The greatest portion of Iceland is mountainous ; the highest elevations appearing on the cast and west sides, while extensive .plains appear, with little interruption, to extend from north to south across the island. The high est mountain, the height of which has been ascertained, is said to be about 5500 feet. The Sneefell Jokul, which bounds the western extremity, and has been esteemed by the natives the highest in the island, was measured trigo nometrically by Sir George Mackenzie, and ascertained to be 4558 feet above an extensive beach formed near Stappen. It is probable that none of the Jokuls exceed 6000 feet. Many of them are very extensive, forming Iong ridges ;a shape which, were it not for their charac teristic covering of per petual snow, would tend to lessen their apparent elevation to the eye. The height of the curve of congelation may be taken for Iceland, at 2829 feet as a m:an ; and thus a tolerable idea of the heights of the mountains may be had, when an opportunity occurs of ob serving the snow line on the Jokuls. From what has been actually observed, and from the best information, it appears that all the Jokuls are of volcanic formation ; in other words, that they are either active or extinct volcanoes. The greatest height of the other mountains which are not volcanic does not much exceed 2000 feet. They are all of the trap formation ; and this does not appear to reach a great height in the northern hemisphere. In Faroe, its greatest elevation appears to be somewhat above 3000 feet. As far as has yet been discovered, the formations in Ice land are limited to the ficetz trap, volcanic, and alluvial, and those of Faroe to the first and last. The fleetz trap of Greenland, that of Iceland, and of Faroe, are probably connected ; and the two last may be supposed to rest on primitive rocks, as the Greenland formation has been as certained to do.
The beds of trap are inclined for the most part at a small angle to the horizon, dipping to the eastward of the meridian; and in this respect they agree with those of Faroe.
The beds consist chiefly of amygdaloid, containing zeolite, chalcedony, opal, quartz, arragonite, and calcare ous spar. The base of the amygdaloid is for the most part wacke. There are many varieties of greenstone, from the most compact to that whose component pacts are of large size ; felspar and augit occurring in crystalline masses of from a quarter to half an inch. Greenstone oc curs also in veins, and it is in this form only that basalt has been observed in Iceland and Faroe. The veins of Ice land, and some of those in Faroe, were found to have their sides vitreous, somewhat resembling black pitchstone, and gradually passing into the character assumed by the mass of the veins. Greenstonc occurs columnar. In the is land of Vic6c, there arc columns formed of tables several inches in thickness, and from three to five feet in diameter, and in some instances, from decomposition, easily separa ble from each other. Some of the tabular columns have the appearance of the tables being composed of oval CDR cretions, rendered visible by decomposition.
Beds of tuf of large dimensions are frequent ; and if they are to be denominated from the formation in which they occur, they may be called trap tuf. The resemblance which they bear to volcanic tuf is, however, in many cases remarkably strong. They occur most frequently among a class of rocks, which have been distinguished by Sir George Mackenzie by the name submarine lavas ; and we shall now quote from that gentleman's account of Iceland, the description of these rocks. He has selected a parti cular mountain which presented a precipitous face about 2000 feet high, and which, but not without considerable difficulty and hazard,lie and his fellow travellers ascended, taking specimens from each bed, a series of which is pre served in the cabinet of the Royal Society of Edinburgh. 66 The mountain of Akkrefeil stands perfectly detached, be ing separated from the mountains of Esian by the Hval fiord, and from those towards the north by a flat swampy country extending severaLmiles. It is bounded on the south and west partly by the Faxefiord, and partly by the Borgarfiord. On the southern side the structure of the mountain is exposed almost from top to bottom, in a pre cipice about 2000 feet high. From Indreholm the beds appear horizontal ; but on going round the mountain we found that they dipped towards the north-cast, forming a considerable angle with the horizon. From the sea at In
drehohn to the foot of the mountain, there is a flat swamp extending more than a mile.
To the height of about 800 feet, the mountain of Akkre fell is composed of beds generally from ten to twenty, and sometimes extending to forty feet thick, of varieties of amygdaloid and tuffa. The latter occurred sometimes not more than a foot in thickness, and was interposed.between the beds of amygtlaloid. When it was so thin, it much resembled red sandstone. While scrambling among the loose stones, we had met with quantities of slags, for which we were at a loss to account, as we had been told that no thing like lava existed on or near this mountain. Its ap pearance indicated nothing volcanic ; and our surprise on finding any undoubted productions of fire in such a place was increased, when, at the height above mentioned, we saw the under part of a bed completely slaggy, and hear ing the must unequivocal marks of no slight operation of fire ; and on continuing to ascend, we found every bed, ex cepting those of tuffa, one of which was at least forty feet thick, presenting the same appearances, and many of them having an amygdaloidal character. Our astonishment was not lessened on discovering a vein of greenstone, about four feet thick, cutting these beds, and having a vitreous coating on its sides, which seems to be common to all the veins of the country. Similar beds occur in the mountains w of Esian, where the most important fact connected with them was found in a mass of slag w hich contained calca reous spar." The same traveller, in a memoir concerning the Faroe Islands, printed in the 7th volume of the Edinburgh Tran sactions, thus speaks of the trap rocks of that country. " Tile surfaces of many lavas which I passed over in Ice land were not unlike coils of rope, or crumpled cloth ; an appearance which we should expect to be assumed by any viscid matter in motion. On our first visit to the island of Naalsoe, we observed the surface of a bed of amygdaloid, which had been exposed to a considerable ex tent by the removal of the bed above, exhibiting an exact picture of the lavas I had seen in Iceland. At first sight this discovery fixed instantaneous conviction on the minds of those who were with me, some of whom had never seen lava, that heat must have caused the appearance be fore us. We brought away a number of specimens, which are now before the Society, find which speak a language not to be misunderstood. We afterwards discovered va ried examples of this crumpled surface in different parts of the country. In the vicinity of Eyde, in Osteroe, there were many instances in which the matter appeared as if, in a viscid state of fusion, it had flowed and spread itself out." These appearances were seen in different parts of Faroe. Mr Allan, in the same volume, has described the indivi dual minerals found in the trap of Faroe, and expressed his entire conviction of its igneous origin ; while at the same time he appears at a loss to account for the manner in which heat had operated. Sir George Mackenzie has observed, that " while the great point, the action of heat in the formation of trap rocks, seems to be demonstrated, forming a theory of the manner in which heat has operated in particular cases, is, in a general view, perhaps not abso lutely necessary in the present state of geology." Indeed the farther we advance in the study the farther we find our selves from the object of our search while we continue re solved that either heat or water shall be the sole agent. To us, the most reasonable mode for geologists to follow appears to be, to consider both heat and water as agents which have been employed in the formation of mineral substances. And we are of opinion that a third agent ought to be added as one which universally and continu ally operates, we mean gravitation. The attraction of co hesion and electricity may also with propriety be taken into the service ; and probably, to form a just theory, we should give a share to every agent which can in any degree influence the economy of the globe ; the atmosphere, the motion of the earth, magnetism, &c. But at present we must refrain from entering on this subject.