332 Transactions. 



them, the calcium and magnesium going towards the formation of the 

 one, the alkalies and alumina to the other mineral. 



The internal decomposition of the augite seems in no way associated 

 with the formation of uralite on its margins. The border of uralite is 

 quite distinct, and shows no gradation towards the products of decom- 

 position in the interior of the crystals. 



Th* Fringe of Uralite. — Dana says in his " System of Mineralogy," 

 " The crystals, when distinct, retain the form of the original mineral, 

 but have the cleavage of amphibole. The change usually commences on 

 the surface, transforming the outer layer into an aggregation of slender 

 amphibole prisms, parallel in position to each other and to the parent 

 pyroxene. When the change is complete the entire crystal is made up of 

 a bundle of amphibole needles or fibres." 



Present Observations. — The fringe of uralite varies in width with the 

 size of the crystal, indicating that the amount of change varies as the 

 surface exposed. Usually, however, it is noticed that the change has 

 taken place more rapidly in the direction of the vertical axis than in 

 the other directions, for the fringe bordering the terminations of the 

 crystals is wider than that bordering prismatic faces. 



Statements about the parallel arrangement of the prisms of uralite 

 do not find support in an examination of the sections of the Green Point 

 crystals, for the fibres are seen to be arranged in radiating groups which 

 show 7 no signs of systematic arrangement. Between crossed nicols some 

 of these groups, or parts of the groups, are extinguished, while other 

 groups are not. Each fibre extinguishes at an angle that varies from 

 15° to 18° to the direction of the longest axis, so that a dark wave 

 traverses the group as the nicols are rotated. 



In sections parallel to the clinopinacoid the groups commonly make 

 an angle of 45° with the edge of the crystal, measured either in a + 

 or — direction. In sections parallel to the macropinacoid and to the 

 base these groups are commonly parallel, and the fibres show straight 

 extinction. This seems to indicate that the fibres are arranged in 

 fan-shaped aggregates parallel to the clinopinacoid, and making angles 

 of about 45° with the macropinacoid. 



Rosenbusch* states that the fibres are parallel, and that the vertical 

 axis is the same in the parent mineral as in the new one. Also, that in 

 the case of a twinned crystal the fibres of uralite stand in twinned 

 position on opposite sides of the twinning-plane. With regard to this 

 latter statement, an occurrence in one section shows quite a different 

 state of affairs. The twinning-plane is distinct enough in the augite, 

 but disappears completely on the verge of the uralite friuge. 



Chemical Changes. — So far as present knowledge goes, the composi- 

 tion of uralite is believed to conform nearly to that of actinolite. The 

 most prominent change in passing from the original pyroxene is that 

 corresponding to the difference existing between the two species in 

 general — that is, an increase in the amount of magnesia and a decrease 

 in that of calcium. Analyses of the Bluff minerals are compared below 

 with results given in Dana's " System of Mineralogy." In the absence 

 of other means of separating the materials the following process was 



* Rosenbusch-Iddings, " Microscopical Physiography of the Rock-making Minerals," 

 4th ed. (1900), p. 271. 



