We will now pass on to the volcanic rocks of which Iceland is constituted. The foundation of Iceland is palagonitic tuff of sub-aqueous origin, disturbed and at times metamorphosed by enormous masses of amygdaloidal basaltic lava whose cavities abound with zeolites, being traversed by dykes and layers of interjected basaltic and trachytic lava at all times dislocated and confused by the various earthquakes which from time to time have shaken Iceland to its nethermost stone. These rocks are overlaid by lava streams of sub-ærial origin, pumiceous tuffs and agglomerates that have been formed by debacles and atmospheric influences. Concerning the strike and dip of the various layers of trap and basalt there is no general inclination, no uniformity—all is confusion. The loose soil of Iceland is entirely composed of disintegrated and decomposed volcanic rocks and decayed vegetable matter, and would be very fruitful if it were in a lower latitude. The vast period of time which it must have taken to decompose the huge lava streams that we find almost entirely converted into humus may be appreciated when we look upon pre-historic lava fields, grey with lichens, like that of Thingvellir, while the actual decomposition of its surface scarcely amounts to half-an-inch. We may divide the lavas of Iceland, like those of most other volcanic districts, into two classes; first, the basalts passing into dolerites, and secondly, the trachytic lavas. The more ancient basalts occur most frequently as intruded masses of amygdaloidal character; the doleritic lavas of Iceland are the more recent products of its volcanoes, varying only in this respect, that the earlier erupted lavas contain crystals of olivine, in addition to the felspar and augite which occur in most of the lavas of our own time.
Trachytic lava occurs but sparingly in the parts of Iceland that I have visited. It has for a long time been assumed that a trachytic band was disposed upon a fissure which bisected Iceland from N.E. to S.W., namely from Cape Langanes to Reykjanes upon which the principal centres of eruption were supposed to be situated. This, however, is a presumption unwarranted by investigation. A glance at the map will show us that there are many other centres of volcanic activity which do not occur in this imaginary trachytic band. True most of the more recently active volcanoes occur upon this rectilinear, but there are Myrdals Jökull, Eyjafjalla and Örœfa Jökull, all volcanoes that have erupted comparatively recently, and a host of more ancient volcanoes distributed over other portions of the island, which might lead us to surmise that there were a dozen instead of one great fissure in the superficial rocks of Iceland.
Trachytes, principally I believe in an altered condition, have been found around and between Hekla and the Geysers, and notably at the volcano of Rauðarkamb. I was informed, however, that we must look for the greater part of the trachyte of Iceland other than in a pumiceous form upon the peninsulars of Snæfells Jökull. Certainly I found that trachytic lava almost died out upon the north side of the Vatna Jökull, or else it is so covered up with recent volcanic productions as to be undiscernible. The obsidians of Iceland, which are found so universally distributed in fragmentary forms upon the sides of the volcanoes are seldom to be met with in situ, indeed the only instance that I have met with of obsidian in situ was at Mount Paul, in the heart of the Vatna Jökull. That mountain, as we have already seen, is entirely composed of obsidian, varying from the vitreous to the grey stony variety.
The obsidians of Iceland seldom contain the beautiful felspar crystals, so characteristic of the Arran pitch-stones, but some of them are of a porphyritic nature, showing under the microscope crystals of quartz much fissured and split about, no doubt during the process of cooling. We must also regard the greater part of the pumice which was ejected last year from the Öskjugjá as an obsidian, in spite of its remarkably vesicular character. The fine dust which was carried to Norway during the eruption of last Easter-day resembled powdered glass, and led geologists there to come to the conclusion that the mountain which was erupting must have been pouring out great quantities of obsidian. As compared with the lavas of Vesuvius, I cannot help suggesting that many of the more ancient lavas in both instances are of a more trachytic and porphyritic character. In the Vesuvian lavas especially, the crystals contained by the older rocks have crystallized out of the uncrystalline or semi-crystalline mass. A prevalent mineral in the older Vesuvian lavas is leucite, which corresponds to the olivine that occurs so frequently in the older erupted lavas of Iceland, while those minerals are seldom to be met with in the more recent lavas of either Iceland or Italy.
I must now bring these few pages to a close. I dare say they contain a great deal of what is not worth reading; but as they give the only account of the Vatna Jökull and the part of the Ódáðahraun which I traversed, I trust those that may take the trouble to read them, will accept them as the best and the most accurate account of those districts that I am able to give.
INDEX.
| PAGE. | |
| Agglomerates | [183] |
| Akreyri | [143] |
| Alderjufoss | [159] |
| Arnarfell-hið-Mikla | [166] |
| Ásberg | [136] |
| Askja, plain of | [86] |
| Birch-tree | [20] |
| Borðeyri | [138] |
| Breiðamerkr Glacier | [17], [24] |
| ” ” Sandr | [23] |
| Climate | [180] |
| Crater Lakes | [122] |
| Dettifoss | [119] |
| Dyngjufjöll | [83] |
| ” ascent of | [101] |
| Equipment | [28] |
| Eyrarbakki | [8] |
| Glaciers | [190] |
| Grafalandá River | [76] |
| Grímstaðir | [70] |
| Heiði | [11] |
| Hekla | [181] |
| Hengill, volcano of | [6] |
| Herðubreið | [65], [103] |
| Hot Springs | [185] |
| Hraun | [7] |
| Hrossaberg | [75] |
| Húsavík | [133] |
| Iron Pyrites, sublimation of | [174] |
| Jökulls | [186] |
| Jökulsá-á-fjöllum | [61] |
| Kaldbakkr | [14] |
| Kálfafellsfjall | [30] |
| Kálfafellstaðr | [25] |
| Kiðagil | [160] |
| Kistufell | [54] |
| Kötlugiá | [181] |
| Krafla | [123] |
| Lœkjarbotn | [5] |
| Lake Grœnavatn | [126] |
| ”Mývatn | [111] |
| Lavas of Iceland | [183], [193] |
| ” ”Mývatns Orœfí | [108], [155] |
| Laxá River | [131] |
| Lindá River | [77] |
| Lómagnúpar | [19] |
| Mount Paul | [36] |
| Mud-pools | [115] |
| Mývatns Orœfí, eruption of | [146] |
| Námufjall | [113] |
| Núpstað | [15] |
| Núpstaða-skógr | [21] |
| Núpsvatn River | [18] |
| Obsidian | [36], [124] |
| Ódáðahraun | [95] |
| Ölfusá, river of | [8] |
| Öskjugjá, crater of | [88] |
| ” volcano of | [84] |
| Pemmican, preparation of | [26] |
| Perlite | [38] |
| Provisions | [29] |
| Pumice | [63], [78] |
| Querkfjall | [63] |
| Reykjahlíð | [112] |
| Rivers, diversion of | [57] |
| Seljalandsfoss | [13] |
| Shark-liver oil | [144] |
| Skeiðarár Sandr | [18] |
| Skjaldbreið, ascent of | [83], [96] |
| Skjálfandifljót | [145] |
| Skógarfoss | [11] |
| Snow, camping in | [32] |
| Solfataras | [174] |
| Spherulite | [38] |
| Sprengisandr | [165] |
| Storms on the Vatna | [46] |
| Sulphur mines | [112], [129] |
| Svartfugl | [23], [25] |
| Swans | [13] |
| Thjórsá | [10], [167] |
| The “Great Kettle” | [129] |
| Trachytic lava | [170] |
| Vaðalda Hills | [61] |
| Vatna Jökull | [32], [186] |
| ” ” Housie | [41] |
| Volcanic mud | [84], [88], [92] |
| Water, eruption of | [89] |