K S. Dana — Barium Sulphati from Perkins* Mill. <'»:; 



On the contrary, the extinction- direction was found to bisect 

 the obtuse and acute angles as exactly as it could be deter- 

 mined; the individual measurements rarely varied more than 

 from 30' to 1" from this position. So tar as the specimens 



under examination go, then, there LS no variation in the position 

 of the axec of elasticity from that required by normal barite. 



The relative values of the axes of elasticity were also found to 

 conform to those of barite, and in a section cut normal to the 

 line bisecting the interior obtuse angle (102°) of the cleavages 

 //' and /> the optic axes were visible, the axial plane being par- 

 allel to the shorter diagonal of the rhombic section. Optically, 

 therefore, the specimens examined conform to normal barite. 



In the nature of the cleavages the supposed monoclinic charac- 

 ter seems to gain more support. The pearly luster of one cleav- 

 snrface (A 1 of Lacroix), parallel to which the specimens read- 

 ily separate into thin plates or scales, is, as has been remarked, a 

 striking feature of the mineral. Parallel to the cleavage which 

 is obliquely inclined to this pearly face (j) of Lacroix), the 

 fracture takes place usually with difficulty, and the surface ex- 

 posed then -hows a multitude of fine lines, which are the edges 

 of the plates parallel to // The remaining cleavage is usually, 

 as noted by Lacroix, less difficult than that of p and the sur- 

 face is generally vitreous in luster. 



An examination of a large number of specimens, however, 

 shows that these characters are not constant. Occasionally a 

 mass is found in which there is no surface of pearly luster at 

 all. and in which the character of the cleavage faces could not 

 be distinguished from ordinary barite — such specimens are 

 rare. On the other hand, we find specimens in which the 

 pearly luster and the tendency to separate into thin plates be- 

 longs to both surfaces of oblique cleavage (prismatic faces of 

 barite) so that it is impossible to distinguish between them. 

 The pearly luster is also sometimes present over a portion of 

 one of the surfaces and absent over the remainder ; again, in 

 >'>ine specimens this character belongs also to the third cleav- 



■ direction (</). Moreover, a close examination of the pearly 

 surface shows an iridescence between the plates due obviously 

 to their slight separation, while scales of a foreign substance 

 can often be detected between them. 



In short, the writer regards it as clear that the apparent easy 

 cleavage (laminated structure) and the accompanying pearly 

 luster of one of the prismatic face- are secondary in origin and 

 have been called out by pressure — a force the action of which 

 can easily be understood in the case of masses enclosed as these 

 are in crystalline limestone. 



Several thin sections have been examined, cut parallel to 

 each of the three cleavages. The sections parallel to what 



