50 GEOLOGY 



Incipient crystallization. A more pervasive metamorphic 

 change in sedimentary rock is incipient crystallization. Some 

 common limestones and dolomites are made up largely of small 

 crystals, though the mass was originally a calcareous mud or ooze. 

 New crystals are also developed in shales and other sediments, 

 out of materials already present, or with such additions as ground- 

 water may make. This process is a kind of incipient metamorphism, 

 and takes place even under ordinary conditions of heat and pres- 

 sure, through the agency of circulating ground-waters. 



Change in chemical composition. Besides simple deposition 

 in pores and cracks, the mineral matter in solution may enter into 

 combination with other mineral matter, giving rise to new, and 

 often to more complex and more compact mineral substances. 

 The changes effected in this way go on slowly, but in the long 

 course of time, they may go so far that none of the original rock 

 material remains in its original condition all having entered 

 into new combinations. Soapstone or steatite, for example, is a 

 rock composed essentially of such secondary material. Serpentine is 

 a rock made up of a secondary mineral (serpentine) apparently 

 derived from magnesian minerals. Chloritic rocks are rocks com- 

 posed largely of the soft, green, hydrated mineral chlorite, derived 

 from the pyroxene, amphibole, biotite, and perhaps other silicates 

 of the parent rock. Other igneous rocks become talcose from the 

 development of talc, a very soft, unctuous, hydrous magnesian 

 silicate, developed from the magnesian minerals of igneous rock. 

 All of these rocks may occur in large bodies. All are metamorphic 

 rocks, developed primarily through the chemical rearrangement 

 of the mineral matter of the original rock, with the addition of 

 some matter brought in by ground water, and with the abstraction 

 of some soluble materials from the original rock. The metamor- 

 phism may be said to be largely chemical. 



By these and similar processes fragment al deposits are solidified 

 into firm rock, and undergo internal changes which reorganize 

 more or less the matter of which they are composed. The process 

 is a very slow one usually, and takes place much more slowly under 

 some conditions than under others. Some of the sands and muds 

 of very early geologic ages are even now imperfectly solidified. 



