2i0 Penfield and Ford — Developments of Calcite Crystals. 



described, is represented by iig. 7. In the Union Springs 

 crystals representing this twinning law the reentrant angle at 

 the tojD wholly fails, and a peculiar, pointed, spear-head devel- 



opment, fig. 8, results from the extension of the two front 

 lettered faces of fig. 7 and the corresponding faces at the 

 back. The crystals observed have always been attached at the 

 lower end. Several crystals of this peculiar type were observed 

 on the specimens sent by Mr. Clarke, and they are said to be 

 quite common at the locality. On a crystal with a broken 

 point the reentrant angle measured from the rhombohedral 

 cleavages was found to be 35° 38', calculated 35° 27'. 



Levy, in fig. 6 of the atlas to his work already cited, gives a 

 representation of a crystal from Kongsberg in Norway, of 

 identically the same habit as fig. 8 of this article ; however, the 

 habit is apparently a very unusual one, and it is interesting to 

 record it at a new locality. 



On a single specimen or even at one locality, as a rule, 

 all crystals of a certain mineral have the same or nearly 

 the same habit, resulting undoubtedly from crystallization 

 under uniform conditions, and therefore it seems a matter 

 of more than usual interest to note on a single hand specimen 

 from the Union Springs locality, the occurrence of simple 

 scalenoheclrons, fig. 2, and of three distinct types of twinning, 

 figs. 3, 6 and 8. The calcite crystals seem to be all of one 

 generation. Associated with them are a few crystals of dolo- 

 mite, apparently of later growth. 



2. Butterfly Twins from Egremont, Cumberland, England. 



The so-called butterfly twins from Egremont are well-known 

 and are figured in many mineralogies. Levy in his work, 

 already cited, gives three figures of them, No. 17, 68, and 69 

 of his atlas. A few words concerning them and new figures 



