CONTRIBUTIONS FROM THE MIXERALOGIC LABORATORY 13 



those of m (ioio) brilliant but somewhat rounded and covered 

 with vicinal prominences. 



In the combination shown in figure 23 which is represented by 

 small, bright transparent crystals of the first generation, the pris- 

 matic zone is developed as a narrow band encircling the crystal, 

 both a (1120) and m (1010) being present. The second order 

 pyramid y (8.8.16.3) which is here present as a characteristic 

 form is beveled on alternate edges by the positive rhombohedron 

 M(404i)lying in the zone [8.8.16.3. 16.8.8.3]. A new scalenohedron 

 X (8 1. 4 1. 1 2 2. 40) very near the common v (2 131), is present as a 

 prominent form. This combination is quite similar to that figured 

 by Penfield and Ford 1 ; it is common on a number of specimens 

 and, as previously noted, appears genetically to form a connecting 

 link between the pyramidal type of the first generation and the 

 scalenohedral type of the second. 



Figure 24 shows a combination which mainly differs from the 

 preceding in that the scalenohedron v 1 (7.4.1 1.3) takes the place of 

 X as a predominating form, the latter form being present only as 

 a subsidiary modification and frequently passing into the commoner 

 form, v (2 131). This combination is distinctly scalenohedral in 

 habit and occurs mingled with secondary crystals of the form 

 shown in figure 25 which latter frequently shows the suppression 

 of the pyramid y. The type shown in figure 26 is found in crystals 

 of the second generation which occur deposited on a thin layer of 

 first generation calcite of rhombohedral habit [see figure 28]. These 

 differ from all which have been previously described in two essen- 

 tial characteristics: they are opaque and milky white in color and 

 show a complete absence of all marcasitc or pyrite inclusions. In 

 crystallization this type is also unique showing the scalenohedron 

 U (10.4. 14.3) in the zone [4041. 8. 8. 16. 3] as a highly developed form. 

 A negative rhombohedron -q (0441) is present as a narrow face 

 beveling the alternate pyramidal edges. M, X, r and y are all 

 present as bright, well defined faces. 



The rhombohedron r (1011) which is present as a modification 

 on the combinations shown in figures 21, 23, 24, 26 is developed 

 to the extent of a crystal habit in the case of the types shown in 

 figures 27 and 28 which represent crystals of the first generation. 

 Of these figure 27 may be regarded as a rhombohedral phase of 

 figure 23 showing an additional scalenohedron C (6178) in the zone 

 [01 12. 101 1.1 1 20]. The crystals, which contain the marcasite in- 



'Penfield. S. L. & Ford, W, E. Some Interesting Developments of Calcite Crystals. 

 Am. Jour. Sci. igoo, 10: 237, fig. 1. 



