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VI.—The Microscopic Characters of a Silo-carbacid Rock from Ceylon; and their Bearing on the Methylotic Origin of The Laurentian “Limestones.”

Published online by Cambridge University Press:  01 May 2009

William King
William King
Affiliation:
Professor of Mineralogy and Geology in Queen's College, Galway.
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Extract

The rocks usually called metamorphic may be conveniently divided into two great groups—Silacids and Silo-carbacids, depending on the nature of their respective acid constituent. The first comprises syenites, gneisses, and ordinary crystalline schists— all essentially composed of mineral silicates. The second includes ophites, urkalks, and some others, which, in addition to siliceous minerals, contain more or less mineral carbonates.

Type
Original Articles
Information
Geological Magazine , Volume 10 , Issue 103 , January 1873 , pp. 19 - 27
Copyright
Copyright © Cambridge University Press 1873

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References

page 19 note 1 The term metamorphic is herein restricted to rocks that have become structurally changed or mineralized. I propose the term methylosis (from meta, change; and hyl, substance) to express the various chemical changes which certain rocks have undergone since their formation by eliminations of. or additions to (both processes acting simultaneously, or one alone) their original substances to a more or less extent The changes are analogous to those of pseudomorphism in minerals. This last term has occasionally been applied in the case of rocks; but some have, with much reason, urged that it ought to be confined, as originally understood, to chemical changes in crystals unaccompanied by any alteration of their form A term similar to methylosis (but coming from different roots) is in use among chemists: geologists, however, are not likely to confound the two. Methylotic rocks, I feel persuaded, will prove to be commoner than appears to be known. They are not altogether confined to metamorphic associations; as some-such as coal, magnesian limestones chloritic, and serpentinous argillites, etc.—are intimately associated with unaltered rocks. A deposit in Co. Antrim, agreeing with typical beauxite in being a hydro-oxyferriferous alumina (and which has been worked by Dr. Ritchie, of Belfast, who has kindly furnished me with speeimens), is seemingly a methylotio product, derived from basalt by the elimination of silica, and the addition of water, etc.

page 20 note 1 I adopt this name from Werner, though it is restoring one about as objectionable as gneiss and some others current in Geology. I have long thought of proposing the name akerlyte for a similar crystalline calcareo-siliceous rock, occurring at Aker in Sweden; but probably one of Brongniart's names, calciphyre and hemitrene, of a later date than Werner's, will prove to be a synonym.

page 20 note 2 Apatite and magnetite occasionally prevail to such an extent amongst the Laurentian metamorphics of Canada as to form phosphatic and oxidic sections of the silo-carbacid group. There are other rocks present, in which mineral carbonates occur so sparsely as to merge them closely into the silacid group; on the other hand, it is conceivable that others exist in which the mineral silicates may be reduced to a very small per-centage.

page 20 note 3 See Quarterly Journal of the Geological Society, vol. xxii., 1866; and vol. XXV., 1869; also Proceedings of the Royal Irish Academy, vol. x., 1870; and Id., new series, vol. i., 1871.

page 23 note 1 Proceedings of the Royal Irish Academy, vol. x., 1870.

page 24 note 1 Whatever particular view may be taken of pseudomorphic processes, I am strongly inclined to believe that the various minerals mentioned in the text are secondary products, resulting from chemical changes in syenites, gneisses, etc.

page 24 note 2 Kalk-diorite and kalk-glimmerschiefer of the Germans seem to belong to this group. Kalk-diabase and lherzolite appear to be younger rocks, whose calcite may be original, or it may have been derived from limestones older than themselves; and I am disposed to admit that much of the calcitic urkalk (not the dolomitie) of Connemara, apparently Lower Cambrian in age, may be a simply metamorphosed calcareous rock.

page 24 note 3 It is even probable that chemical changes commenced while the Laurentian sediments were undergoing metamorphism. Mr. J. Arthur Phillips has made known some highly valuable and suggestive facts, which seem to show that Cornish Killas or slate rock is marked by a decrease of silica and alumina, and an increase of water and magnesia (or lime in some cases), the more it assumes a metamorphic condition—thus passing into a substance approaching to serpentine. (See Philosophical Magazine, February, 1871.)

page 26 note 1 Farey's “Derbyshire,” contents, p. xxviii.