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On Rocks; Their Chemical and Mineral Composition, and Physical Characteristics*
Published online by Cambridge University Press: 17 March 2016
Extract
I. “Il suffit de parcourir des parties très circonstrites de la surface du globe, pour voir qu'elle est composée de matériaux très-variés.” The most striking of these differences were, of course, noticed even in the least civilized times; and, as observation advanced, and experience accumulated, the minor and less obvious variations of constitution and texture were gradually reduced to order, and classified into the science of Mineralogy.
II. So far our science concerned itself merely with what we should now call a correct description of the mineral character of rocks, without entering into the question of their genesis or mode of origin. But at the dawn of geology—that is, when men first conceived that the various rocks and strata at the surface of our earth were formed at different, and often immeasurably remote periods—then the study of rocks entered on a new phase. The age and succession of these formations were judged, in the first place, from the order of their superposition; and when it was found, as a general rule, that each formation, or stage of this succession, was marked by rocks of a specific mineral character, it was hastily concluded that this mineral character was typical of their age, and that the latter could be deduced from the former: nor was this generalization devoid of broad principles of truth; the ancient and more recent formations are each undoubtedly composed of rocks of very different mineral character—the former being made up such of rocks as granite and the so-called primary limestones and clay-slates, while the latter consists of clays, chalk, or slightly consolidated sandstones.
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Footnotes
*
In case any of my readers, being young geologists, should misapprehend the exact geological meaning of the word rock, I give Sir Charles Lyell's definition:—“The materials of the earth's crust are not thrown together confusedly; but distinct mineral masses called ‘rocks’ are found ib occupy definite spaces. The term rock is applied indifferently by geologists to all these substances, whether they be soft or stony; for clay and sand are included in the term, and some have even brought peat under this denomination.”—“Elements,” p, 2.
References
page 416 note † D'Aubuisson's “Geognosie” par Burat.
page 417 note * Recent experience has shown what very erroneous conclusions as to the age of rooks we may draw from mineral character alone. “The hillocks of slightly coherent mud, marl, and sand, near St. Petersburg, are truly of the same age as the slaty mountains of North Wales,” although they differ little in mineral character from the London tertiaries; while on the flanks of the Rigi, in the Alps, at the height of 8,000 or 9,000 feet, “deposits formed at the same time as our slightly consolidated London clay have been in many parts converted into schists and slates, as crystalline as many of the so-called primary rocks of our islands.”—“Siluria,” pp. 18, 503. Still these are exceptions ; in nine cases out of ten, the crystalline rocks will be found to belong to the old formations, and the slightly consolidated to the new.
page 419 note * Cotta, .—Die Gesteinslehre. Freiberg, 1856, pp. 255 Google Scholar.
page 419 note † Senft, .—Classification und Beschreibung der Felsarten. Breslau, 1857, pp. 442 Google Scholar. An abstract of the leading chapters of this work may be found in the “Quarterly Journal” of the Geological Society, Vol. XIV., part II, miscell. p. 1., &c.
page 420 note * Naumann, .—Lehrbuch der Geognosie. Leipsic, 1858. (New Edition). Vol. I., pp. 381–776 Google Scholar. In French, I may refer to Professor Coquand's Traité des Roches. (Paris, Baillière, 1857)Google Scholar, and D'Orbigny's article Roches in the Dictionnaire Universale d'Histoire Naturelle.
page 420 note † Elements of Chemical and Physical Geology. Translated by the Cavendish Society. Two Vols.