them. He has furnished me with a revised classification ([p. 418]), based on purely petrographical considerations. The doleritic and basaltic series may be grouped into two divisions, one with, and the other without, olivine, and each division may be further separated into a dolerite group, which presents an ophitic or sub-ophitic structure, and a basalt-group in which the groundmass is made up of felspar and granular augite, and possesses the "intersertal structure" of Rosenbusch, or consists of idiomorphic augite embedded in felspar substance. The term "sub-ophitic" is employed by Mr. Watts "to imply that the augite grains are neither very large nor very continuous, optically, and that they rarely contain entire felspar-crystals imbedded in them, merely the ends of a group of these crystals as a rule penetrating into them."

[444] This classification was given in my Presidential Address to the Geological Society, 1892, Quart. Journ. Geol. Soc. vol. xlviii. p. 129. See Report of Geological Survey for 1896.

Transitional forms occur between many of the following types by the increase or diminution in the relative proportions of the constituents. Thus it is not easy to draw a line between 2b and 2c; the latter again shades into 2d and 2e by the decrease of the felspar.

Mr. Watts has further observed that the rocks containing no olivine offer greater difficulties in classification than those in which that mineral is present. "The very distinction," he remarks, "between dolerites and basalts is less marked, the types are much less sharply distinguished, and decomposition and masking of the structure are more common. While using the term Dolerite for such rocks as have a sub-ophitic structure, I have extended it to those rocks in which evidence exists that a great part of the crystallization took place under intratelluric conditions. Although not quite holocrystalline, the crystals of felspar, augite and magnetite are large and the structure coarse-grained, while the groundmass is confined to comparatively small interstitial patches. In these rocks there is usually no one dominant porphyritic ingredient."

I. The Olivine-bearing Series
1. Olivine-Dolerites
1a.	Porphyritic elements inconspicuous, olivine being the principal, and felspar of secondary importance; groundmass sub-ophitic.	}	Jedburgh Type.
1b.	Strongly porphyritic; felspar-phenocrysts large; olivine smaller; groundmass sub-ophitic.	}	Kilsyth Type.
1c.	Porphyritic olivine, but not felspar; groundmass sub-ophitic.	}	Gallaston Type.
2. Olivine-Basalts
2a.	Porphyritic olivine, augite and felspar; groundmass of felspar-laths imbedded in granules of augite.	}	Lion's Haunch Type. (See [Fig. 207].)
2b.	Porphyritic olivine and augite; groundmass of felspar-laths imbedded in granules of augite. More rarely the groundmass is made of idiomorphic augite imbedded in felspar-substance.	}	Craiglockhart Type.
2c.	Porphyritic olivine abundant, augite much less common, and felspar very rare or absent; groundmass with granular or idiomorphic augite (one of the most common types).	}	Dalmeny Type.
2d.	Porphyritic olivine more common than augite; groundmass of granules of augite set amongst lath-like felspars which are much fewer in number than in 2c.	}	Picrite Type.
2e.	Porphyritic olivine more common than augite; groundmass of idiomorphic augite imbedded in felspathic material which is not abundant.	}	Limburgite Type.
2f.	Porphyritic olivine and felspar, without augite; groundmass of granular or idiomorphic augite, with lath-shaped felspars.	}	Kippie Law Type. (For analysis see [p. 379]).
II. The Non-olivine-bearing Series
3. Olivine-free Dolerites
	Felspar, augite, magnetite in coarse-grained aggregate usually ophitic or sub-ophitic; groundmass not plentiful.
3a.	Groundmass absent		Ophitic Type.
3b.	Groundmass micropegmatitic		Ratho Type.
3c.	Groundmass an unstriated felspar (not orthoclase) and occasionally some interstitial altered glass or a little quartz.	}	Burntisland Sill Type.
4. Doleritic Basalts
	Felspar, augite and magnetite in coarse-grained aggregate; groundmass rather more plentiful and often in large patches.
4a.	Felspar and augite, related sub-ophitically where together, but augite showing crystalline contours in contact with the groundmass; some interstitial quartz and unstriated felspar.	}	Bowden Hill Type.
5. Basalts
	Finer-grained rocks, generally with a porphyritic ingredient and much scattered interstitial matter in the groundmass.
5a.	Porphyritic felspar, and occasionally a little augite; groundmass of granular augite, felspar needles and magnetite with some interstitial matter.	}	Binny Craig Type.
5b.	Porphyritic felspars not conspicuous and small; the rock mainly made up of a mesh of fine felspar-laths set amongst granular augite, magnetite and base.	}	Tholeiite Type.
5c.	Similar to the last but even finer-grained, and with the } - base in a cryptocrystalline condition.	}	Cryptocrystalline Type.

