■^O NEW YORK STATE MUSEUM 



"basal edges of the predominating rhombohedron. Indications 

 pointed to a second scalenohedron in this zone giving the 

 indexes (lo. 16.26.3) ^^'^ bevehng the basal edges of q: as thin 

 lines from which measurements were obtained with great difficulty. 

 The form must be regarded as doubtful. 



Type III. Calcite crystallizing in forms of this type differs from 

 those previously described both in mode of occurrence and habit. 

 They occur for the most part embedded in masses of byssolite and 

 are often free or so loosely attached that doubly terminated indi- 

 viduals are readily obtained. They are of a later generation than 

 those of type I, being contemporary and closely associated with 

 secondary quartz, hematite and albite derived from the minerals 

 accompanying type I. In habit they are essentially pyramidal, 

 the simpler development showing the predominance of two pyra- 

 mids in the same series, (8.8.16.3) and (2243) figure 7a, 7b. More 

 ■complex variations of this habit [fig. 8a-8b] are found 

 associated with these secondary minerals and, indeed, the remain- 

 ing types to be discussed may be said to represent phases of the 

 same conditions of deposition, as they are, at the same time, modi- 

 fied expressions of the same crystal habit. The combination 

 «hown in figure 7a, 7b represents this habit in its simplest develop- 

 ment and is found in crystals varying from 2 to 5 mm in vertical 

 length. The pyramid y (8.8.16.3) occurs as a series of bright, 

 sharp faces. The faces of the pyramid r (2243) ^-^^ o^ the rhom- 

 hohedron Y (0.19. 19. 13) are of fair brilliancy but frequently 

 roughened by naturaf etchings. The planes of v (2 131) are often 

 present on this combination but of relatively small development. 

 On two crystals a terminating scalenohedron in the zone [0,19.19.- 

 13. 1 9. 1 9. 0.1 3] gave measurements roughly corresponding to (7.2.9.1 1) 

 but on account of the imperfect nature of the reflections the form 

 must be regarded as doubtful. 



The combination shown in figure 8a-8b represents a modification 

 ■of this habit in which the planes of the scalenohedron v (2 131) 

 partly replace those of y and a second negative rhombohedron 

 I (0445) terminates the crystal partly replacing the planes of r. 

 The alternate polar edges of y are beveled by the scalenohedron 

 W: (14.12.26,5) in the zone [8.8. 16.3. 16.8.8.3]. This combination 

 ■which seems to indicate a slower and more perfect stage of crystal- 

 lization occurs in larger crystals than that previously described 

 under this type, detached crystals measuring from 4 mm to 30 mm 

 in vertical length. 



