52 GEOLOGY 



the free aid of water. In these cases the chief effects are chemical 

 recombination and crystallization. In the limestones and sand- 

 stones the changes are simple, and in the shales more complex. 

 In pure limestones and dolomites little chemical change takes place, 

 but the molecules are rearranged into larger and more perfect 

 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 

 attendant water circulation. Crystals an inch or two across are 

 sometimes formed in the zone of contact between the intruded 

 lava and the limestone where the attendant water action is im- 

 portant. If impurities, as silica, alumina, iron, etc., were present 

 in the limestone, various silicate minerals (such as tremolitc and 

 actinoliie, p. 74) may be formed in the marble. In pure quartzose 

 sandstones, the effect is to cause the building up of the quartz 

 grains until the interspaces are essentially filled, and the whole 

 becomes a massive quartzite. Here, as in the marbles, impur- 

 ities form adventious crystals, a very common one being 

 hematite formed from the segregation of the ferric oxide of the 

 sandstone. 



In the 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 often some potash, soda, 

 iron, compounds of calcium, magnesium, etc., for the muds from 

 which shales arise frequently 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 

 composition permits. The result is the development of complex 

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

 hornblende, etc. There is usually a predisposition to form mica 

 in preference to other silicates if the proper const it units arc present, 

 and the result is that mica schists are common products of the 

 metamorphism of shales by contact with bodies of lava. Mica 

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

 on the composition of the shales, are formed about intrusions 



