﻿714 PROF. T. G. BONNET AND MISS C. RAISIN ON THE [Nov. 1905, 



of a single mineral — antigorite. Both forms usually afford straight 

 extinction, but it is occasionally oblique, though the angle is small. 

 Thus, either the mineral is dimorphous, or its optical characters 

 have been affected by pressure, or it is really monoclinic. Flakes, 

 however, do occur which, while extinguishing obliquely at low 

 angles, have a slightly-higher index of refraction. These probably 

 represent a form of hornblende. 



(3) That it is doubtful whether any hard-and-fast line can be 

 drawn between the rather fibrous forms of the mineral in the 

 ordinary serpentine-rocks and the mica-like (antigorite) of certain 

 others. 



(4) That the most typical antigorite occurs when the rock has 

 been considerably affected by pressure, but that it becomes smaller 

 and rather less typical when the pressure has been very great (that 

 is, in the most slaty serpentines). 



(5) That the 'gestrickte struktur ' of the antigorite, so far 

 as it exists (and it is mostly subjective), has no connection with the 

 nearly-rectangular prismatic original cleavage of augito ; the latter 

 structure, curiously enough, being worse preserved than any other 

 in the process of serpentinization. Typical antigorite, however, 

 appears to be produced rather more readily from augite than from 

 the allied ferromagnesian silicates, but its characteristic form is 

 more directly a consequence of pressure ' than of chemical com- 

 position, or, in other words, when this has acted, the mineral 

 serpentine occurs in mica-like plates, instead of in rather irregular 

 or flame-like flakes. 



EXPLANATION OF PLATE XLV. 



Fig. 1. Diaraantiferous breccia ('Hard Blue') from Kimberley (specimen 

 obtained by Sir J. B. Stone in 1894). (a) unchanged olivine ; (b) 

 shell of serpentine; (<?) feather-like growth of serpentine. (X 20, 

 crossed nicols.) (See p. 693.) 



2. Band of almost pure augite in a serpentine from north-east of Bonhomme, 



Vosges. (a) augite ; (b) serpentinized olivine (shaded). ( X 20.) (See 

 pp. 697-98.) 



3. Antigorite-serpentine, Sprechenstein, Brenner. From one of the less- 



crushed lenticles, about 7 feet from the edge of the mass, representing 

 the so-called 'thorn-structure.' ( X 25, crossed niculs.) (See p. 702.) 



4. Antigorite-serpentine with puckered foliation : (a) antigorite. Of the 



non-flaky materials, the very dark (b) are iron-oxide (mostly magnetite) 

 and the dusky (c) residual augite. ( X 25, crossed nicols.) (See p. 704.) 



5. Antigorite-serpentine, from a boulder with a slightly-streaky structure 



on the right moraine of the Fee Glacier, Saas Fee, consisting largely 

 of augite. (a) antigorite ; (6) augite, represented not only by the 

 granular clear mineral, but also by much of the dusty part not showing 

 a mica-like structure ; (c) magnetite. ( x 25.) (See p. 708.) 



6. Antigorite-serpentine, from the summit of the Biffelhorn, Zermatt. 



(a) antigorite; (b) residual augite; (0) magnetite. (X 25, crossed 

 nicols.) (See p. 709.) 



1 F. Becke called attention to the connection of antigorite with pressure (as 

 already stated) in 1894; but our conclusions were formed independently, as 

 one of us had noticed it some time before that date. 



