File:The Quarterly journal of the Geological Society of London (13960201533).jpg
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[edit]DescriptionThe Quarterly journal of the Geological Society of London (13960201533).jpg |
igneous rocks after their consolidation. 185 8. Bearing of these considerations on the problem of the Origin of Foliation in the Metamorphic Hocks. It is scarcely necessary to point out the important "bearing of this principle of the growth of crystals — and especially of felspar-crystals — in solid rock-masses upon the great question of the origin of folia- tion in rocks, a question in the discussion of which the Fellows of this Society have taken so prominent a part. That, as the result of contact-metamorphism, many well-defined mineral species are developed in the midst of solid rocks, the crystals growing at the expense of and deriving their materials from the surrounding detrital fragments, has long been recognized. These phenomena find many beautiful illustrations in the so-called "spotted schists." I hope to be able to show, on a future occasion, how large a part a similar action plays in producing the characteristic structures of many fresh and apparently unaltered igneous rocks. The founders of the theory of dynamo-metamorphism — Scrope and Darwin — very clearly perceived that in the study of igneous rock-masses which have been subjected to movements and internal stresses, we find alike the clearest analogies and the simplest and most readily studied examples of the processes which go on during the production of foliation in rocks. Those who have done most towards establishing the theory on its present firm basis, by tracing with the aid of the microscope the actual changes which the minerals of rocks undergo while the development of foliation is in progress — and I especially refer to the beautiful researches of Lossen and Lehmann and those of students of petrography who, like Dr. Reusch in Scandinavia, Prof. Williams in America, and Mr. Teallin this country, have sought to follow in their steps — have skilfully and patiently pursued the same methods. Charles Darwin was able to show, by the aid of a pocket-lens and blowpipe only, that in a noncrystalline lava in the Island of Ascension which had been subjected to powerful internal mechanical stresses, the felspar, augite, and magnetite had separated from the glassy mass in distinct and parallel folia *. He clearly perceived, what some in more recent years have quite failed to realize, that mechanical force 'per se is wholly incompetent to produce a true foliation in rocks ; but that the mechanical force becomes really effective by determining and controlling the operations of chemical affinity and crystallization; and it is by these that the metamor- phoses of the minerals in rocks are brought about which result in the development of the foliated structure. The more carefully these metamorphoses of the minerals in rocks are studied, and the more clearly it is perceived that the whole structure of deep-seated rocks may undergo a complete transforma- tion, during such metamorphoses of their constituent minerals, the less difficulty will be felt in accepting the teachings of Scrope and Darwin's ' Volcanic Islands,' published in 1844, p. 56 (p. 64 of reprint issued in 1876). I have studied Darwin's original specimens with the microscope, and can confirm the wonderful accuracy and acumen of hisresearches. |
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Source | https://www.flickr.com/photos/biodivlibrary/13960201533 | ||
Author | Geological Society of London | ||
Full title InfoField | The Quarterly journal of the Geological Society of London. | ||
Page ID InfoField | 36939974 | ||
Item ID InfoField | 113696 (Find related Wikimedia Commons images) | ||
Title ID InfoField | 51125 | ||
Page numbers InfoField | Page 183 | ||
BHL Page URL InfoField | https://www.biodiversitylibrary.org/page/36939974 | ||
Page type InfoField | Text | ||
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Flickr posted date InfoField | 21 April 2014 | ||
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26 August 2015
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current | 05:52, 26 August 2015 | 1,166 × 2,104 (579 KB) | Fæ (talk | contribs) | == {{int:filedesc}} == {{BHL | title = The Quarterly journal of the Geological Society of London. | source = http://www.flickr.com/photos/biodivlibrary/13960201533 | description = igneous rocks after their consolidation. 185 <br> 8.... |
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