756 DYNAMICAL GEOLOGY. 



2. Loss of Water or other Vaporizable Ingredients. — The water 

 contained in the original material of a rock is sometimes wholly, and 

 sometimes but partly, expelled. Gneiss, granite, and mica schist, are 

 examples of the former ; hydromica schist, chlorite schist, and serpen- 

 tine, of the latter. The volatile portions of bituminous coal have been 

 wholly or partly driven off* by the process, and anthracite and semi- 

 bituminous coal formed (p. 400). 



Carbonic Acid is expelled from carbonate of lime, or limestone, as 

 is well known, in a heated lime-kiln. But, in the metamorphism of 

 limestone, it is retained. It has been shown by experiment that the 

 carbonic acid is not given out, if the material is under heavy pressure. 

 If this be true of carbonic acid, it will be so also of other ingredients 

 less easily expelled. 



3. Loss of Material by Chemical Solvents. — Limestones suffer most 

 in this way, often losing much of their thickness, as recognized by 

 Lyell, and, since the foreign ingredients are mostly left behind, the 

 beds become by the process increasingly impure. 



4. Change of Color. — Rocks of black, gray, and other colors, have 

 been altered to white, grayish-white, and clouded gray, as in the change 

 of limestones to marble; those of brown and yellow, and sometimes 

 black kinds, to red ; those of green, brown, and other shades, to black. 



5. Obliteration of Fossils. — This fact has already been illustrated 

 by examples. In the progressing obliteration, the fossil generally be- 

 comes flattened or distorted, and indefinite in outline, and finally is re- 

 duced to a patch of white crystalline limestone. 



6. Crystallization without a Change in the Combinations present. — 

 The Carrara marble, the marble of Rutland, and other pure crystalline 

 limestones, have undergone in their metamorphism, no change of com- 

 position, but only that of crystallization. The same is true of the 

 purer crystalline magnesian limestones, or dolomite. Some sandstones 

 are made up of pulverized granite or gneiss ; and if changed to granite 

 or gneiss by metamorphism, as has often happened in such cases, the 

 process would be simply one of crystallization. The steps and the re- 

 sult are analogous to those in the tempering of steel (p. 627), where 

 the crystalline texture is changed from coarse to fine or the reverse. 

 The beds of hematite; (Fe 2 3 ) of Archaean rocks may be simply the 

 dull-looking earthy hematite beds of sedimentary formations crystal- 

 lized. 



In the metamorphism of common limestone, another change takes 

 place besides that in the texture, which at the same time is not a 

 change in the elements present, although it is one in the mineral nature 

 of the material. Ordinary compact limestone, being made from pul- 

 verized shells, corals, or other organic secretions, has its calcium car- 



