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VI. An Account of some Geological Facts observed in the Faroe Islands

Published online by Cambridge University Press:  17 January 2013

George Steuart Mackenzie
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Extract

The singular appearances which were presented to my view by the Trap Rocks of Iceland, and the interest which they excited, made me resolve, as soon as I had given an account of that country to the public, to visit the Islands of Faroe. This expedition was undertaken, for the purpose of ascertaining, whether, in a Trap Country, where no traces of external volcanoes existed, any thing similar to the peculiar features of the rocks of Iceland was to be found. In the latter country were discovered a series of rocks, lying above the beds of Trap, which bore the most striking marks of igneous origin; in some instances having a perfect resemblance to ordinary Trap, and in others to the common Lavas of the country. The beds of Trap, and those above them, being separated by mechanical depositions of Tuffa (Trap-Tuff), led me to the conclusion, that the whole of the beds had been formed at the bottom of the sea, by successive eruptions of a submarine volcano.

Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 7 , Issue 1 , 1815 , pp. 213 - 227
Copyright
Copyright © Royal Society of Edinburgh 1815

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References

page 214 note * Landt, Translation, Lond, 1810, p. 5,

page 215 note * Landt, p. 11.

page 217 note * The name of this mountain, and some other names, are variously spelled; but the mode adopted in this memoir conveys the pronunciation.

page 218 note * We had a view of Myggenæs, near enough to distinguish that it was composed of beds, which rose at an angle considerably greater. The coal is probably in the same position as that found in the Isle of Skye, near Talisker, where it occurs between beds of trap.

page 218 note † I have preferred the term Tuffa to that of Trap-tuff, because I wish to employ a generic term, and one that has no allusion to theory. Trap-tuff is, no doubt, used as generic by the Wernerians; but it is in reality a specific term. It is a question whether the specific terms should be derived from the basis or the included masses. As it is not in any case easy, I may say possible, to determine to what specific substance we ought to refer the basis, which varies in several respects, it may perhaps be best, when there is any uncertainty, to describe both it and the included masses; for prolixity is, in every description, preferable to want of perspicuity. Trap-tuff implies, either that the basis is derived from Trap, or that the included masses are Trap, or that both have the same origin. Now, as the rock in question often contains sandstone, and sometimes wood; and as in Iceland and other volcanic countries, it contains lava and other volcanic substances; the term Trap-tuff conveys none of these important particulars. Correctness seems, therefore, to demand some reform. I would propose that the basis should be understood generally, as Professor Jameson has described it, to be “rather a loose, spongy, “clayey basis,” and that we should relinquish the term trap, and use, as a general term, the word tuff, or the original and more harmonious Italian Tuffa, and describe the included masses. Thus, in Iceland, I met with tuffa, the component parts of which were so minute, that it resembled red sandstone; tuffa including rounded masses of trap, from a size very minute, to many feet in thickness; tuffa including masses of trap, lava, slags, and mineralized wood. Here the description appears to be absolutely necessary to prevent misapprehension; and instead of being at all offensive, it affords perfect satisfaction: whereas, were I to use for them all merely Trap-tuff, many important particulars would be omitted.

Objections also arise to the term Calc-tuff, which may be understood to mean a calcareous basis, cementing together substances, the nature of which we are left to imagine. The included masses being sometimes entirely calcareous, and sometimes heterogeneous, the term, by itself, conveys nothing satisfactory. For fragments connected by a solid basis, the old term breccia seems to be unobjectionable, when qualified, as circumstances may require, by a description of the basis and the included masses.

page 223 note * “When the lava is very hot, and consequently very liquid, the steam will have less difficulty in penetrating it than when it is viscid. We may conceive cases in which the lava burst forth in such a high state of liquidity, as to permit the whole of the moisture to pass through it in the form of steam; in such a state of viscidity, as to admit of its escaping very slowly, so that the lava may become solid, and, by confining the steam, more or less vesicular†; and, lastly, so tough, that the exertions of the elastic vapour shall be confined entirely to the lower surface of the lava. In the first ease, a compact mass of stone would be formed, having no appearance of the action of heat; in the second, on account of the pressure of the superincumbent water being sufficient to prevent the escape of carbonic acid and other volatile ingredients, a vesicular and amygdaloidal mass would be produced; and from the last would result a mass entirely compact, excepting at the under surface.”—Travels in Iceland, chap. IX.

To this passage of the text the following note is subjoined: “In such a case, it is possible that the steam, when condensed, would, in some instances, remain confined in the stone in the form of water; and thus the fact of water being found in the vesicles of basalt and other rocks, may be accounted for. It must be observed that the steam, in such circumstances, must have been very much condensed, so much, indeed, as to be almost in the state of water greatly heated; much more so than in the familiar experiment made with Papin's digester.”