In the typical peridotites the varieties present are enstatite and hron/ite. 

 These minerals usually occur without any definite crystalline outline. When 

 cleavage cracks are present those parallel with the prism faces of the typical 

 form, intersecting at angles of about 92 and 88, may be readily detected in 

 the unaltered mineral. Sections at light angles to these cleavage planes 

 (cross sections of the prism) extinguish, in parallel rays of polarized light, 

 when the short axes of the nicols bisect the angles formed by the traces of the 

 cleavage planes, and give a positive bisectrix in convergent polarized light. (1) 

 Tn addition to the prismatic cleavages one frequently observes a cleavage 

 parallel to the brachypinacoid (010), and this is especially evident in the 

 altered forms. 



The commencement of alteration is seen in a fine striatioii parallel with 

 the vertical axis. This striation frequently begins in the neighbourhood of 

 transverse cracks which traverse the crystal and finally affects the 

 entire mass. The development of this striation is accompanied by 

 the formation of a perfect cleavage parallel to 010, and by the 

 entire or partial obliteration of the prismatic cleavages which are usually 

 so characteristic of the unaltered mineral. It is also accompanied by the 

 addition of water and by a change in the ellipsoid of elasticity. Tn the 

 unaltered or slightly enstatite or bronzite the optic axial plane lies in the 

 plane of easiest pinaeoidal cleavages (010) ; in the modified enstatite or 

 bronzite (bastite) it lies in the plane of the macropinacoid (100). This 

 furnishes a ready method of distinguishing bronzite with cleavage parallel to 

 (010) from bastite. A cleavage flake of the latter mineral which may be 

 detached with a pen-knife shows a negative bisectrix in convergent polarized 

 light ; whereas a corresponding flake of the former mineral gives no definite 

 optic picture. (2) 



The striation above referred to is due to a fine fibrous structure. The 

 long axes of the fibres are approximately parallel with the vertical axis of 

 the prism. A further stage of alteration is represented by the formation of 

 serpentine, in which the fine fibrous structure is often more or less 

 presented (see fig. 2, plate I.) 



Monoclinic Pyroxenes. Several varieties occur in this group of 

 rocks. One of the most interesting is the green augite (chrome- 

 diopside) of the Iherzolites, the composition of which was first 

 determined by DAMOUK. A similar augite occurs in the divine-nodules in 

 certain basalts, and in the picrite of the Schwarze Stein, near Dillgegend, in 

 Nassau. Professor Jriw describes an augite which is bright green by 

 reflected light, and pale green by transmitted light, in the picrites and rocks 

 associated with the picrites on the flanks and summits of Halival, Haiskeval 

 and Tralival, in the island of Rum. Dr. HEBDLE has analysed this green 

 augite, and his analysis shows a close relation between this mineral and the 



(1) Corresponding cross sections of a monoclinic pyroxene show an optic axis. Hence a 

 ready method of distinguishing- between these two minerals. 



(2) The only other mineral that may be confused with bastite and bronzite is diallage. 

 This may be instantly determined by the examination of a minute cleavage flake in convergent 

 light, when an optic axis will be seen, 



