File:The Quarterly journal of the Geological Society of London (1904) (14598489058).jpg

Identifier: quarterlyjourna601904geol (find matches)
Title: The Quarterly journal of the Geological Society of London
Year: 1845 (1840s)
Authors: Geological Society of London
Subjects: Geology
Publisher: London (etc.)
Contributing Library: Smithsonian Institution Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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Text Appearing Before Image:
btained formicroscopic examination. (See figs. 2 & 3, pp. 404 & 405.) The Grey Gneiss.—The dominant member of the MoineGneisses in this area is an evenly colour-banded and markedly-granular, acid gneiss, containing a variable, but often considerable,amount of brown mica. White mica is frequently present inthe typical gneiss, but as a rule in smaller quantity than brown,while it is in many bands absent altogether. Though the amount ofbrown mica varies incessantly, taking the group as a whole, it is forthe most part evenly distributed through small thicknesses of thegneiss, varying from a fraction of an inch to sometimes as much as >07> %«* xv7> rjf O l-T-A e>rJtfe o ^IM *•-*• ~ > T (75 m _ f-l < « ^ a w u f=i ■ S C o ■H - D rt ^ Z fc o c oo P a c3 0 ri r- X r- £ o <: <u c3 pc, 0 H< to o ! cM o DC — r^ £ ct o eg o o O fin o r-> , , d rt <1) O PH T3 X w (Jill GlciPoo 3 t~ oc Ci
Text Appearing After Image:
Vol. 60.) MOINE GNEISSES OF THE EAST-CENTRAL HIGHLANDS. 403 2 feet, or even more. This increase and decrease, in different bands,of evenly-disseminated brown mica imparts different shades of grey,pale-grey, or pinkish-grey to the banded gneiss, and is the principalcause of the evenly colour-banded aspect, which is its mostcharacteristic feature. The bedded aspect of the series is oftenintensified by the arrangement of the individual crystals ofbiotite parallel to the colour-banding. But it is still furtheraccentuated by the presence of films of felted dark mica, whichare always rigidly parallel, and appear on a cross-fractured surfaceas fine black lines. These films decompose more readily than the rest of the rock,and give rise to planes of diminished coherence, so that whenfragments become detached from a scar-face they break away alongthose parallel surfaces. Further, this decomposed material weathersout, leaving a series of minute parallel grooves that have theappearance of dark li

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