Stones for Building and Decoration

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1903 Excerpt: ... crystals elongated in the direction of their principal axes, and standing at approximately right angles to the plane of deposition, as noted by Sorby in deposits of similar origin. The characteristic banding or grain of.the stone is due to lines of accretion, comparable with the lines of growth upon the trunk of a tree, each layer representing successive surfaces Quarterly Journal of the Geological Society of London, vol. xxxv, 1879, p. 73 over and upon which the lime-holding solutions have deposited new materials. In some instances the successive layers vary more or less in character of crystallization and color, due to a slight change in contents of organic matter or metallic oxides, or physical conditions, whereby the material is rendered more or less opaque. The characteristic translucency of the stone is a purely physical quality. As a rule the crystallization, in sound blocks, continues uninterruptedly upward through the successive layers for a distance of several millimeters, so that there is no tendency toward separation along these layers until a point is reached where, owing to impurities in the water, or, it may be, a temporary cessation of deposition, crystallization ceases. On beginning once more such lines not infrequently form lines of weakness. As a natural consequence of its mode of deposition the surface structure is usually botryoidal. Cut across the plane of deposition, the structure is then as shown in Fig. I of Plate xxV. Cut at right angles to this, as in Fig. 2 of this same plate, the structure, owing to the wavy, botryoidal nature of the original surfaces, is often wonderfully beautiful and always interesting. The colors continually appear and reappear in varying degrees of intensity accordingly as they lie upon the im...