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NEW YORK STATE MUSEUM 



elusions characteristic of the younger calcite in this locality, often 

 show elongation parallel to the rhombohedral zone assuming a 

 somewhat prismatic aspect. They apparently fill a gap in the 

 genetic series between the pyramid scalenohedral habit [fig. 23] 

 and the distinctly rhombohedral habit shown in figure 28. 



Crystals of this last type [fig. 28] occur in a loosely compacted 

 mass deposited on a layer of crypto-crystalline carbonate of lime 

 occupying the space between the crystallized calcite and the lime- 

 stone wall of the cavity or vug to the depth of about 5 milli- 

 meters. The calcite crystals are piled upon this crystalline layer 

 to the depth of from 10 to 15 millimeters, the largest individuals 

 lying in the top layers. The order and manner of deposition sug- 

 gest the possible derivation from a solution which originally com- 

 pletely filled the space and deposited its dissolved carbonate of 

 lime first from a rapidly then from a slowly cooling medium. The 

 crystals of this type which are remarkably clear, brilliant and well 

 developed, range in size from 5 to 20 millimeters in diameter. All 

 the faces give excellent reflections. The middle edges of the 

 rhombohedron r (1011) are beveled by the scalenohedron X 

 (8 1. 4 1. 1 2 2. 40) which throughout this occurrence replaces the com- 

 mon form v (2 131) which it approaches very closely. It was only 

 after repeated measurement of a number of crystals, both of this 

 and of the foregoing types that the form was considered as estab- 

 lished. The prism a (11 20) is present as a small face in this zone. 

 In the zone of the pyramidal faces [16.8.8.3.8.8. 16.3] occur the 

 forms M (4041) and S' (19. 10. 29. 6) both lying well within the zone 

 and agreeing as to measured angles well within the limits of ac- 

 curacy. 



The twin crystals of the second generation have been so amply 

 described by Messrs Penfield and Ford that there is little to add. 

 The twin crystal shown in figure 29 occurs on several specimens 

 in milky individuals of about 20 millimeters diameter, which sug- 

 gest in their rather peculiar development the familiar types oc- 

 curring at Rossie, St Lawrence co 1 . 



Figure 30 shows the prevailing type of scalenohedral twin, the 

 habit being that of the scalenohedron v. (7. 4. 11. 3) and the twin- 

 ning plane parallel to the rhombohedron e (01 12). The reentrant 

 angle or "gash" is chiefly formed by the planes of y (8.8.16.3) 

 and m (1010). 



■Nason, F. L. Some New York Minerals and their Localities. N. Y. State Mus. Bui. 4. 

 1888. 



