The Quarterly journal of the Geological Society of London . Fig. 3.—Microscopic Section of Radiolarian Rock, showing Radio-larian casts and a cube of pyrites {the dark spot). From theLower Culm Measures at Carzantic Quarry, near Launceston,Cornwall. From a photograph taken by Mr. J. J. H. Teall, (Enlarged about 32 diameters.). Vol. 51.] RADIOLARIAN ROCKS IN THE LOWER CULM MEASURES. 631 and minute crystal-needles of rutile and zircon. The siliceousgroundmass in polarized light, between crossed nicols, usuallyexhibits the faint speckled aspect of cryptocrystalline silica similarto that of
The Quarterly journal of the Geological Society of London . Fig. 3.—Microscopic Section of Radiolarian Rock, showing Radio-larian casts and a cube of pyrites {the dark spot). From theLower Culm Measures at Carzantic Quarry, near Launceston,Cornwall. From a photograph taken by Mr. J. J. H. Teall, (Enlarged about 32 diameters.). Vol. 51.] RADIOLARIAN ROCKS IN THE LOWER CULM MEASURES. 631 and minute crystal-needles of rutile and zircon. The siliceousgroundmass in polarized light, between crossed nicols, usuallyexhibits the faint speckled aspect of cryptocrystalline silica similarto that of a section of a flint from the Chalk ; sometimes it is almostentirely dark, in other instances certain bands of the rock in whichthe radiolaria are very crowded show the lively tints of radiolaria in the rock generally have been infilled with clear,nearly transparent silica, free to a great extent from the rutile-crystals and the dark substances disseminated in the groundmass,and either cryptocrystalline or microcrystalline in the radiolarian casts the silica has not unfrequently a radiatefibrous arrangement, and in this case a black cross is shown inpolarized light. The more distinctly crystalline structure of theradiolarian casts facilitates their recognition in the rocks with aclear groundma
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Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
