Concretions.—A notable phase of this internal reconstruction is the assembling together of like kinds of matter. For instance, silica that was probably deposited in the form of the silicious shells and spicules of plants and animals, and was disseminated through the sediments as originally formed, is aggregated into nodules of chert or flint ([Fig. 361]); similarly, concretions of ferrous carbonate or calcium carbonate grow in sands, silts, or muds; clusters of crystals of pyrite (FeS2), of sphalerite (ZnS), and galenite (PbS) are formed in clayey layers, pressing the clay back as they grow; and in many other cases, kind comes to kind. Some concretions probably form during the accumulation of the beds in which they lie.
Replacements and pseudomorphs.—So also there are replacements, sometimes resulting in imitative or false forms. Frequently the calcium carbonate of corals, molluscan shells, etc., is replaced by silica, and this substitution is brought about so gradually, particle by particle, that the minutest details of structure are sometimes fully preserved. This is often of great service in their study, since the limestone in which they are imbedded may often be dissolved away, while the silicified fossil is unaffected. So woody matter is sometimes replaced by silica, forming silicified wood. Similarly, the molecules of one crystal are sometimes replaced by different material, as the molecules of calcite by zinc carbonate, giving a pseudomorph of zinc carbonate after calcite.
Fig. 361.—Nodule of chert. About half natural size. (Photo. by Church.)
Incipient crystallization.—A more general change is incipient crystallization. Some common limestones and dolomites are now largely made up of small crystals, though the mass was originally a calcareous mud or ooze. Incipient crystals are formed in shales and other sediments. This process, like the preceding, is a kind of incipient metamorphism or reconstruction, but it is a pervasive process, taking place under ordinary conditions of heat and pressure, and through the agency of circulating ground-waters.
By these and similar processes the fragmental deposits are solidified into firm rock and undergo internal changes which more or less reorganize the matter of which they are composed. The process is a very slow one usually. Some of the sands and muds of very early geologic ages are yet imperfectly solidified; e.g., much of the St. Peter’s sandstone, a very ancient formation, is yet so incoherent as to break down into sand in being dug out, and is used for mortar sand much more than for building stone. Some of the Hudson River shales of scarcely less age are more nearly clay than hard rock. But these are examples of excessive slowness and slightness of change. In general, all but the most recent deposits show notable progress in reconstruction.
Fig. 362.—Figure showing the elongation of pebbles resulting from pressure. Carboniferous formation, Bancroft Place, Newport, R. I. (Walcott, U. S. Geol. Surv.)
Reconstruction under Exceptional Conditions.
Two special conditions greatly influence changes in rocks, viz., pressure and heat. Their action gives rise to three general cases, but these blend indefinitely: (1) exceptional pressure without great heat, (2) great heat without exceptional pressure, and (3) great heat and great pressure conjoined. Exceptional pressure may arise from the weight of overlying rocks, or from lateral thrust due to the shrinkage of the globe, and occasionally from other causes. Exceptional heat may arise from pressure, from the intrusion of hot lavas, and occasionally from other sources. In the case of intruded lavas there may or may not be exceptional pressure. Thrust usually gives heat as well as pressure, but if lateral thrust acts on rocks near the surface, they may be mashed into new forms without becoming very exceptionally heated, though some rise of temperature is inevitable.