THE ORIGIN AND DESCENT OF ROCKS. 447 



strata that lie beneath the surface, and bring to bear upon them intense 

 heat for a long period. So also, when a vent or fissure is the passage- 

 way for lavas that continue to come to the surface for long periods, as 

 in the case of persistent volcanoes, the rocks which form the walls of the 

 vent or fissure are heated for a long time, and this gives rise to metamor- 

 phism through heat, without very unusual pressure, but usually with 

 the free aid of w^ater. In these cases the chief effect is chemical recom- 

 bination and crystaUization. In the hmestones and sandstones it is 

 simple; 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, and marble is the result. The 

 coarseness of the crystals is, in a general way, a 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 contact zone, where the attend- 

 ant water action is important. If impurities, as silica, alumina, iron^ 

 etc., are present, various minerals, such as tremolite and adinolite, 

 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, impurities form adventitious 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 an aluminum silicate, there is usually much free quartz, 

 not a httle potash and iron, and more or less of lime, magnesia, soda, and 

 other ingredients, for the muds from which the shales arose "contained 

 not only the fully decomposed matter of the original crystalhne rocks, 

 but the 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 constitution permits. The potash chiefly unites with 

 alumina and sihca, and forms potash feldspar (orthoclase chiefly) and 

 potash mica (muscovite). The iron often unites with magnesia, alumina, 

 and silica to form biotite or one of the ferromagnesian minerals, chiefly 

 an amphibole. The lime usually aids in the formation of other siHcates 

 of either the feldspar or the ferromagnesian group, while the surplus 

 siHca crystaUizes into quartz. There is usually a predisposition to form 



