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

by Archibald Geikie

[174] Geol. Mag. 1889, p. 22.

Llandovery.		Pentamerus grit. Conglomerate (Mulloch Hill).
Caradoc.		Shales, sandstones, grits, etc. (Ardmillan, Balcletchie).
		Thick conglomerate (Byne Hill, Bennane, etc.).
		Thick fossiliferous limestone (Stinchar, Girvan). (On this horizon come the perlitic felsites and soda-felsites of Winkstone and Wrae.)
		Sandstone (Orthis confinis) passing down into thick conglomerate.
[Unconformability.]
Upper Llandeilo.		Green mudstones, grits and greywackes.
		Thin band of dark mudstone with Upper Llandeilo graptolites.
Arenig and Lower and Middle Llandeilo.		Group of Radiolarian cherts (about 70 feet) with alternating tuffs.
		Tuff or volcanic conglomerate, with occasional lava-flows.
		Black shale (10 feet) with Arenig graptolites.
		Volcanic breccias around local centres (Knockdolian, etc.).
		Thick group of porphyrite and diabase lavas.
		Red flinty mudstones with Arenig graptolites.
		Porphyrites, etc.
		Fine tuffs, etc., with Lower Arenig fossils.
		Diabase lavas, etc. (base not seen).

It will be noticed from this table that the bottom of the volcanic series is not reached, so that no estimate can be formed of its full thickness, nor on what geological platform it begins. Possibly its visible portions represent merely the closing scenes of a long volcanic history, which, over the area of the south of Scotland, extended into Cambrian time, like the contemporary series of Cader Idris.

Among the lowest lavas there are interstratified courses of fine tuffs, flinty shales and thin limestones, which sometimes fill in the hollows between the pillow-like blocks above referred to. Among the characteristic Lower Arenig graptolites of these intercalated layers are Tetragraptus bryonoides, T. fruticosus, T. quadribrachiatus, and T. Headi together with Caryocaris Wrightii. Considerable variation is to be seen in the development of the upper part of the volcanic series. In some places the lavas ascend almost to the top; in others, thick masses of breccia or agglomerate take their place. These fragmentary materials are locally developed round particular centres, which probably lie near the sites of active vents whence large quantities of pyroclastic material were discharged. One of the volcanic centres must have been situated close to the position of Knockdolian Hill already referred to. The exceedingly coarse breccia of that eminence is rudely stratified in alternations of coarser and finer material, which was probably to some extent assorted under water around the cinder-cone that discharged it. The date of the explosions of this hill has been ascertained by Mr. Peach from the intercalation of black shales containing Arenig graptolites among the breccias. Another vent lay somewhere in the immediate neighbourhood of the Mains Hill agglomerate, if not actually on part of the site of that rock. Though probably not more than a mile from the Knockdolian volcano, and belonging to the same epoch of eruption, this vent, to judge from the peculiarities of its ejected material, must have been quite distinct in its source. A third vent lay somewhere in the immediate vicinity of Bennane Head, and threw out the extraordinary masses of agglomerate and the sheets of lava seen on the coast at that locality. A fourth may be traced by its separate group of fine tuffs on the coast three miles south of Ballantrae.

Fig. 53.—Section across the Lower Silurian volcanic series in the south of Ayrshire (B. N. Peach).
B, Interstratified lavas in Arenig group; t, tuffs; r, radolarian cherts; b², Llandeilo group; b³ Caradoc group. Σ, Serpentine. G, Gabbro.

A feature of singular interest in the material erupted from these various centres of activity consists in the evidence that the explosions occurred at intervals during the deposition of the Lower Silurian formations, and that these formations were successively disrupted by submarine explosions. Mr. Peach has found, for example, abundant pieces of the peculiar and easily recognized radiolarian cherts imbedded in the volcanic series. That these cherts were deposited contemporaneously with the volcanic eruptions is proved by their intercalation among the breccias. Yet among these very breccias lie abundant fragments of chert which must have already solidified before disruption. It is thus evident that this siliceous ooze not only accumulated but set into solid stone on the sea-floor, between periods of volcanic outburst, and that such an occurrence took place several times in succession over the same area.

These facts derive further interest from the organic origin of the chert. It is now some years since Mr. Peach and his colleagues observed that between the Glenkiln Shale with its Upper Llandeilo graptolites and the top of the volcanic group in the central part of the Silurian uplands, alternations of green, grey or red shaly mudstones and flinty greywackes are interleaved with fine tuffs, and are specially marked by the occurrence in them of nodules and bands of black, grey and reddish chert. This latter substance, on being submitted to Dr. Hinde, was found by him to yield twenty-three new species of Radiolaria belonging to twelve genera, of which half are new. It thus appears that during the volcanic activity there must have been intervals of such quiescence, and such slow, tranquil sedimentation in clear, perhaps moderately deep water, that a true radiolarian ooze gathered over the sea-bottom.[175]

[175] Ann. Mag. Nat. Hist. (1890), 6th ser. vi. p. 40.

That the deposition of this ooze probably occupied a prolonged lapse of time seems clearly indicated by the evidence of the fossils that occur below and above the cherts. The graptolites underneath indicate a horizon in the Middle Arenig group, those overlying the cherts are unmistakably Upper Llandeilo. Thus the great depth of strata which elsewhere constitute the Upper Arenig and Lower and Middle Llandeilo subdivisions is here represented by only some 60 or 70 feet of radiolarian cherts. These fine siliceous, organic sediments probably accumulated with extreme slowness in a sea of some depth and over a part of the sea-floor which lay outside the area of the transport and deposit of the land-derived sediment of the time.[176]