phalen] the rocks of nugsuaks peninsula ^93 



Another feature of the hornblende is a perfect parting parallel to 

 the plane (loi). This is rather indistinctly shown in plate liv, 2, 

 in the right portion of the figure, and in the same crystal of horn- 

 blende there is shown the dark hornblende associated with the 

 bleached material. This parting is far more perfect than any 

 cleavage possessed by amphibole, not excepting the prismatic. It 

 is present as a series of rather fine, sharp black lines, parallel to 

 each other, and making an angle of 75.8° with the prismatic cleav- 

 age on the brachypinacoid. Though rather rare, the phenomenon 

 is not unique; it is first mentioned by Jermejew,^ and incidentally 

 referred to by Williams^ and Vom Rath." Cross* and Miigge^ have 

 also remarked it. 



In more recent articles, Williams® and Weidmann" have described 

 the phenomenon at some length, and have noted in connection with 

 it twinning lamellae. At times in the sections under discussion a 

 faint white band could be seen between the parting planes. The 

 highest powers, however, failed to resolve any twinning lamellae, 

 and it is probable that none exists in these particular sections. 

 Though it cannot be positively stated that the parting planes are 

 concentrated near fractures and are hence of dynamic origin,- the 

 evidence which points to this view is very strong. As a rule they 

 are concentrated at the boundaries of the hornblende crystals and 

 tend to pinch out toward their centers. If now it is borne in mind 

 that the rock under discussion has been sheared, and that its present 

 crystalline nature is most evidently the result of dynamic meta- 

 morphism, it will not be difficult to assign a similar explanation for 

 the observed parting planes. An analysis of the separated amphibole 

 is as follows : 



Si02 4279 



AI2O3 15-04 



Fe^Os 5-44 



FeO 11.61 



' Neties Jahrbuch f. Min., etc., 1872, p. 405. 



^ Am. Jour. Sci. (3), xxix, 1885, p. 486. 



^ Sitziingsher. d. Niederrh. Gcs. f. Nafiir. u. Heilkundc. July 7, 1886 



* Tschermaks Min. u. Pet. Mitth., iv, p. 386, 1881. 

 ^ Neues Jahrbuch f. Min., etc., 1889, i, 243. 



^ Am. Jour. Sci., xxxix, 1890, p. 352. 



''Am. Jour. Sci. (4), xv, March, 1903, p. 229. 



* Williams {Am. Jour. Sci., xxxix, May, 1890, p. 335) and Weidmann 

 (ibid. (4), XV, March, 1903, p. 230) both assign a secondary, dynamic action 

 as the cause of these gliding planes. 



