286 0. C. Farrington — Datolite from Guanajuato. 



appear as well-defined faces, while Z, 130, is always a minute 

 triangular face at the edge of e. The negative hemi-pyramid 

 n, 111, is always well-defined, and the positive hemi-pyramids, 

 v, 111, and e, 112, are usually prominent. The succeeding 

 faces of this zone round off in a manner which has been 

 noted on datolite from other localities. The clinodomes 

 can usually be distinguished by being duller in luster than 

 the other faces and by their showing minute striations 

 diagonal to their edges. The habit of the crystals is deter- 

 mined by the predominance of the orthodome x, 102, the 

 crystals being tabular with respect to this face, and of the clino- 

 domes t, g and m x . To these faces the prisms and pyramids 

 are quite subordinate and, as mentioned later, may become 

 very minute. The habit produced by predominance of the 

 faces mentioned may perhaps best be described as an arrow- 

 point habit. It is illustrated by fig. 1. As here indicated, 



only half the crystal is present in nature and that the left half, 

 the point of attachment being near the symmetry plane. This 

 development is very constant and by being familiar with it one 

 can recognize the prominent faces at a glance. 



Crystals approaching these in habit, in that they are tabular 

 with respect to x, have been described by E. S. Dana from 

 .Bergen Hill, N. J.,* and by Franzenau from the Swiss Alps in 

 Tyrol. f The Guanajuato crystals differ, however, from those 

 described by Dana in that they only rarely have a rhombo- 



* This Journal, III, vol. iv, p. If, and fig. 14, pi. L. 

 f Handbuch der Mineralogie, C. Hintze, p. 113. 



