SCHISTS AND FOLIATED ROCKS. 



the Laurentian gneiss rocks of Loch Maree in Ross, and the struc- 

 ture also appears wherever intrusive rocks come in contact with lime- 

 stones ; but the most familiar example is furnished by the Carara marble 

 of Italy, which is probably of Carboniferous age. When sand becomes 

 converted into quartzite, the grains of sand are more or less obliterated 

 and blended together, as grains of sago might blend when cooked. 

 The fracture of the rock, when the sand was fine, often resembles that 

 of horn, hence these altered rocks are often called hornstones, but 

 exposure to the weather usually develops on the surface a laminated 

 structure. Here the temperature may not have been very high, since 

 heated water has considerable power of dissolving silica, and all rocks 

 contain more or less water, the temperature of which may be raised 

 by pressure. Schists are rocks 

 which consist chiefly of fine 

 layers of crystalline quartz, ar- 

 ranged in short irregular parallel 

 films, which are separated by 

 films of some other mineral. Thus 

 mica schist consists of quartz and 

 mica; talc schist is quartz and 

 talc ; chlorite schist is quartz and 

 chlorite ; hornblende schist is 

 quartz and hornblende. There are 

 a few other schistose rocks, the 

 most important being gneiss, 

 which is made up of the same 

 minerals as granite, quartz, felspar, 

 and mica, only the minerals are arranged in parallel layers. All these 

 rocks contain various other minerals in small quantities, but schists 

 especially abound in garnets. The arrangement of the minerals in 

 parallel layers was 

 named by Mr. Dar- 

 win foliation. These 

 rocks were originally ^sT~-< 

 all fine sandstones ^c 



C^WTiC 



^ ^t\03Sfei^,(Rj-J- 



^MKJX<n\\ 



Fig. 4. Mode of Occurrence of Schists in 

 Scotland. 



and sandy clays ; and 

 under the influence 

 and action of heat, 



Fig. 5. Contorted Schist. 



pressure, and the 

 water contained in 

 the rock,the chemical 

 substances have re- 

 combined into the minerals already mentioned, being arranged in 

 layers, which do not usually correspond with the layers in which the 

 rock was deposited. The schistose rocks are always crumpled and 

 contorted, and commonly occur on the flanks of the older mountain 



**J 9 



Slates were clays, often more or less sandy and micaceous, which 



