The American journal of science (1917) (18153578185)

Title: The American journal of science

Identifier: americanjourna4441917newh (find matches)

Year: 1880 (1880s)

Authors:

Subjects: Science

Publisher: New Haven : J. D. & E. S. Dana

Contributing Library: Smithsonian Libraries

Digitizing Sponsor: Biodiversity Heritage Library

Text Appearing Before Image:

416 Tarr—Origin of the Chert in the Burlington Limestone. placed, and still others were unaltered. In all instances of alteration the calcite was replaced by quartz. The most coarsely crystalline areas and the largest grains of quartz are in the silicitied fossils. Scattered through the grains of quartz there are many very small grains of calcite, these grains being the unreplaced residue of the original fossil. These calcite grains are so minute that they do not show any cleavage planes or other crystalline characteristics, except the interference tints. Fig. 1. Fig. 2.

Text Appearing After Image:

Fig. 1 (x40). Fresh chert, Burlington limestone, Columbia, Mo. Shows fine mosaic of quartz and chalcedony ; the larger areas of the latter show the interference cross. Fig. 2 (x40). Altered chert. Burlington limestone. Shows fine mosaic of chalcedony in the altered outer portion of a chert nodule. When a fossil is only partially replaced the quartz crystals form a border around the calcite. This border may occupy fully one-fourth of the area of the fossil. The dense white chert consists almost entirely of a mosaic of chalcedony and quartz. The grains which compose this mo- saic are very minute. Probably not more than one per cent of the chert is amorphous silica. The amorphous silica remains dark during a complete rotation of the stage of the microscope between crossed nicols. Since it may show an interference cross at times, it may be more abundant than is stated. By re- flected light it has a pale milky-white luster. Much of the chert in the Burlington has an outer zone

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1917

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