62 E. 8. Dana — Barium Sulphate from Perkintf Mill. 



ter; this is made the clinopinaeoid, g x (010). The measured 

 angles were pti = 102° to 103°, pg l = tig 1 = 90°. Further a 

 polysynthetic twinning is described, resembling the triclinic 

 feldspars, the face ti being the twinning plane and composition 

 face; "macles par interpenetration " are also mentioned, but 

 not minutely described. 



Optically, one axis of elasticity is normal to g\ this is taken 

 as probably the bisectrix, while the axial plane makes angles of 

 134° and 124° with ti and p respectively in their obtuse angle ; 

 in other words the axes of elasticity in g 1 make angles of 5° 

 with the diagonals of the section formed by ti and p. The re- 

 fractive indices are given, but their values are very nearly iden- 

 tical with those accepted for barite. The S23ecific gravity (4*39) 

 and composition are those of barite. 



In conclusion, therefore, we find that the monoclinic nature 

 of the Perkins' Mill barium sulphate rests upon the following 

 grounds : the difference in character between the two cleavages, 

 ti and p\ the presence of enclosed twinning lamellae seen in 

 sections parallel to g 1 ; and the deviation of the axes of elas 

 ticity of about 5° from the diagonals of the rhombic section 

 formed by the two cleavages ti and p. 



Through the kindness of Mr. G. Christian Hoffmann, of the 

 Canadian Geological Survey, to whom we are indebted for 

 many important contributions to our knowledge of Canadian 

 minerals, the writer has received a considerable supply of 

 specimens (upwards of fifty) of the barium sulphate from Per- 

 kins' Mill. The spot from which these were obtained corre- 

 sponds in position with that described by Lacroix, and the 

 individual, who furnished Mr. Hoffmann with the specimens, 

 remembered having given others from the same place to a 

 French gentleman (whose name he did not recall) in the sum- 

 mer of 1888. 



Upon the first examination of the specimens the observer is 

 struck with the beautiful pearly luster of one of the cleavage 

 surfaces, corresponding to the face called ti (100) by Lacroix. 

 The mineral occurs in cleavable forms only, and varies all the 

 way from those which show cleavage surfaces of several square 

 inches to those which are coarsely granular only. Portions of 

 the enclosing granular crystalline limestone are often seen. 



Attention was directed first to the most important variation 

 from the requirements of the orthorhombic system reported, 

 viz : the position of the axes of elasticity in the cleavage section, 

 which is normal to the two oblique cleavages (prism of barite), 

 and parallel to the face g 1 of Lacroix (base of barite). A large 

 number of carefully selected cleavage fragments were taken, 

 upwards of thirty from a dozen or more different specimens. 

 These failed, however, to confirm the measurements of Lacroix. 



