THE ORIGIN AND DESCENT OF ROCKS. 439 



back as they grow; and in many other cases,, kind comes to kind. Some 

 concreuons probably form during the accumulation of the beds in which 

 they lie. 



Replacements and pseudomorpbs. — So also there are replacements, 

 sometimes resulting in imitative or false forms Frequently the cal- 

 cium carbonate of corals, molluscan shells, etc., is replaced by sihca, 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 



Fig. 361. — Nodule of chert. About half natural size. (Photo, by Church.) 



are imbedded may often be dissolved away, while the silicified fossil 

 is unaffected. So woody matter is sometimes replaced by silica, form- 

 ing silicified wood. Similarly, the molecules of one crj^stal are some- 

 times replaced by different material, as the molecules of calcite by zinc 

 carbonate, giving a pseudomorph of zinc carbonate after calcite. 



Incipient crystallization. — A more general change is incipient crystal- 

 lization. Some common limestones and dolomites are now largely made 

 up of small crystals, though the mass was originall}^ a calcareous mud 

 or ooze. Incipient crystals are formed in shales and other sediments. 

 This process, hke 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 reor- 

 ganize the matter of which they are composed. The process is a very 

 slow one usually. vSome of the sands and muds of very early geologic 

 ages are yet imperfectly solidified; e.g., much of the St. Peter's sand- 

 stone, a very ancient formation, is yet so incoherent as to break down 



