We have here an intercalated group of strata upwards of 40 feet thick, consisting partly of tuffs and partly of fine clays, which may either have been derived from volcanic explosions or from the atmospheric disintegration of basaltic lavas. Through some of these strata abundant carbonaceous streaks and other traces of plants are distributed, while among them lies a band almost wholly composed of compressed vegetation. Unfortunately none of the strata at this locality seem to have preserved the plant-remains with sufficient definiteness for identification. There can be no doubt, however, that they were terrestrial forms like those of Mull and Antrim.
This coal, with its accompanying sedimentary deposits, has been traced through Suderö, and another outcrop, possibly of the same horizon, occurs on Myggenaes, the extreme western member of the group of islands, at a distance of some 40 miles.[266]
[266] See in particular Prof. J. Geikie, Trans. Roy. Soc. Edin. vol. xxx. (1880), p. 229.
CHAPTER XL
THE MODERN VOLCANOES OF ICELAND AS ILLUSTRATIVE OF THE TERTIARY VOLCANIC HISTORY OF NORTH-WESTERN EUROPE
From the facts stated in the foregoing chapters concerning the structure of the basalt-plateaux of North-Western Europe, it is evident that in none of these areas have the eruptions come from one great central volcano like Etna or Vesuvius. On the contrary, in every instance there is abundant evidence that the basalt has flowed from many scattered points of eruption. The uniformity of the lava-sheets in petrographical characters, their continuity when viewed in mass, their general horizontality, and their constant thinning away in different directions, show that the eruptive vents must have been distributed over the whole plateau-areas.
The conditions under which such eruptions took place can be most readily understood by a comparison of the phenomena with those observable in modern volcanic tracts where extensive outflows of lava have taken place without the existence of any great central cones. Of these regions the most instructive is undoubtedly to be found among the recent lava-deserts of Iceland. There the parallels to the structures described from the British and Faroe plateaux are so numerous and so close that an account of the Icelandic region may appropriately be inserted here.
The evidence furnished by Iceland is of special value in our present enquiry, inasmuch as that island, besides its modern eruptions, includes vast basaltic plateaux of Tertiary age. These areas of nearly level sheets of basalt belong to the same geological period as those of the British and Faroe Islands, and display the same internal structure and external features. But they have this distinguishing peculiarity that the volcanic fires beneath them are not yet extinguished. They have been broken through again and again in recent times by volcanic eruptions which have repeated many of the characteristics of their Tertiary predecessors. The old and the new development of the same volcanic type are thus visible side by side.
The Tertiary volcanic series of Iceland reaches a thickness of upwards of 3000 metres, or nearly 10,000 English feet, but as its base is nowhere seen, it may be still thicker. Its successive sheets, piled over each other in parallel layers, form terraced hills and bold escarpments along the coast, whence they slope gently inland. The plateau, as in the Faroe Islands and in Scotland, has been extensively eroded, and has been trenched by many long valleys and fjords The composition of the basalts remains remarkably uniform over the island. The lava sheets are often decomposing, amygdaloidal, and filled with zeolites; while higher in the series compact basalts abound, the uppermost fine-grained sheets being especially constant in structure and composition. Numerous dykes traverse the plateau, and some of them cut even its highest members. The parallel with the geological structure of the Inner Hebrides is continued in Iceland by the appearance of intrusive masses of gabbro and granophyre, which represent the deeper parts of the Tertiary volcanic series, while the basalts were poured out at the surface. Thus, at Papafjord, the gabbro rises into mountainous peaks and, like the similar rock in Mull and Skye, is intersected by dykes of a coarse-grained granitoid liparite or granophyre. Large dykes and ramifying veins of the same acid material, often with a thoroughly granitic aspect, extend into the basalts.[267]
[267] Mr. Thoroddsen, Dansk. Geografisk Tidsskrift, vol. xiii.
A long series of eruptions has taken place in Iceland since the Glacial Period. There were likewise pre-glacial eruptions. The glaciated lava-streams are found underneath the modern lavas. So far indeed as is known, no evidence exists of any important cessation of subterranean activity there since Tertiary time.[268] The existing volcanic phenomena may with probability be regarded as the survival of those which were so widely manifested over the Icelandic area and the north-west of Europe in the older Tertiary ages. A careful study of them may therefore be expected to throw light on the history of the Tertiary basaltic plateaux; while, on the other hand, the thorough dissection of these plateaux by the denuding agencies will not improbably be found to explain some parts of the subterranean mechanism of the modern Icelandic volcanoes.