Taking first the superficial lavas, I know of only one locality where picrite occurs in such a position that it may be included among the surface outflows. This is the quarry near Blackburn, to the east of Bathgate, where I originally observed it.[445] The rock occurs there on the line of the basalt-flows from the Bathgate Hills, and I mapped it as one of them before the microscope revealed the remarkable composition of the mass. I still believe it to be a lava like the "leckstone" described on [p. 443], though the other known examples of this rock in the basin of the Firth of Forth are intrusive sheets. The rock locally known as "leckstone" or "lakestone" has long been quarried for the purpose of constructing the soles of bakers' ovens, as it stands a considerable temperature without cracking. Its microscopic structure is now well known. As exposed in Blackburn quarry, an interesting difference is observable between the lower and upper parts of the sheet. The lower portion is a picrite, with abundant serpentinized olivine, large crystals of augite, and a considerable amount of ores. The upper portion, on the other hand, has plagioclase as its most abundant definite mineral, with a minor quantity of minute prisms of augite and of iron-ores, and scattered crystals of olivine. Here, within the compass of a few yards and in one continuous mass of rock, we have a transition from a variety of olivine-basalt into a picrite.

[445] Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 506.

The great majority of the puy lavas belong to the olivine-bearing series. A few of them are dolerites, but most are true basalts of the Dalmeny type, of which typical examples may be seen at the Kirkton quarries, Bathgate, and in the coast section between Pettycur and Kinghorn. Occasionally they present transitions towards picrite, as in the sheet overlying the lowest limestone at Kirkton, and in the lowest lava of King Alexander's Crag, Burntisland. These puy lavas exhibit considerable variety of structure as seen in the field. Some are solid, compact, black rocks, not infrequently columnar and weathering into spheroidal exfoliating forms. Others are somewhat granular in texture, acquiring green and brown tints by weathering, often showing amygdaloidal kernels, and even passing into well-marked amygdaloids. Many of them exhibit a slaggy structure at their upper and under surfaces (Figs. [153], [170], [171]). These external differences are an index to the corresponding variations in composition and microscopic structure enumerated in the foregoing tabular arrangement.

As a rule, the basic rocks which occur intrusively in connection with the puys, especially where they form a considerable mass, have assumed a much more coarsely crystalline texture than those of similar composition which have been poured out at the surface. They are generally dolerites rather than basalts. But with this obvious distinction, the two groups have so much in common, that the geologist who passes from the study of the subterranean phenomena of the Plateaux to that of the corresponding phenomena of the Puys is at once impressed with the close relationship between the material which, in the case of the puys, has consolidated above ground, and that which has been injected below. There is no such contrast between them, for example, as that between the basic and intermediate lavas of the plateaux and the more acid intrusions associated with them.

By far the largest number of the basic sills, bosses and dykes associated with the puys are somewhat coarsely crystalline dolerites without olivine. They include, however, olivine-dolerites and basalts, and even some extremely basic compounds. Of these last, a typical example is supplied by the now well-known picrite of Inchcolm, in the Firth of Forth, which occurs as an intrusive sheet among the Lower Carboniferous Sandstones.[446] In recent years one or two other picrite-sills have been observed in the same district. An interesting example has been described from a railway cutting between Edinburgh and Cramond where the rock invades and alters shales. More detailed reference to it will be made in the account of the sills connected with the puys. Another instance of the occurrence of this rock is in a railway cutting immediately to the west of Burntisland where it has been intruded among the Calciferous Sandstones below the Burdiehouse Limestone.

[446] Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 506. Teall, British Petrography, p. 94.

