292 MATERIALS AND THEIR ARRANGEMENT 



boring rock, and the changes effected by a given mass of lava are 

 more considerable. Furthermore, the time during which the 

 adjacent rock is hot, and therefore the time during which thermal 

 waters are operative, is usually longer than in the case of extruded 

 lavas, and the chemical and crystalline changes are greater. The 

 changes are greater the greater the mass of the lava and the higher 

 its temperature. 



In limestones and sandstones the changes are simple, and in 

 shales more complex. In pure limestones and dolomites little chemi- 

 cal change takes place, but the molecules are rearranged into larger 

 crystals, making marble. The coarseness of the crystals is a rough 

 sort of measure of the length of time during which the heat acts, 

 and of its intensity; but much depends on the freedom of the attend- 

 ant water circulation. If impurities, as silica, alumina, iron, etc., 

 were present in the limestone, various silicate minerals may be 

 formed in the marble. In pure quartzose sandstones, the effect is 

 to bring about more complete cementation, converting the sandstone 

 into quartzite (p. 285). Here, as in marbles, impurities form adventi- 

 tious crystals. In shales, the material to be acted upon is more 

 complex, for, while the main mass is composed of hydrous aluminum 

 silicate, there is usually much free quartz, and in many cases some 

 potash, soda, iron, compounds of calcium, magnesium, etc., for many 

 muds from which shales arise contain not only the fully decom- 

 posed matter of the original crystalline rocks, but some fine matter 

 worn from them by wind and water without decomposition. When 

 this mixed matter is acted upon by high heat and moisture, it tends 

 to return to its original crystalline state, so far as its changed com- 

 position permits. The result is the development of complex sili- 

 cates, similar to those of igneous rocks, such as feldspar, mica, 

 hornblende, etc. Mica schists are common products of the meta- 

 morphism of shales by contact with bodies of lava. Mica schists 

 also are formed in other ways, and other schists, dependent on the 

 composition of the shales, are formed about intrusions of igneous 

 rock. In all such cases pressure probably attends the heat, and is 

 a factor in the development of the schists. When the change in- 

 duced by the heat is less considerable, the shale is baked, with 

 incipient re-crystallization, and may take the form of argillite, a 

 compact, massive sort of shale. 



Beds of hydrous iron oxide (limonite) or of iron carbonate (sider- 

 ite) may be converted by heat into hematite or magnetite (p. 255). 



