CHAPTER XVII.

VOLCANOS OF ICELAND.

Other indications of volcanic action on the confines of the Atlantic Ocean

in parts appertaining to Europe - Ireland-Scotland-Hebrides—Faroe. Iceland. The latter island particularly described—its general structure-direction of its active vents-classification of its volcanic products-cavernous lava.-Active volcanos—their number and position in the south of the island.- Eruption of 1785—1846.-Volcanos in the north.-Character of the Iceland lavas.--Sulphur mountains.—Geyser springs—their constituents—their high temperature.—Minerals of Iceland.-Surturbrand. Island of Jan Mayen.

In the preceding chapters, the volcanic phænomena which were brought before the notice of my readers existed for the most part in countries which had been visited by the author; so that although a large proportion of the facts related do not claim to be the result of original observation, yet it has at least been in my power in most cases to appreciate the value of the information, in consequence of having compared it with my own recollections of the region referred to.

In the remaining portions of my work I shall not have the same advantages, and accordingly, if the descriptions introduced are able to put forth, with any kind of justice, higher pretensions than those belonging to a mere compilation, they must derive them from that general familiarity with the subject, which may have enabled the author to select from the mass of materials before him the parts best calculated to enlighten us with regard to the real nature of the operations from which they arise, and not from any personal acquaintance possessed by him with the localities themselves.

In pointing out then the parts of Europe in which volcanic products exist, not yet brought before our notice, there will be a natural propriety in alluding first to those placed within the verge of that Ocean, the confines of which, in treating of Portugal, we had already reached; for although Ireland, the Hebrides, the Faroe Islands, and Iceland, are now separated one from the other, as well as from the Iberian Peninsula, by vast tracts of sea, still the late researches of Professor Edward Forbes * have rendered it probable, that they were once connected by a continuous tract of land, ranging from the Azores, along the line of that belt of Gulf-weed which exists between the fifteenth and forty-fifth degrees of north latitude.

It is not my purpose however to treat of the geological structure of any portion either of Great Britain or of Ireland, first, because the details are already before the world in treatises readily accessible to the English public t, and secondly, because the volcanic products seem in these regions mostly submarine, and are apparently in no cases of more modern date than the age of the chalk.

In accordance indeed with this great antiquity, and with the almost total cessation of volcanic action in the country subsequently (unless indeed the slight earthquake-shocks perceived at Cumrie be allowed to establish the contrary), we observe throughout these districts an entire absence of thermal springs, as well as of those other minor exhibitions of igneous action, which occur in most other localities, where equally wide-spreading manifestations of the same forces have taken place.

It is true that many of the basalts which I have noticed as occurring in Germany were similarly circumstanced, if we may judge by their characters and structure, but then they are associated with other igneous products more nearly approaching in these respects to those produced under actual circumstances, and it would have been difficult to have described the latter without introducing some notice of the first.

In Ireland, on the other hand, as well as in the Hebrides, we have an example of volcanos, which during the whole of the extended period of time embraced within the tertiary epoch, no less than within the compass of historical times, have given no token of vitality-a circumstance, as it appears to me, more reconcilable with that theory which attri

* Memoirs of the Geological Survey of Great Britain, vol. i.

+ Macculloch's Western Islands-Jameson's Mineralogical Travels— Berger on the Geological Features of the North-Eastern counties of Ireland, with an Introduction and Remarks by the Rev. W. ConybeareDescriptive Notes referring to an outline of Sections of the same Coast, by Messrs. Conybeare and Buckland, &c.

butes volcanic action to certain chemical processes taking place within the interior of the earth, than to the idea of its arising merely from the contraction of the crust upon its fluid contents, which latter being inexhaustible, ought, it should seem, according to this hypothesis, to be protruded periodically, and to afford a fountain of igneous matter as unfailing as the source from which it proceeded.

The same reasons for omission exist with respect to the Faroe Islands likewise, which consist, as it would appear, entirely of compact and amygdaloidal trap, neither of which can be referred to subaërial volcanos *.

The only rock which forms an exception to that igneous origin which we now are all agreed to ascribe to basaltic rocks, is the coal of the island of Suderoe, which, like that lately discovered in Kerguelen's Island, and the vein met with at Ballinty on the coast of Antrim, is imbedded in trap.

And with the idea that the whole had been erupted at the bottom of a deep sea and subsequently upheaved, the only fact at variance, seems to be the occurrence of an amygdaloid having its upper surface filled with small insulated perpendicular cavities, as if caused by the escape of a gaseous fluid whilst the rock was in a soft pasty condition—a fact analogous to what I have noticed in my account of the volcanic rocks near Frankfort (p. 105).

