HEAT — METAMORPHISM. 313 



A trap dike intersecting the clayey layers, sandstones, and coal-beds of 

 the island of Nobby, near Newcastle, New South Wales, has baked the clayey 

 layers to a flint-like rock to a distance of 200 yards from the dike, the whole 

 length of the island. (D., 1849.) 



In the Spanish Peaks region, southeastern Colorado, the injection of ig- 

 neous rocks across coal-beds has produced, according to R. C. Hills, a dense 

 natural coke or an impure powdery graphite. The outcrop of coke thus 

 made near Trinidad is probably two miles long; and at other places similar 

 outcrops are four to five miles in length. 



A region of igneous eruptions is often also, as a consequent or concurrent 

 fact, a region of steaming fissures and of hot springs, conveying the heated 

 moisture widely through the strata of the region ; and in this way probably 

 the sand-beds of the Mesozoic formations of eastern America were generally 

 reddened as well as consolidated. 



Baking effects, and sometimes crystallizations, have been occasioned by 

 the burning of coal-beds. (See page 266.) 



In the Tyrol, near Monzoni and Predazzo, a Peruvian limestone, in the vicinity of 

 masses of igneous rocks, has been crystallized, and near the contacts occur garnet, ido- 

 crase, gehlenite, epidote, spinel, mica, anorthite, magnetite, hematite, and apatite. (Dcel- 

 ter, 1875.) In the White Mountains, near Crawford's, alongside of granite, an argillitic 

 mica schist is much altered and penetrated with crystals of orthoclase and tourmaline. 

 (Hawes, 1881.) 



These examples of alteration illustrate not only local but also regional 

 metamorphism, for the minerals formed are among those that figure exten- 

 sively in metamorphic rocks. Chlorite, garnet, tourmaline, are among the 

 most common of such minerals ; and if these and other species can be made 

 under the rather rapid and coarse conditions afforded by the eruption of an 

 igneous rock, the results of slow-working metamorphism should be much more 

 complete. 



It is observed, also, that these minerals are made by selecting and 

 combining the needed elements. The iron of the epidote, chlorite, garnet, 

 tourmaline, must be the iron that gives the red color almost everywhere else 

 to the enclosing rock, or is present in occasional grains of magnetite. The 

 tourmaline crystals seem to show that marine waters (or, perhaps, borate 

 springs, made earlier from the ocean's waters) may supply boracic acid which 

 they require. The hematite crystals (FejOg) may be derived from dissemi- 

 nated red hematite coloring the rock, or from the oxidation of grains of 

 magnetite (Fe304) . The quartz crystals were made out of silica taken from 

 the siliceous minerals (feldspar, etc.) that were decomposed by the steam to 

 furnish material for the new crystallizations ; and the heat, as far as it 

 reached through the sand-beds, even if of low degree, in the same way made 

 the siliceous solutions that produced the consolidation of the rocks adjoining. 



Special metamorphic power is often attributed to granite in the dike-like condition, 

 and the minerals in the rock adjoining are regarded as contact minerals when the granite 



