[87] In some recent borings around Hartlepool the Magnesian Limestone has been found to be interstratified with thick bands of gypsum and anhydrite, and to be overlain by more than 250 feet of the latter substance. Nothing could show more forcibly the exceedingly saline and insalubrious character of the Permian lakes or inland seas.
The vegetation of the land surrounding these basins was still essentially Palæozoic in character. It presented a general resemblance to that of Carboniferous time, but with some notable differences. The jungles of Sigillaria seem to have disappeared, while on the other hand, conifers increased in number and variety. The sediments of the water-basins have handed down only a scanty remnant of the animal life of the time. Along the sandy shores walked various amphibians which have left their footprints on the sand. A few genera of ganoid fishes have been found in some of the shales, and a comparatively poor assemblage of crinoids and molluscs has been obtained from the Magnesian Limestone. To the geological period distinguished by these geographical and biological characters the name of Permian is assigned.
In his survey of the progress of volcanic history in the area of Britain, the geologist finds that the long period of quiescence indicated by the deposition of the Coal-measures, and probably also by the unconformability between the Coal-measures and the Permian formations, was at length terminated by a renewed volcanic outbreak, but on a singularly diminished scale and for a comparatively brief period of time. Whether, had the Permo-Carboniferous strata which connect the Coal-measures with the Permian formations on the Continent been found in this country, they would have filled up the gap in the geological record, and would have supplied any trace of contemporaneous volcanic action, cannot even be surmised. All that we know is that, after a vast interval, and during the deposition of the breccias and red sandstones which unconformably overlie the Coal-measures, a few scattered groups of little volcanoes appeared in the area of the British Isles.
It is unfortunate that in those districts where these volcanic relics have been preserved, the stratigraphical record is singularly imperfect, and that on the eastern side of England, where this record is tolerably complete, there are no intercalated volcanic rocks. The latter occur in tracts where the strata are almost wholly destitute of fossils, and where therefore no palæontological evidence is available definitely to fix the geological age of the eruptions. Nevertheless there is usually ample proof that the strata in question are much later than the Coal-measures, while their geological position and lithological characters link them with the undoubted Permian series of the north-east of England. They may, however, belong to a comparatively late part of the Permian period, if indeed some of them may not be referable to the succeeding or Triassic period.
The comparatively feeble and short-lived volcanoes now to be described are found in two regions wide apart from each other. The more important of these lies in the south-west and centre of Scotland. A second group rose in Devonshire. It is possible that a third group appeared between these two regions, somewhere in the midlands. The evidence for the history of each area will be given in a separate section in the following pages.
i. GENERAL CHARACTERISTICS—NATURE OF MATERIALS ERUPTED
The chief district for the display of volcanic eruptions that may be assigned to the Permian period lies in the centre of Ayrshire and the valleys of the Nith and Annan. But, for reasons stated below, I shall include within the same volcanic province a large part of the eastern half of the basin of the Firth of Forth (see Map V.).
Unfortunately the interesting volcanic rocks now to be considered have suffered severely from the effects of denudation. They have been entirely removed from wide tracts over which they almost certainly once extended. But this enormous waste has not been wholly without compensations. The lavas and tuffs ejected at the surface, and once widely spread over it, during the deposition of the red sandstones, have been reduced to merely a few detached fragments. But, on the other hand, their removal as a superficial covering has revealed the vents of discharge to an extent unequalled in any older geological system, even among the puys of the Carboniferous period. The Permian rocks, escaping the effects of those great earth-movements which dislocated, plicated and buried the older Palæozoic systems of deposits, still remain for the most part approximately horizontal or only gently inclined. They have thus been more liable to complete removal from wide tracts of country than older formations which have been protected by having large portions of their mass carried down by extensive faults and synclinal folds, and by being buried under later sedimentary accumulations. We ought not, therefore, to judge of the extent of the volcanic discharges during Permian time merely from the small patches of lava and tuff which have survived in one or two districts, but rather from the number, size and distribution of the vents which the work of denudation has laid bare.
The evidence for the geological age of the volcanic series now to be described is less direct and obvious than most of that with which I have been hitherto dealing. It consists of two kinds. (a) In the first of these comes the series of lavas and tuffs just referred to as regularly interstratified with the red sandstones, which, on the grounds given in the next paragraph, it is agreed to regard as Permian. (b) Connected with these rocks are necks which obviously served as vents for the discharge of the volcanic materials. They pierce not only the Coal-measures, but even parts of the overlying bedded lavas. So far there is not much room for difference of opinion; but as we recede northward from Ayrshire and Nithsdale, where the intercalation of the volcanic series in the red sandstones is well displayed, we enter extensive tracts where these interstratified rocks have disappeared and only the necks remain. All that can be positively asserted regarding the age of these necks is that they must be later than the rocks which they pierce. But we may inferentially connect them with the interstratified lavas and tuffs by showing that they can be followed continuously outward from the latter as one prolonged group, having the same distribution, structure and composition, and that here and there they rise through the very highest part of the Coal-measures. It is by reasoning of this kind that I include, as not improbably relics of Permian volcanoes, a large number of vents scattered over the centre of Scotland, in the East of Fife.
The red sandstones among which the volcanic series is intercalated cover several detached areas in Ayrshire and Dumfriesshire. Lithologically they present a close resemblance to the Penrith sandstone and breccias of Cumberland, the Permian age of which is generally admitted. They lie unconformably sometimes on Lower and Upper Silurian rocks, sometimes on the lower parts of the Carboniferous system, and sometimes on the red sandstones which form the highest subdivision of that system. They are thus not only younger than the latest Carboniferous strata, but are separated from them by the interval represented by the unconformability. On these grounds they are naturally looked upon as not older than the Permian period. The only palæontological evidence yet obtained from them in Scotland is that furnished by the well-known footprints of Annandale, which indicate the existence of early forms of amphibians or reptiles during the time of the deposition of the red sand. The precise zoological grade of these animals, however, has never yet been determined, so that they furnish little help towards fixing the stratigraphical position of the red rocks in which the footprints occur.