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II.—On the Origin of certain Banded Gneisses

Published online by Cambridge University Press:  01 May 2009

J. J. H. Teall
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Extract

The term gneiss as generally used by geological writers signifies a rock of granitic composition in which a parallel structure in the arrangement of the constituents is more or less apparent. For our present purpose it is important to note that other plutonic rocks besides granite (e.g. diorite, gabbro, and peridotite) have their gneissose equivalents, so that, if we use the term gneiss in a structural rather than in a mineralogical sense, we may speak of diorite-gneiss, gabbro-gneiss, and so on. Now the parallel structure of gneissose rocks is of two kinds. It may consist (1) of a parallel arrangement of certain constituents (e.g. mica plates or porphyritic felspars), or (2) of an alternation of bands of varying chemical and mineralogical composition. It is agreed on all hands that a parallel structure of the first kind may be due either to the deformation of a mass of half-consolidated plutonic rock at the time of intrusion, in which case it is strictly analogous to the flow structure in many volcanic rocks, or to deformation produced by earth-stresses operating on the mass after consolidation.

Type
Original Articles
Information
Geological Magazine , Volume 4 , Issue 11 , November 1887 , pp. 484 - 493
Copyright
Copyright © Cambridge University Press 1887

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References

page 484 note 1 Roth has recently (Sitz. d. k. preuss. Akad. d. Wissen. Ueber den Zobtenit, vol. xxxii. 1887, p. 611) proposed that Leopold von Buch's term Zobtenfels should be revived under the form Zobtenite for those gabbros which are associated with crystalline schists and are often foliated.Google Scholar

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