THE TEEEESTEIAL PERIDOTITES. — DUNITE. 119 



Webster, North Carolina. 



5135. A crystalline-granular rock, of a yellowish-bro^-n color on the fresh fracture. 

 Lustre resinous and greasy, fracture uneven-conchoidal. Weathered to a granular pale- 

 yellow mass, on the exterior portions. Contains grains and crystals of picotite. 



Section : of a pale-yellow color ; composed of olivine, enstatite, diallage, picotite, and 

 serpentine. The olivine forms the chief portion of the rock, and is in irregular fissured 

 grains. It is clear-transparent, and holds grains of picotite, some of which are in minute 

 lenticular forms. The picotite is very abundant, but mostly in minute microscopic 

 grains of a coftee-brown color. The macroscopic picotites are opaque, except in the thin- 

 nest portions. 



The enstatite and diallage are in small, transparent, irregular masses, lying between the 

 olivine grains. Both are traversed by longitudinal fissures, but in general the enstatite 

 cleavage is better marked aud more finely fibrous than that of tlie diallage. The latter 

 mineral was observed sometimes to have the irregular, approximately right-angled cleav- 

 age of augite. Although the enstatite could sometimes be separated from the diallage by 

 its cleavage, in general the distinction was made solely by optical metliods. 



The serpentine is mainly of a pale-yellowish color, althougli in some places a darker 

 or brownish color was observed. It follows the fissures, making a network, envelop- 

 ing the fragments of olivine, enstatite, and diaUage. Many of the olivine grains now 

 separated by Serpentine are seen by their optical characters to be parts of the same 

 original crystallographic mass. Tlie serpentine is plainly a secondary product, formed 

 from the alteration of part of the original minerals, comprising this peridotite. In some 

 parts of the section in which the mineral fragments were small, the minerals have been 

 changed entirely to serpentine, forming ganglion-like masses in this plexus of serpentine. 

 The serpentine shows the common fibrous polarization, the fibres standing perjDendicular 

 to the walls of the channel. 



Plate IV., figure 3, shows well the yellowish and greenish serpentine alteration along 

 the fissures, and surrounding the clear olivine grains. The brown spots are picotite 

 grains. 



Dr. r. A. Genth, in 1862, made an examination of some Webster (Jackson Co., N. C.) 

 peridotite, and stated that they gave " evidence that chrysolite is probably the mineral 

 from which talc .slate and many of the serpentines have been formed." * 



Dr. Alexis A. Julien regards the North Carolina peridotite as formed by consolidated 

 olivine sand — a detrital deposit derived from the wearing down of older eruptive rocks. 

 He describes tlie rock as occurring in long lenticular masses, that show a laminated struc- 

 ture ; giving his reasons why he regards this lamination as due to the sorting of sediments 

 deposited in water. His reasons for holding that the dunite is a sedimentary rock are 

 good so far as they go, but they do not appear to be conclusive ; since the same condition 

 of things could readily exist in an eruptive rock. The rock, when altered near the surface 

 of disintegration, is, according to Julien, bound together by a network of quartz or actino- 

 lite fibres. 



The alterations in the rock-mass, as traced out by Dr. Julien, are very interesting. 

 Briefly, they are as foUo^vs : 1. Chalcedonic ; 2. Hornblendic ; 3. Talcose ; 4. OphioUtic ; 

 5. Dioritic. 



In the first, the silicates are decomposed, the silica forming chalcedonj' or chert, while 

 the bases remain as soft ochreous grains, or are entirely removed. 



* Am. Jour. Sci., 1863 (2), xxsiil. 199-203. 



