File:The Quarterly journal of the Geological Society of London (1904) (14598620827).jpg

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

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
a film of tough schist occurs between the Limestone and the PinkFelspathic Bock. This (10,549) is a highly-micaceous rock, builtup of alternating films of quartzose and micaceous material ; thelatter consist largely of white mica and chlorite, often envelopinglarge cracked and decomposed garnets. Iron-ore is abundant, bothin good-sized grains and as fine dust, and it is often embedded in aclear, almost glassy material, which is known to be plagioclasealthough it here shows no striatum. There can be little doubtthat this is a siliceous modification of the Felspar-Rock, and itillustrates a difficulty that occurs repeatedly. As the area isapproached where the more sandy material, now forming the MoineGneisses, was deposited, the Dark Schist tends to become slightlymore siliceous, and differs slightly in appearance from the rocks ofthe type-area. If the siliceous material increases beyond a certainpoint, the zone can, of course, be no longer identified. So far as is 1 See Map, PI. XXXITI.
Text Appearing After Image:
Vol. 60.] 3JOINE GNEISSES OF THE EAST-CENTRAL HIGHLANDS. 431 known at present, this increase is more marked in the lower part ofthe Schist than in the upper. Of all these patches of Dark Schist, the most interesting is thatwhich lies apparently on the top of the Limestone in the river-bank at the first bend below Marble Lodge. (See fig. 5, p. 430.)Here the Limestone is shown resting in an eroded hollow of theattenuated Parallel-Banded Rocks into which the Moine Gneisseshave slowly passed, while lying apparently above the Limestoneis the patch of Dark Schist now mentioned. It is a dark, somewhatmassive rock, evidently rich in biotite, but containing a numberof very minute lenticular films of quartz, suggesting that, aswe approach an area of more sandy deposition, the Dark Schist Fig. 6.âDiagram showing the Dark Schist and ParallelBandedRocks apparently on opposite sides of the Limestone, while inreality they are on the same side. A B â¢â â¢â¢ it ii ii H H n ii M ii â¢â » »â

Note About Images