The Quarterly journal of the Geological Society of London . Illinois, Iowa, and Wisconsin, cannot bedetermined from the hand-specimens examined, since they show notrace of dolomite-crystals, even in thin sections under the micro-scope. It seems more probable that the first change was due tosilicification, in the form of a gradual molecular replacement of thecalcareous stereom^ of the constituent fossils, beginning with theouter layer of each ossicle or fragment. Thus, in the case of acrinoid stem, the silicifying fluid bathed the outside of the stem,the walls of the axial canal from which the
The Quarterly journal of the Geological Society of London . Illinois, Iowa, and Wisconsin, cannot bedetermined from the hand-specimens examined, since they show notrace of dolomite-crystals, even in thin sections under the micro-scope. It seems more probable that the first change was due tosilicification, in the form of a gradual molecular replacement of thecalcareous stereom^ of the constituent fossils, beginning with theouter layer of each ossicle or fragment. Thus, in the case of acrinoid stem, the silicifying fluid bathed the outside of the stem,the walls of the axial canal from which the soft contents hadpreviously decayed, and the narrow interstices between thecolumnals formerly filled with ligament. The supply of silica thendecreased, and acidulated water passing through leached out anyremaining stereom from the inner portions of each fragment. Therewere thus left a number of hollow boxes, as it were, formed of silica. Fig. 2.—Microscope-section of matrix of Petalocrinus mirabilis,specimen y {Cut hy Mr. F. Chapman. Brit. Mus. E6635.)c. X 30 diam. [The drawing is diagrammatic, especially in so far as the crinoid-fragmentshave been selected from different parts of the original preparation ; it repre-sents the section as seen by polarized light, nicols crossed, a, columnalcut longitudinally, showing axial canal; b, brachial cut transversely;c, brachial and covering-plates cut transversely; d, chalcedony; e, columnalcut transversely. Between all these, and in the cavities formed by solution,are minute crystals of quartz, while minuter crystals are in the meshes ofthe stereom, which also is silicified. The black areas represent brachials certainly, and the columnals possibly, do not belong toPetalocrinus itself.] ^ Stereom, any hard tissue, forming skeletal structures in MetazoaInvertebrata and in Protozoa: Nature, vol. xliii (Feb. 12th, 1891) p. 345. Vol. 54.] MR. F. A. BATHER 0?f PETALOCRINUS. 405 With the disappearance of all the carbonate of lime,
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Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
