CONTRIBUTIONS FROM THE MINERALOGIC LABORATORY 17 



following notes were collected by the writer, through the courtesy 

 of the Helderberg Cement Co., from the mine which furnishes 

 natural cement rock to this company. The crystals which vary 

 in size from 40 millimeters in diameter to microscopic individuals 

 are of uniform habit and jire invariably characterized by a marked 

 twinning parallel to e (01^2). They are frequently associated with 

 tufted aggregates of acicular aragonite which appears, in one 

 instance at least, to have been derived from the re-solution of the 

 calcite. In the instance noted a geodic mass almost completely 

 filled with crystallized calcite yielded on fracture several fine tufts 

 of aragonite deposited on calcite crystals of the prevailing habit 

 which latter were found to be deeply pitted with natural etchings. 

 Calcite crystals of a second type were found in the Helderberg 

 limestone, overlying the Rondout, lining the fossil remains of 

 Rhynchonella wilsoni with which portions of this forma- 

 tion are thickly studded. These latter differ somewhat from the 

 type of the principal occurrence and will be discussed as a supple- 

 mentary type. 



The crystals of the principal type shown in figure 31 exhibit a 

 complex combination of forms occurring in several clearly defined 

 zones; the relation of these is shown in the spherical projection 



[fig. 33]. 



Rhombohedrons. The rhombohedron r (ion) infrequently pres- 

 ent alternating with the low scalenohedron q (5167) which latter, 

 although clearly defined, is without question a built up form more 

 or less vicinal in character. The planes of r are smooth but rather 

 dull. The rhombohedrons M (4041) and r (7071) occur as nar- 

 row but extremely brilliant faces, giving excellent reflections and 

 beveling the edges of U (10.4.14.3) and V (6281) respectively. 

 In the zone with these is also found the negative rhombohedron 

 $> (0.14.14.1) occurring as small triangular faces of fine brilliancy. 



The rhombohedron e (0112) is universally present as brilliant 

 faces which make excellent points of reference in this zone. 



Scalenohedrons. As previously noted the scalenohedron q (5167) 

 is present in many instances as a built up form with deeply striated 

 faces giving poor reflections. The basal edges of q are modified by 

 the common scalenohedron v (2131) the obtuse polar edges of 

 which are terminated by the scalenohedrons U (10.4.14.3) and V 

 (6281). Owing to the fact that the indexes of these latter forms 

 are quite near those of v and to one another their intersection 

 edges are not distinctly marked, the successions of forms tending 

 to produce a slight rounding of the crystal toward the rhombo- 



