168 PERIDOTITE. 



In the terrestrial peridotites we commence with dunitc — a clear granular 

 mass of fissured olivine grains, which are either colorless or slightly tinged 

 yellow or green. Sprinkled through the olivine mass are dark to brownish, 

 opaque to translucent, grains and crystals of chromite or picotite, and mag- 

 netite, as well as sometimes a few enstatite plates, either colorless or of a 

 greenish tinge (Plate IV. figure 2). 



In these a gradual change to serpentine begins by its production along 

 the fissures of the olivine, forming a yellowish or greenish network, and this 

 change goes on until the olivine is completely altered, and only the network 

 structure remains (Plate IV. figures 3, 4 ; Plate V. figures 1, 2, 4 ; Plate 

 VI. figure 2). Again the change extends so far that not even this trace 

 of the original structure remains (Plate VI. figures 5, 6; Plate VII. figure 2). 

 In the process of alteration to serpentine, that formed first along the fissures 

 generally takes a different structure and color from that later produced by 

 the alteration of the interior portion of the olivine grains — thus showing 

 two, and sometimes three, stages in the progress of alteration. While the 

 mode of alteration is thus consj)icuous in the earlier stages, the final result 

 is to produce a pure, clear, homogeneous serpentine, of a uniform ^^ellowish 

 or greenish tint, or even colorless ; in which no trace remains of the original 

 structure, to tell its derivation. The proof of these changes is found in fol- 

 lowing out the various gradations in the different peridotites, particularly 

 in different portions of the same rock-mass. 



By the gradual increase in the amount of enstatite present, we pass into 

 the succeeding variety — saxonite, and this again, by the addition of diallage, 

 gradually passes into the Iherzolite variety, and this into the succeeding form 

 (buchnerite) by the addition of augite. Again, we pass gradually to those 

 forms in which the enstatite has disappeared, and only diallage (cid^sitc) or 

 augite {picrite) remains with the olivine to form the rock. Every gradation 

 exists between these forms, and they are closely allied in physical and 

 chemical characters. 



The enstatite appears as a clear, colorless mineral, or else as one slightly 

 tinged with green. Sometimes it occupies a subordinate portion of the rock, 

 then again it forms the chief part, holding the olivine inclosed in and 

 subordinate to it. The enstatite is sometimes feebly pleochroic, and shows 

 a well-marked longitudinal cleavage ; the development of which, histead of 

 forming a smooth fracture, usually occasions the tearing of a rough line, 

 with stringy fibres extending from one side to the other. Besides the longi- 



