Fig. 354.
Fig. 355.
Fig. 354.—Quartz crystal enlarged by secondary growth. The shaded outline represents the outline of the sand grain; the solid lines, the outline after secondary growth. Magnified 67 diameters. (Van Hise.)
Fig. 355.—Sandstone and quartzite texture. The shaded outlines represent the surfaces of the sand grains before growth, the intervening white portions, the added quartz, and the black portions, unfilled spaces. Open spaces characterize sandstone. When the spaces are filled with quartz, the rock becomes quartzite. Magnified 35 diameters. (Van Hise.)
The cementing process works at times in specially interesting ways. In quartz sandstones, the grains are worn fragments of quartz crystals, formed originally in quartz-bearing rock. The crystalline force in these remnants controls the arrangement of the new molecules of silica deposited about them. The result is that the new deposits tend to build up the original forms of the crystals from which the sand grains were derived ([Fig. 354]). Sometimes a film of iron oxide has formed about the grain of sand before the addition of the new silica. This, or some difference of color, may clearly distinguish the original grain from subsequent additions. Sometimes the adjacent grains of sandstone are rebuilt in this way until the interstices are completely filled. When this has been accomplished, the sandstone becomes a quartzite ([Fig. 355]). Most quartzites indeed appear to have been formed in this way, but mainly under special conditions that promote the deposition of silica. Grains of other minerals, such as feldspar, are subject to similar secondary enlargement ([Fig. 356]).
Fig. 356.—Feldspar crystals enlarged by secondary growth. Magnified 50 diameters. AA = original grains; BB = enlargements; D = unfilled spaces. (Van Hise.)
Sometimes the new material is deposited in the form of concentric shells about the particles of sediment, building them up into little spheres. Rock formed of such spherules is known as oolite, from the resemblance of the grains to the roe of fish ([Fig. 357]). Sometimes the nuclei of the concretions are grains of quartz sand, and the added concentric layers are of calcium carbonate. In this case the structure is quite obvious; but perhaps more frequently the nuclei are minute and difficult to identify, and the concentric shells make up the main mass of the grains. Certain formations, as the oolitic limestone of Indiana and elsewhere, and the Upper and Lower Oolites of England, are characterized by this structure. In most cases these accretions probably grew in depositing waters that gently rolled the grains while layers were being added. They thus do not fall under the head of cementation after the beds were formed; but concentric additions to the grains appear sometimes to have taken place after they were formed into beds.