I shall proceed then to Iceland, where volcanic operations have been carried on on a more gigantic scale perhaps than in any other part of Europe; for although there be no one mountain in this island which rivals Mount Etna in magnitude and height, yet evidences of igneous action pervade a much larger area than in Sicily, and have generated in the course of time a much greater amount of volcanic products.

Indeed, whilst the utmost length of Sicily is about 100 miles from Messina to Cape Passero, and its breadth 150 from Messina to Trapani, Iceland measures at least 240 miles from its most northern to its most southern point, and as much from east to west; and whilst of the former island not a tenth of the surface is volcanic, the whole of the latter is derived from igneous operations either of an early or of a recent date.

According to Krug von Nidda *, one of the latest geological travellers who have visited this island, the whole surface, embracing an area of 1800 square miles, presents only two principal rock-formations, one seeming to occupy the bottom of that northern ocean out of which the islands of Iceland and Faroe have risen, and consisting of trap rocks of the ordinary kind; whilst the other, which forms the nucleus of the former island, and may be regarded as the principal cause of its existence as an upraised tract of land, is trachyte, with its accompaniments of tuffs and lava currents. If, as Krug von Nidda thinks, there are any Neptunian beds in the island, they are at least so metamorphosed by the action of heat as to put on the characters of an indurated tuff or obsidian. The trachyte traverses the island in a broad band from S.W. to N.E., and has produced in the line of its elevation an immense fissure, along the sides of which the accompanying traps are seen to be upheaved.

Accordingly the active volcanos of Iceland are placed, if we believe this writer, on the borders of the above fissure, or at the junction of the trachyte with the trap.

The coasts of Iceland, as may be seen by the map, are intersected by deep gulfs, caused, as this geologist supposes, by that rupture of the strata which was brought about by the elevation of the trachyte and the contiguous trap. These gulfs, which are seldom more than half a mile in width, extend high up into the mountains flanked by precipitous and overhanging cliffs.

The arrangement of the beds of trap is always nearly parallel to the horizon, but there is a slight inclination towards the interior of the island, or towards the supposed trachytic nucleus. This, which is the very reverse of what might have been expected, is explained by the giving way of the older rock along the line of fracture.

Such is the view of the general structure of Iceland taken by the author above-quoted; but a more recent geologist, M. Robert, who accompanied the scientific expedition des

Karsten's Annalen, vol. ix., partly translated in Jameson's Journal for April 1837.

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patched by the French Government to Iceland and Greenland, under the direction of M. Gaimard, disputes the general occurrence of trachyte throughout the island, and the linear arrangement represented as belonging to it, maintaining that in some instances M. Krug von Nidda has mistaken for trachyte some other rock, as at Smiorfäll, a lofty mountain which consists in reality not of trachyte, but of basanites * and mimosites having a porphyritic character.

Be that as it may, there can be no doubt that Krug von Nidda is correct in assigning to the volcanic operations now proceeding in Iceland a direction from N.E. to S.W., bounded on the west by the course of the Huitaa, and extending from Faxifiord on the south to Eyafiord on the north, and having for its eastern boundary Oræfa-jökull and the valley of Langor Fleot to its termination in the sea on the coast in the pro vince of Mulé Syssel.

Within this area all the volcanic action now going on in Iceland is circumscribed, whilst the country to the right and left of it consists of trap rocks indeed, but not of modern volcanic products.

We may therefore revert to the distinctions laid down many years ago by Sir George Mackenzie as substantially correct, there being nothing to controvert his views with respect to the existence in Iceland of two classes of volcanic products, submarine and terrestrial, although there may be reasons for differing from that writer as to the particular rocks which he has assigned to the one or to the other of these divisions.

Thus, whilst he refers the trap rocks which constitute so large a portion of the surface of Iceland to eruptions which had taken place under the pressure of the ocean, he distinguishes between the latter and those lavas which he calls submarine, but which appear to include many cellular and even vitreous rocks, such as pearlstones, pumices, and even obsidian,

Much as we are indebted to this author for the clear views he had the merit at this early period of promulgating with

* Basanite, according to Cordier's nomenclature, is a mixture of augite and felspar, with distinct crystals of augite imbedded. Mimosite is also a mixture of felspar, augite and titaniferous iron, with a granitic structure.

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