WARWICKSHIRE

Some fifty miles to the north-east of the Malvern Hills, in the heart of the rich Midlands, and among the coal-fields and the New Red Sandstone to which these Midlands owe so much of their manufacturing industry and their agricultural fertility, another little tract of Cambrian rocks rises to the surface on the east side of the Warwickshire coal-field between Nuneaton and Atherstone. So unobtrusively do these ancient strata take their place among their younger peers, that their venerable antiquity was for a long time undetected.[125] They were actually regarded as parts of the Carboniferous series, which at first sight they seem to underlie conformably. It was not until 1882 that the mistake was corrected by Professor Lapworth, who proved the rocks to be Cambrian by finding undoubted Upper Cambrian fossils in them.[126] Subsequent investigation enabled him to work out the detailed sequence of these strata. He found that the supposed "Millstone Grit" is a thick-bedded quartzite perhaps 1000 feet in thickness, and resembling the well-known quartzites of the Lickey and Caer Caradoc. The "Coal-shales" proved to be a series (possibly 2000 feet thick) of purple, green, grey and black shales, which from their fossils could be paralleled with the dark shales of the Upper Cambrian series of the Malvern Hills.[127] These shales are immediately overlain by the Coal-measures.

[125] Their antiquity was recognized by Yates as far back as 1825 (Trans. Geol. Soc. 2nd series, vol. ii. p. 261). They had been confounded with "Millstone Grit" and "Coal-shale" by Conybeare and Phillips, and this mistake was adopted on the maps and memoirs of the Geological Survey.

[126] Geol. Mag. (1882), p. 563.

[127] Op. cit. (1886), p. 319.

For our present inquiry, however, the chief feature of interest in these discoveries is the recognition of a group of volcanic rocks underneath the quartzite. This group was named the "Caldecote Volcanic Rocks" by Professor Lapworth, who first recognized its nature and relations. Its rocks have been studied by Mr. T. H. Waller[128] and Mr. F. Rutley,[129] and have been traced upon a revised edition of the Geological Survey map by Mr. A. Strahan.[130] They consist of a thin series of well-stratified tuffs apparently derived from andesitic lavas. Their base is not seen owing to the fault which brings down the New Red Sandstone against them. They are surmounted by the quartzite, which at its base is conglomeratic and contains blocks of the tuff. A mass of quartz-felsite is possibly intrusive in these strata, and is associated with a diabase-porphyrite. In these rocks, but still more in the shales which overlie them, numerous sills of diorite and diabase occur. The total thickness of rocks from the lowest visible part of the Caldecote volcanic series to the base of the Coal-measures is probably between 2000 and 3000 feet.

[128] Op. cit. p. 323.

[129] Op. cit. p. 557.

[130] Geol. Mag. (1886), p. 540. In this paper full references will be found to the previous papers on the geology of the district. Jukes had recognized that the rocks below the coal-bearing strata were "older than the Upper Silurian, perhaps older than any Silurian," Mem. Geol. Survey, "South Staffordshire Coal-field" (1859), p. 134.

There can be no doubt as to the geological position of the dark fossiliferous shales and their underlying quartzite. The fact that the basement conglomerate of the quartzite is partly made up of the underlying volcanic series may possibly mark a wide difference of age between them, and may indicate that the eruption of the tuffs took place long before Upper Cambrian time. On the other hand, the tuffs have the same strike and angle of dip with the quartzite, and as Professor Lapworth admits, the break between them may not be of great moment. It is at least certain that the intrusive sills of the district are later than the tuffs, and later also than the sedimentary Cambrian groups.

BOOK IV
THE SILURIAN VOLCANOES

CHAPTER XII
CHARACTERS OF THE SILURIAN SYSTEM IN BRITAIN. THE ARENIG VOLCANOES

The Land and Sea of Silurian time—Classification of the Silurian System—General Petrography of the Silurian Volcanic Rocks—I. The Eruptions of Arenig Age.

The next great geological period, to which Murchison gave the name of Silurian, has in Britain a fuller record than the period which preceded it. The rocks that tell its history are more varied in origin and structure. They are displayed at the surface over a far wider area, and, what gives them special interest and value, they contain a much larger assemblage of organic remains. For the immediate subject of the present volume, they have likewise the additional attraction that they include a singularly complete and widespread volcanic chronicle. They display in many admirable sections the piled-up lavas and tuffs of scores of volcanoes, scattered all over the three kingdoms, from the headlands of Kerry to the hills of Lammermuir. They thus enable us to form a truer conception of what the early Palæozoic volcanoes were than is possible from the more limited evidence furnished by the Cambrian system.

