Wild. — Geology of the Bluff. 331 



Nevertheless, since perhaps the majority of authorities give pro- 

 minence in their definition of this mineral to a fibrous or acicular 

 structure, this distinction will be observed in the present paper, and 

 the definition adopted here is as follows : Uralite, pale-green slightly 

 pleochroic fibrous variety of hornblende, derived from pyroxene. The 

 hornblende of the norite, therefore, though of secondary origin, is not 

 in this paper referred to as uralite, for it is a compact variety, rather 

 dark in colour, and strongly pleochroic. 



Uralite was first described by Gustav Rose from a green porphyritic 

 rock at Mostovaya, near Ekaterinburg, and at Kaminskaya, near Miask, 

 in the Ural Mountains. It has since been observed from many localities. 

 The microscopical study of rocks has shown the process of " uralitiza- 

 tion " to be very common, and some authors regard many hornblendic 

 rocks and schists to represent altered pyroxene rocks on a large scale. 



The crystals obtained from the dyke at Green Point afford abundant 

 and excellent material for the investigation of the changes that take place 

 when augite is converted into uralite. These crystals are usually short 

 and stout, and show an equal development of the unit prisms (110), 

 tin' orthopinacoids (100), and the clinopinacoids (010), while the usual 

 terminal faces, the plus pyramids (111), are also perfectly formed. 

 Twinned forms, with the orthopinacoid (100) as twinning and composi- 

 tion plane, are also quite common. 



The results obtained from an examination of sections of some of these 

 crystals cut in various directions will now be given. 



The Core of Augite : The internal core of augite is colourless, except 

 where recrystallization has commenced. 



Cleavage : The usual cleavage-lines are not very distinct in sections 

 in the zone of the prisms, though they are seen well enough in cross- 

 sections. What is very distinct in sections parallel to the ortho- and 

 clino-pinacoids is a series of parallel lines which intersect the cleavage- 

 lines at angles approximating 70° in sections parallel to the clinopina- 

 coid, and at right angles in sections parallel to the orthopinacoid. 

 These lines thus represent a series of parting-planes parallel to the 

 base (001), a not uncommon feature in augite. 



Refractive Index : A rough surface in polarized light indicates the 

 usual high value. 



Pleochroisin : Not noticeable. 



Crossed nicols : 



Interference colours : Bright tints of second order. 

 Extinction : In sections || a (100) - 37°. 



b (010) = 0°. 



Alteration- products tvithin the Mineral. — These are feldspar and 

 hornblende in about equal amount and a little olivine. The decom- 

 position begins at points on the cleavage-lines and proceeds most rapidly 

 in the direction of them. The hornblende is dark green and strongly 

 pleochroic. It extinguishes when the cleavage-lines of the augite arc 

 parallel to the vibration-directions of the nicols. The feldspar extin- 

 guishes at small angles. Olivine occurs in small grains; it is very 

 rare. There is no trace of calcite, epidote, or chlorite, minerals that 

 are commonly reported as associated with such changes as are here 

 described. The fact that hornblende and feldspar are always associated 

 as decomposition-products in the interior of the crystals suggests that 

 the material derived from the decomposing augite is divided between 



