CONTRIBUTIONS FROM THE MINERALOGIC LABORATORY 7 



Penetration twins on twinning axis c are quite common result- 

 ing in the forms shown in figures 5 and 6. Measurements of a 

 number of crystals yielded the following results: 



ZONE (lOlo), (lOIl) 

 Forms Measured angle on 1010 Calculated angle on 1010 



The presence of vicinal forms in the zone [1010.1011] inter- 

 fered materially with the measurement of this zone. 



Calcite. The vein calcite of Rondout occurs, for the most part, 

 as a secondary deposit on dolomite and presents types of crystal- 

 lization of marked variety and unique development. The asso- 

 ciated pyrite which is here present in extremely minute crystals, 

 occurs in many cases included in the larger calcite individuals ar- 

 ranged along the crystallogenetic lines of the latter mineral in dis- 

 tinct bands on the surface of, or as phantoms within, the crystals 

 of the calcite. These structure lines as outlined by the pyrite in- 

 clusions are of notable interest in their relation to the development 

 of the calcite. Somewhat similar inclusions have been noted in 

 the calcite from Phoenixville Pa 1 . A careful study of some 200 

 specimens has led to the division of the observed combinations of 

 crystal forms into nine types, of which the second and third may, 

 be regarded as variations of the same type with respect to crystal 

 habit. 



DISCUSSION OF TYPES 



First type. The simple combination of the prism (1010) with 

 the rhombohedron (0112) which is the prevailing type throughout 

 the vein calcite of the Siluric limestones of the Hudson valley, is 

 here present in crystals which occasionally reach a diameter of 3 

 cm. As elsewhere noted in this region, this type appears to mark 

 a condition of regular and uninterrupted deposition as distinct from 

 the disturbed and brecciated vein structure characteristic of some 

 of the types to be ^subsequently discussed. The planes of the 

 rhombohedron e (0112) are dull whereas those of the prism are 

 fairly brilliant and marked with natural etchings. Twins occur 

 parallel to c (oooi). 



1 Smith, J. L. Am. Jour. Sci. Ser. 2. 20:251. 