At the beginning of the Silurian period most of the area of the British Isles lay under the sea. But if we may judge from the sedimentary strata which represent the floor of that sea, the water, during most of the time, was of no great depth. There is evidence, indeed, that during a part of the period the sea was deep enough to admit of the accumulation of wide tracts of radiolarian ooze, with but little admixture of mechanical sediment. But, for the most part, sand and mud were drifted from neighbouring lands, the more important of which probably lay to the north, over what are now the Highlands of Scotland and the north and north-west districts of Ireland. No general change in topography or in physical conditions took place at the close of Cambrian time. The older era glided insensibly into the newer, unmarked by any such catastrophe as was once supposed to have intervened at the end of each great geological period. There are traces, indeed, of slight local disturbances, but these only make the general gradual transition more marked.

Of the vegetation which covered the Silurian lands hardly anything is known. Traces of lycopods and ferns have been detected, and these probably formed the chief constituents in what must have been rather a sombre and monotonous flora. The character of the terrestrial fauna is still hidden from us, though we do know that insects winged their way through those green flowerless forests, and that scorpions likewise harboured there. That these primeval arachnoids were air-breathers is shown by their breathing stigmata; and from the fact that they possessed a well-developed poison-gland and sting, we may believe that there were already living at the same time other land-animals, possibly of higher grade, on which they preyed. But of these ancestral types no actual relics have yet been discovered.

It is the life of the sea-floor that has mainly been chronicled among the sedimentary formations. Taking the Silurian system as a whole, we find it to be the repository of a remarkably varied assemblage of organisms. Among the simpler forms, Radiolaria deserve especial notice, from their wide range in space and time, and the comparative indestructibility of the highly-siliceous, fine-grained, flinty strata, which have preserved them in abundance and have a wide distribution over the British Isles. The Graptolites, so specially characteristic of the system, range entirely through it, and by their successive differences of specific and generic forms, furnish a basis for the division of the whole series of rocks into more or less definite stratigraphical zones. Hardly less important for purposes of correlation are the Trilobites which in the Silurian period reached the culmination of their development in regard to number of species and genera. These interesting extinct types of crustacean life must have swarmed over some parts of the sea-bottom, for their remains abound in its hardened silts. The Brachiopods are likewise numerously represented among Silurian strata; and since the vertical range of the species is generally not great, they serve as useful guides in fixing stratigraphical horizons. Lamellibranchs, Gasteropods, and Cephalopods become increasingly numerous and varied as we follow the succession of strata from the base to the summit of the Silurian system. That there were fishes also in the Silurian seas is proved by the occurrence of their remains, more particularly in the higher formations.

From the organic remains which have been preserved in the rocks, it may be inferred that the animal life of the globe became more varied in Silurian time; higher types made their appearance, until vertebrates took the place of pre-eminence which they have ever since maintained.

The volcanic activity that had marked the passage of Cambrian time in Britain was prolonged into the Silurian period. In North Wales, indeed, it is clear that though the eruptions began in the earlier era of geological history they continued to be comparatively feeble until they broke out into full activity in the succeeding epoch. There is no hiatus or essential difference between the volcanic phenomena, any more than there is between the sedimentary deposits, of the two periods.

Although it may be only owing to the fact that the Silurian formations come much more extensively to the surface of the land than the underlying Cambrian are permitted to do, yet it is at least noteworthy that the relics of Silurian volcanoes are spread over a far wider area of the British Isles than those of the earlier period. Throughout a large part of Wales they form some of the most prominent mountains, such as Cader Idris, the Arans, Arenig Fawr, Moel Wyn, Moel Siabod, and Snowdon. They rise into the picturesque hill-groups of the Lake District, they appear at many detached places throughout the south of Scotland, and form conspicuous eminences in Carrick. In Ireland they abound all down the east side of the island, and even reappear on the far western headlands of the Dingle coast-line.

To the same pioneers, by whom the foundations of our knowledge of the Cambrian volcanoes were laid, we are indebted for the first broad outlines of the history of volcanic action in Silurian time. The writings of Sedgwick and Murchison, but still more the detailed mapping of De la Beche, Ramsay, Selwyn, Jukes, and the other members of the Geological Survey, have given to the Silurian volcanic rocks of Wales a classic interest in the history of geology. To these labours further reference will be made in subsequent pages.[131]

[131] For references to the older literature see ante, [p. 142].

The amount of material being so ample for the compilation of a record of volcanic action in Britain during Silurian time, it will be desirable to arrange it in stratigraphical order. For this purpose invaluable assistance is afforded by the evidence of organic remains, whereby the whole Silurian system has been subdivided into sections, each characterized throughout the whole region by certain distinctive fossils. The following tabular statement exhibits the chief stratigraphical divisions of the system, and the short black lines in it mark the positions of separate volcanic platforms in each of the three kingdoms:—

EnglandWalesScotlandIreland
Upper SilurianLudlow Group............
Wenlock Group.........---
Llandovery Group?.........
Lower SilurianBala and Caradoc Group------------
Llandeilo Group------------
Arenig Group------------

It will be most convenient, following the combined stratigraphical and geographical arrangement of this table, to discuss first the volcanic history of the Lower Silurian period as recorded in each of the three kingdoms, and then that of the Upper Silurian.