HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 531 



only those which have originated by a series of chemical changes known 

 as metasomatosis, a process of indefinite substitution and replacement, 

 in simple mineral aggregates occurring associated with the older uieta- 

 morphic rocks. Such are the serpentines derived from nonalumiuous 

 pyroxenes, like those of Montville, New Jersey (39038), and Moriah, 

 New York (70084), and those from Easton, Pennsylvania, derived from 

 a massive tremolite rock (70109). 



Several varieties of serpentine are popularly recognized. Precious 

 or noble serpentine is simply a very pure compact variety of a deep oil- 

 yellow or green color (39038). Amianthus, or chrysolite, as noted above, 

 is the name given to the fibrous variety(37645). WUUamsite is a deep 

 bright green, translucent, and somewhat scaly granular variety, occur- 

 ing associated with the chrome iron deposits in Fulton Township, Lan- 

 caster County, Pennsylvania (36041). D.eweylite is a hard, translucent 

 variety occurring in veins in altered dunite beds. Bowenite is a pale 

 green variety forming veins in limestone at Smithfield, Rhode Island 

 (36763). Picrolite, Marmolite, and Retinolite, are varieties of minor im- 

 portance, and may be found in the mineral collections. Serpentine 

 alone, or associated with.calcite and dolomite, forms a beautiful marble, 

 to which the names verdautique, ophite, and ophiolite are given. (See 

 Building and Ornamental stones.) The name serpentine is from the 

 Latin serpentinus, a serpent, in allusion to its green color and often mot- 

 tled appearance. The so-called " Eozoon Canadeuse," a supposed fos- 

 sil rhizopod, is a mixture of serpentine and calcite or dolomite (speci- 

 men 70434 from Cote St. Pierre, Canada). Those serpentines which 

 were derived from basic eruptives or complex metamorphic rocks are 

 exhibited with those rocks with which, in their unaltered state, they 

 would naturally be g:ouped. 



The mineral steatite or talc, when pure, differs from serpentine in 

 containing 63.5 per cent, of silica to 31.7 percent, magnesia and 4.8 per 

 cent, of water. Its common form is that of white or greenish inelastic 

 scales, forming an essential constituent of the talcose schists. As is 

 the case with serpentiue it sometimes results from the alteration of 

 eruptive magnesian rocks such as the pyroxenites. 



Here are exhibited but a few forms occurring in veins or masses 

 indicative of an origin by chemical deposition. Specimen 39088 is 

 a compact variety occurring in the form of veins in the limestones 

 of Thomastou and Eockland, Maine. Nos. 36135 and 27654 are schis- 

 tose forms from New York and North Carolina. Rensselaerite is a re- 

 lated variety from St. Lawrence County, New York (specimen 36117). 

 Pyrophyllite or agalmatolite is a hydrous silicate of alumina, somewhat 

 harder than talc, and extensively used in making slate pencils and 

 small images (specimen 37812 from Mexico and 27562 from Japan). 



Kaolin, also a hydrous silicate of alumina, is a chemical product in 

 that it is a residue left by the chemical decomposition of the feldspars. 

 These minerals, as explained elsewhere, consist of silicates of alumina, 



