I'M VCHE, | 



LherzQlite- Serpentine. 



on account of their brilliant cleavage faces. Limonite is also pres- 

 ent in varying amount, sometimes invading the whole mass of the 

 rock, rendering it gray or brown in color, and quite opaque, w ith a 

 decidedly porcellanous aspect. Under the microscope all these 

 varieties of the serpentine have a similar aspect, and are also iden- 

 tical with the serpentinous groundmass of the massive fades next to 

 be described. Their characters will, therefore, be described under 

 that heading. 



The massive facies is most characteristically exhibited in the 

 spheroidal masses, of which mention has been made, but it is not 

 confined to them, several varieties of it being abundantly developed 

 in other forms. The freshest phase of the rock which occurs in 

 those spheroids least affected by shearing is dense and massive, 

 dark green in color, tough and of uneven fracture. Its specific 

 gravity, determined on several fragments, is 2.66. 



Macroscopically it appears to be chiefly composed of serpentine, 

 homogeneous and translucent in thin splinters, imbedded in which 

 are numerous crystals of pyroxene, with brilliant cleavage faces, and 

 grains of magnetite. 



Under the microscope, serpentine is seen to be the predominating 

 mineral. Imbedded in it are numerous pyroxene crystals of vary- 

 ing dimensions, less abundant grains of olivine, and grains of chro- 

 mite and magnetite. 



The serpentine is colorless in most sections, having a slight tinge 

 of green. Its structure is not apparent in ordinary light, but with 

 crossed nicols it is seen to be a colorless felted aggregate, too fine 

 to be studied as individuals with the microscope. The serpentine 

 aggregate exhibits weak double refraction, the interference colors 

 never surpassing a pale yellow, and being more commonly of gray 

 tints. Between crossed nicols it retains nearly the same tint 

 throughout a complete revolution of the stage, owing to compen- 

 satory extinction of the irregularly oriented serpentine fibers. 

 While a large proportion of the serpentine is thus lacking in any 

 definite arrangement of its component fibers, in places narrow 

 bands, in which the fibers are approximately parallel, are seen to 

 inclose areas of confused orientation. By their mode of arrange- 

 ment these bands give rise to the well-known types of structure, 



