ITS MICROSCOPIC CHARACTERS. IGO 



tudinal cleavage, a cross fracture, at right angles to the principal cleavage, 

 is often present. 



The diallage possesses similar characters to the enstatite, and generally is 

 undistinguishable from the latter except optically, but sometimes it shows two 

 cleavages approaching those of augite, breaking the surface into rough, irregu- 

 lar rhombs, which serve to distinguish the diallage in question from enstatite. 

 Again, diallage has not only its proper longitudinal cleavage, but also the 

 well-marked cleavage of angite. This fact indicates that there is no real dis- 

 tinction between diallage and angite, but both form a continuous series. 



The augite is pale-yellow or brown, shows its characteristic cleavage, and 

 is sometimes feebly pleochroic. It occurs in grains and crystals, and some- 

 times encloses olivine Q:rains. 



Since the olivine, octahedral oxides, and their alteration-products in the 

 varieties of peridotite are like those in the first-described variety, it remains 

 simply to trace the alterations in these varieties as they are modified by 

 the addition of other minerals than olivine. The pyroxene minerals, as 

 a rule, are less liable to alteration than olivine, and are frequently deter- 

 minable after the olivine has been entirely changed. This is indicated in 

 Plate TV. figure 6, and in Plate VI. figures 3 and 4. The enstatite usually 

 shoAVS its alteration by the formation of a greenish product along its cleavage 

 planes (Plate V. figure 3; Plate VII. figure 1). As the alteration progresses, 

 the pyroxene minerals are transformed into a yellowish or grayish serpen- 

 tinous mass, which may show aggregate polarization, or may retain that of 

 the mineral from which it has been derived. 



The various changes in the peridotites can perhaps be best followed from 

 the plates. In Plate IV., figure 1 indicates a typical d unite composed of 

 unchanged olivine grains, with only a cloudiness produced by the fissures by 

 which the mass is traversed. Figure 3 shows another dunite containing 

 brown picotites and exhibiting the formation of greenish and yellowish ser- 

 pentine along the fissures of the olivine, with sometimes a complete serpen- 

 tinization of the interstitial olivine grains. Figure 4 shows a saxonite in 

 which the alteration to greenish serpentine has progressed so far as to leave 

 only a subordinate portion of clear olivine grains untouched. Part of the 

 enstatite has been changed to serpentine, while part is only partially altered, 

 as shown in the upper left-hand portion of the figure. 



In Plate V. figure 1, is shown the commencement of alteration in a Iher- 

 zolite, in which the olivine and pyroxene minerals assume a gray tinuv, ;»ud 



22 



