Abstract
THE tract of Devonian rocks which stretches through the north of France and Belgium, and across Rhenish Prussia into Westphalia and Nassau, has furnished ample materials for geological disquisition. Among the problems which it presents to the observer, not the least important is the remarkable metamorphism of certain bands or, areas of its component strata. Dumont first called attention to this feature in the Belgian Ardennes. It was subsequently shown by Lessen to be extensively developed in the Taunus. More recently the question has been attacked anew with all the appliances of modern petrography. M. Renard has subjected some of Dumont's original localities to a critical revision, which has resulted in a confirmation of the accuracy of that remarkable geologist's observations. The latest contribution to the literature of the subject is a paper (Annales Soc. Géol. du Nord, vol. x. p. 194) by Prof. Gosselet, who at first refused to admit the metamorphism contended for by Dumont and corroborated by M. Renard, but who now comes forward with independent evidence in its support, from another locality. He describes the arkose of Haybes and of Franc-Bois de Villerzies on the frontier of Belgium as having undergone such a metamorphism as to be no longer recognisable. M. Barrois reports that on examining microscopically some sections of the altered rocks, he found among them bi-pyramidal crystals of quartz with liquid inclusions and movable bubbles, as in the quartz of pegmatite. These crystals have been broken in situ, with conchoidal fractures, and the surrounding paste appears as if injected into them. This paste is composed of small irregular quartz-grains like those of schists, and is coloured by fibrous chlorite, so arranged as to impart a more or less schist-like structure. The chlorite, arising from alteration of biotite, is predominant in some specimens, while the quartz-grains preponderate in others. M. Barrois compares this altered arkose with some porphyroids and some granitic veins in Brittany recently studied by him. Prof. Gosselet shows that these crystalline intercalations are portions of the true Devonian strata, and he accounts for their highly altered condition by what he terms a metamorphism by friction. A portion of the Devonian rocks has slipped down between two faults and has undergone great lateral pressure, and has in consequence been heated sufficiently that metamorphism has been determined in it. The extent of change has been proportionate to the degree of pressure. The metamorphosed arkose is provisionally referred to the Gedinnian division of the system.
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Metamorphism among Devonian Rocks . Nature 29, 315–316 (1884). https://doi.org/10.1038/029315a0
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DOI: https://doi.org/10.1038/029315a0