HEAT — METAMORPHISM. 319 



chlorite schist or a hornblende schist, and for filling such schists with crystals 

 of the silicates, hornblende, pyroxene, garnet, staurolite, ottrelite ; or if free 

 from iron, it has the elements for making andalusite, sillimanite, cyanite 

 (each Al203Si02), silica and alumina, when no alkali or other bases are free, 

 being ready to form these aluminum silicates. 



Increasing remoteness from a region of crystalline rocks favors the mak- 

 ing of sediments free from alkali, because the alkali becomes leached out of 

 sediments by the transporting waters. This is illustrated a few miles 

 west of New Haven, Conn., where the mica schist gradually changes to 

 the southward, to a chloritic hornblende schist, — hornblende and chlorite, 

 unlike the mica, containing no potash. 



When carbonaceous shales are altered to mica schist, the '' fixed carbon " 

 present (page 315) may become crystallized into graphite; for graphitic 

 mica schists are common. It has been suspected that diamonds, another 

 form of carbon, may have been made in the course of the metamorphic 

 changes of carbonaceous shales or sandstones. 



Again, if a dolomyte, or magnesian limestone, contains some silica finely disseminated 

 through it as impurity, either in the state of quartz or of organic silica (Diatoms, spicules 

 of Sponges), metamorphic action may, while crystallizing the limestone, fill it with bladed 

 or radiating crystallizations of tremoUte (white hornblende) ; for a portion of the dolo- 

 mite (^ Ca^ MgO. COj) might take the silica (SiO^) as a substitute for its carbonic 

 acid (CO2), and thus tremolite (i Ca | MgO. Si02) would result. When the dolomyte 

 contains some iron, as well as the silica, actinolite (green hornblende) may form and in 

 like manner be disseminated through the mass of the rock, instead of tremolite. Under 

 similar circumstances, at a higher temperature, white pyroxene which has the same 

 composition as tremolite, or green pyroxene, which has the composition of actinolite, 

 may be formed in stouter crystallizations. 



If clayey impurities are present in the limestone (these consisting of silica and alu- 

 mina, with or without iron or magnesia), the limestone may become filled with garnets and 

 other silicates. An Eocene limestone, in the Ligurian Apennines, much contorted and in 

 contact with diabase, gabbro, etc., contains crystals of the soda- feldspar alhite ; and inside 

 of the crystals there are the siliceous tests of Radiolarians (genera Ethmospheera, Helios- 

 phsera, and others), suggesting that possibly the silica of the albite was of organic origin. 

 (A. Issel, 1890.) 



Chrysolite consists of silica 41*4, magnesia 50-9, iron protoxide 7-7. In the rocks 

 it is often found changed to serpentine, which consists, in 100 parts, of silica 43-5, 

 magnesia 43-5, water 13. The iron protoxide and some magnesia are here rejected 

 and water received ; and usually the iron stays about or within the serpentine, as a cloud 

 of black grains or a few black crystals of magnetite. So, also, the magnesian silicates, 

 pyroxene, hornblende, chondrodite, chlorite, and other species, occur changed to serpentine. 

 When such a change happens on a large scale, a chrysolite rock, or pyroxenic rock, or 

 hornblendic rock, etc., becomes, in part or wholly, a serpentine rock. In a similar way, 

 pyroxene, or hornblende, or garnet, may be changed to chlorite, or to epidote, etc., labra- 

 dorite, or anorthite (G = 2-7) to saussurite (G = 3 — 3-5). 



The pure amorphous serpentine often has parallel cracks (apparently due to contrac- 

 tion on drying), which are filled with fibrous serpentine (amianthus, or asbestos) ; and 

 when the cracks are very thin and numerous, and are filled with calcite or dolomite, the 

 specimens often have the aspect and general structure of the so-called Eozoon of Archseau 

 rocks. 



