File:The Quarterly journal of the Geological Society of London (1904) (14804981853).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:
Fig. 3.—Typical scar formed of Moine Gneisses, showing the resemblanceto unaltered sandstones : on the River Garry, near Clune, 2 milesnorth-west of Struan.
Text Appearing After Image:
Q. J. G. S. No. 240. 2F 406 ME. G. BAEROW ON THE MOINE GNEISSES (NOV. I904,. fact that, their highly-crystalline character is shown, not by thequartz or felspar, but by the persistently-large size of the rnicas,.when these are present in notable quantity. Years of study haveproved that this is by far the most sensitive test by which to judgeof the degree of crystallization in altered sediments, such as wereoriginally normal sandstones and shales. Microscopic Characters of the Gneisses in the StruanArea.—Great light is thrown on the structure and composition ofthese gneisses by the aid of the microscope. Taking first theprevalent type—the parallel-banded rocks, we find that theyare essentially felspathic gneisses, the felspar being usually in excessof the quartz, and in some cases occurring almost to the exclusion ofthe latter. As the quartz decreases in amount it tends to assume arather rounded form, embedded more or less in the felspar, and con-stituting quartz-bleb structure. It

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