502 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 5U. 



which are similar in development and agree closely with Ungemach's 

 description of the type. As a matter of fact the form P (332) is a 

 comparatively rare form, while v (Til) is much more common on 

 this type of crystals. So far as seen the form b (223), which is 

 almost invariabty present on the crystals of type 1 (Kraus), does 

 not occur on those of type 4 (Ungemach's type 2). On the latter 

 crystals e (112) is often the steepest negative clinopyramid present. 



DISTORTION. 



Distortion, or unequal development of corresponding faces on oppo- 

 site portions of the crystal, is the rule rather than the exception. 

 Symmetrically developed crystals are rare, and various freakish 

 developments are so common and so variable that they can not 

 be described in detail. As typical examples of distortion, crystal 65 

 (fig. 22) and crystal B5 (fig. 13) may be cited. Crystal 65 is pris- 

 matic by elongation parallel to o (120) and is so unsymmetrical that 

 were it not for the characteristic etchings of the various faces its 

 orientation would have been almost impossible. Crystal B5 is 

 unequally developed, so as to give the basal pinacoid the outline of 

 a right triangle and to bring the new positive orthodome 2 (205), 

 which is present as a relatively large dull face, adjacent to the pyra- 

 mid k (115), this unusual angle being beveled by the new form j l 

 (5-6-30), which is dependent for its occurrence on this unusual 

 development. The unequal development of crystals of type 1 is 

 often expressed by the unsymmetrical outline of the basal pinacoid, 

 which is right triangular in outline. Ideal development has been 

 prevented in most crystals by mutual interference during growth, 

 and more than one quadrant of a crystal is rarely developed, so that 

 what form would have been assumed by the missing faces can not 

 be surmised. 



IRREGULARITIES OF THE CRYSTAL FACES. 



Dana writes with regard to datolite: "Faces often wavy and rarely 

 giving good measurements; x (102) commonly dull." The difficul- 

 ties encountered in properly orienting crystals of datolite render cer- 

 tain peculiarities of several of the more prominent faces occurring 

 on this mineral very important. Consequently the several forms, 

 faces of which exhibit peculiarities of surface of sufficiently constant 

 occurrence to be of value as orienting criteria, deserve special descrip- 

 tion. No two crystals are alike in the irregularities of surface shown 

 by the planes, the surfaces, even when highly lustrous, showing an 

 infinite variety of patterns and tracings, the detailed description of 

 which might occupy many pages. Certain features are, however, 

 comparatively constant and serve as a most convenient guide in 

 orienting the crystals. The most useful of these are as follows: 



