400 PROCEEDINGS OF THE AMERICAN ACADEMY, 



idea as to the relative steepness of the figure-faces on account of the great 

 variability of the pits, on the one hand, and, on the other, the deep tint 

 characteristic of the pits. The largest mature pits are perfectly black, 

 blunt on the upper end because of the truncation by a long transverse 

 edge and bulging below but tapering rapidly to the lower end. The 

 angle made by the transverse edge with the cleavage trace was measured 

 at 83°, a value which lies within the limiting values of the angle D A H, 

 in Figure 2 ; the edge is thus oriented the same as the edge D A oi the 

 Wolfsberg sub-type, and the two are regarded as equivalent. 



Now, these pits appear on all four cleavage faces and are also asso- 

 ciated on these according to the law of twinning after (100) ; but with 

 them were found a second set of j^its arranged enantiomorphously to 

 the first. Evidently, no system of twinning can explain their orienta- 

 tion, and it is all the more surprising from the fact that there is no 

 perceptible difference in the young forms of the two sets of figures. 

 Moreover, whole areas of a cleavage surface are covered with pits belong- 

 ing to a third category, in which the straight edges are the exception and 

 a curved outline the rule (Photograph 7). By an optical illusion, these 

 pits have the appearance of projecting from the general surface after the 

 manner of etch-hills ; in so doing, the form of the outline and a pro- 

 nounced shelly structure, exactly imitating the lines of growth in a mol- 

 luscan shell, make each pit extremely organic in look. Very often, the 

 shells are greatly elongated in a direction transverse to the cleavage 

 trace (Photograph 7). Only one probable explanation of these two 

 classes of abnormal pits has suggested itself, namely, that the crystals 

 are in a condition of internal tension, which interferes with the workings 

 of the usual molecular cohesions. If this hypothesis be correct, we 

 should expect all transitions between the normal type and the other two 

 abnormal types of figures. Such is the fact, typified in the observed 

 occurrence of pits intermediate to those of the first and second kinds, 

 where the transverse edge is seen to have all directions within the limits 

 set by the enantiomorphous pits (see Photograph 6). Moreover, we 

 might expect on this hypothesis that there should be distorted pits on the 

 cleavages etched with reagents other than HF. Using caustic soda, I 

 obtained normal pits on (110) illustrated in Photograph 9 ; occasionally, 

 though more rarely than in the case of crystals etched with the acid, the 

 beautifully marked shells were replaced by others elongated transversely. 



A possible cause of this distortion by differential tensions is not far to 

 seek. Several carefully cut sections of the mineral showed that it is 

 strikingly zoned, each zone possessing its own tint of color which is 



