64 E. S. Dana — Barium Sulphate from Perkins* Mill. 



would be the base of barite (g l ) show a nearly uniform devel- 

 opment of the cleavage cracks parallel to the two oblique direc- 

 tions. Between these lines, and in both directions, (A 1 and p, 

 Lacroix), though somewhat more marked in one of these, are a 

 series of inclusions of the same mineral, having nearly uniform 

 extinction and so orientated that one of the prismatic cleavages 

 is very nearly parallel to the line bisecting the obtuse angle of 

 the cleavage faces of the specimen as a whole. These parallel 

 lines of inclusions vary much in width and on the whole are 

 irregular in outline, though showing a tendency to take a crys- 

 talline form. They stand out sharply when the section is ex- 

 amined in polarized light and suggest at once something of the 

 nature of polysynthetic twinning, A careful measurement 

 gave the angle between the similar cleavages in adjacent por- 

 tions as 56°, and, if the two are actually in twinning position, 

 the twinning plane (referred to the barite form) might then be 

 the macroprism 210, which would require 54° 21', or the 

 brachyprism 130, requiring 54° 29'. It is to be noted, how- 

 ever, that the extinction-directions vary a few degrees among 

 themselves, and further, besides these inclusions with nearly 

 regular orientation, there are many others which are totally 

 irregular in position. Moreover, a macroscopic examination of 

 such sf cleavage surface shows long parallel lines of inclusions 

 with cleavages in nearly the same position and yet changing a 

 few degrees from one to the next, not only in the direction of 

 the prismatic zone {ph l ) but also at right angles to it. This 

 last fact sufficiently explains why in the included portions the 

 extinction-directions vary as much as the 5° of Lacroix from 

 the diagonals of the rhombic section of 78° and 102°. 



It seems, therefore, more probable that there is no definite 

 twinning involved in any of these cases. It should be added 

 that these sections show what look at first like very narrow 

 twinning lines following the cleavages, but examination shows 

 them to be, for the most part, only the open cracks between 

 the plates of the mineral. The section parallel to the other 

 faces show also lines of inclusions chiefly in the direction of the 

 prismatic edge (h l /p), but while some of these keep the same 

 optical orientation, most of them are irregular and many are 

 made up of a multitude of minute grains. 



Another feature of this barium sulphate is the presence upon 

 a prismatic face, especially when it shows the pearly luster, of 

 line striations closely crowded together and parallel to the basal 

 edge. This seems to be due to the development, probably by 

 pressure, of a steep pyramidal face inclined from 5° to 8° to 

 the prism. Furthermore, another set of similar lines upon the 

 same face, are often seen inclined about 45° to the basal edge 

 as if caused by the partial development of a macrodome; this 



