no.1726. ON SAND-BARITES FROM EGYPT— POGVE. 21 



the parallel growth of small plates. Its rounded contour is in all 

 probability due to a concretionary tendency; that is to say. its 

 imperfect crystal form is supposed to be the resultant of two mole- 

 cular forces, the one striving for the formation of crystal outline', the 

 other intent upon the development of concretionary structure, and 

 each partly successful. In other words, the completion of the con- 

 cretion was prevented by the development of two crystal planes— 

 the very two planes, indeed, which in barite are potentially the 

 most powerful, as evidenced by their universal occurrence and the 

 perfect cleavage parallel to them. The tablet can not owe its 

 rounded outline to the carving effect of wind-blown sand, for simi- 

 lar shapes are found in protected positions within complex groups, 

 nor is there ever any sign of differential abrasion. 



A second type, more complicated, is shown in figs. 2, ,3, 6, and 7, 

 where two or more plates similar to the one described above inter- 

 penetrate. There is universally a central tablet, to which the others 

 are variously inclined, and the several components apparently pos- 

 sess a &-axis in common. Usually two oppositely-inclined plates 

 (fig. 2) make angles of 30 °° with a central one, though angles of 12° 

 and 45° are not uncommon, and those of 65°, 70°, and even 90° 

 (fig. 7) are occasionally met with. Many of these inclinations may 

 be included in a single group; or by various permutations of two 

 similarly inclined plates with one or more differently inclined even 

 more complex assemblages may be formed. 



Other still more involved growths have, in addition to plates 

 crossing with &-axes in common, other plates variously inclined to 

 these. The common disposition thus effected is shown in figs. 8 and 

 10, where three plates are inclined 30° to the base of a central 

 plate, with angles of about 120° to 150° between their own 

 planes. From above a three-sided, hopper-shaped appearance is 

 presented. Evidently in such occurrences one or more of the plates 

 fail to have even a single axis in common with the other plates pos- 

 sessing a mutual &-axis. Occasionally, too, a four-sided hopper is 

 developed, as shown in figs. 9 and 11, where two of the secondarily 

 inclined plates have opposite inclinations in respect to each other. 

 Not infrequently the two patterns just described will be seen on the 

 same specimen, the one on one side and the other on the other. In- 

 deed, throughout the entire suite the two sides of the central tablet 

 are almost invariably differently modified. Again, there are spher- 

 ical skeleton forms, composed of many thin plates of nearly like 

 diameter, resembling the rosettes from Rockenberg described by 

 Delkeskamp. 6 



"The angles are only approximate, as exacl measurements could nol be made. 

 &Zeitschr. Naturw, Italic, vol. 7. r >, 1902. 