Rocks approaching limburgite occur among the sills and bosses which pierce the Carboniferous Limestone series of Fife between Cowdenbeath and Inverkeithing. One of these is found at Pitandrew, near Fordel Castle. Dr. Hatch observed that it consists of "numerous porphyritic crystals of olivine, with a few grains of augite and an occasional small lath-shaped crystal of felspar imbedded in a groundmass which is composed principally of idiomorphic augite microlites, small crystals of a brown mica, granules of magnetite and prisms of apatite. In addition, there is a considerable amount of interstitial matter, which is partly colourless glass, and partly shows a slight reaction between crossed nicols." Another example of the same type of rock occurs as a plug or boss in the tuff-vent of the Hill of Beath, and a further display of the limburgite type is to be seen in Dunearn Hill near Burntisland.

Although olivine-basalts of the Dalmeny type are most frequently met with as interstratified lavas, they also occur as bosses and sills. The typical example from Dalmeny is itself intrusive. Other illustrations are to be found in the Castle Rock of Edinburgh and in the sheets near Crossgates and Blairadam in Fife. The presence or absence of olivine, however, may sometimes be a mere accident of cooling or otherwise. I have shown that in the same mass of rock at Blackburn a gradation can be traced from a rock largely composed of altered olivine into one consisting mainly of felspar with but little olivine, and another example occurs in the picrite-sill between Edinburgh and Cramond. Dr. Stecher has ascertained that the marginal portions of the sills in the basin of the Firth of Forth, which cooled first and rapidly, and may be taken, therefore, to indicate the mineral composition of the rock at the time of extrusion, are often rich in olivine, while that mineral may be hardly or not at all discernible in the main body of the rock.[447]

[447] Dr. Stecher, Tschermak's Mineralog. Mittheil. vol. ix. (1887) p. 193. Proc. Roy. Soc. Edin. vol. xv. (1888) p. 162.

Of the ordinary and characteristic dolerites without olivine which constitute most of the intrusive masses, the various types enumerated in the tabular arrangement are abundantly developed in Central Scotland. Thus the normal ophitic type is displayed by the uppermost sill of the Burntisland series, and by the rock which forms the plug of the Binns Hill neck in Linlithgowshire. The Ratho type is well seen in the large sill at Ratho, likewise in the extensive intrusive sheets in the west of Linlithgowshire as at Muckraw and Carribber. The Burntisland sill type is shown by the lower sills of Burntisland and by some others in the same region, especially by that of Colinswell, and by another on the shore east from the Poorhouse, near Kinghorn. The great boss among the Bathgate Hills likewise displays it. The Bowden Hill type occurs in well-marked development at Bowden Hill, three miles south-west of Linlithgow, and in the massive sill at St. Margaret's, west from North Queensferry.

The non-olivine-bearing basalts are found in various bosses and sheets in the basin of the Firth of Forth. Thus the Binny Craig type occurs in the prominent and picturesque sill from which it is named, likewise among the intrusive sheets near Kirkcaldy, in Fife. Sometimes the same mass of rock displays more than one type of structure, as in the case of the great Galabraes neck among the Bathgate Hills wherein both the Tholeiite and Burntisland sill types may be recognized.

Some of the sills in West Lothian, as I pointed out many years ago, contain bitumen and give off a bituminous odour when freshly broken. They have been injected into bituminous shales or coal-seams.[448]

[448] Geol. Survey Memoir on Geology of Edinburgh (Sheet 32, Scotland), p. 46.

2. Andesites.—Rocks referable to this series appear to have been of rare occurrence among the puy-eruptions. Examples of them containing as much as 60 per cent of silica occur among the lavas of the Limerick basin. Some of the necks and what may be sills in the same district likewise consist of them.

3. Trachytes and Quartz-bearing Rocks.—Acid rocks, as I have already said, are extremely rare among the puy-eruptions. The only important examples known to me are those around the Limerick basin, where they rise apparently in old vents and form conspicuous rounded or conical hills. These rocks have been examined microscopically by Mr. W. W. Watts. One of the most interesting varieties, which occurs at the Standing Stone near Oola, was found by him to show quartz enclosing ophitically the felspars which, with well-terminated prisms, project into it. Further west, near Knockaunavoher, another boss occurs with conspicuous quartz. These rocks have much in common with trachytes but have a wholly crystalline structure. They will be described in the account of the Limerick basin.