Gneiss. 



Mica-schist. 



Chlorite-schist. 



Talcose-schist. 



Hornblende-schist. 



Clay-slate. 



Quartzite. 



Marble. 



Serpentine. 



METAMORPHIC ROCKS. 225 



Kinds. The adjoining table shows the principal kinds: 

 Gneiss is a rock having much the 

 appearance and mineral composition 

 of granite i. e., quartz, feldspar, 

 mica, and hornblende- differing 

 only in a bedded structure. In many 

 places, as, for example, on Manhattan 

 Island, gneiss can be traced by in- 

 sensible gradations into granite. 

 Schists are rocks having a fissile 

 structure through the abundant 

 presence of scales of some kind. In mica-schist they 

 are mica ; in the other schists they are chlorite, or talc, 

 or hornblende. c 



Quartzite and Marble are both white, crystalline, or 

 granular rocks, looking like loaf-sugar ; but in the one 

 case the granules are quartz, in the other, lime-carbonate. 

 Serpentine is a greenish rock, having usually a schistose 

 structure and a greasy feel like talc. It contains a nota- 

 ble quantity of magnesia. 



Origin of these Kinds. Metamorphic rocks are prob- 

 ably changed sandstones, limestones, and clays, and 

 mixtures of these. The infinite variety which we find is 

 the result partly of the original kind and partly of the 

 degree of change. For example, sandstones and lime- 

 stones are often perfectly pure. Now, a metamorphic 

 pure sandstone is quartzite, and a metamorphic pure lime- 

 stone is marble. But clays are nearly always impure, 

 being mixed with sand and lime and iron and other bases. 

 A moderately pure clay with a little sand by metamor- 

 phosis makes gneiss or mica-schist. If it contains much 

 iron, it makes a hornblende-schist ; if magnesia, talcose- 

 schist or serpentine. Serpentine is, however, often a 

 changed eruptive rock. 



LB CONTE, GEOL. 15 



