CONTRIBUTIONS FROM THE MINERALOGIC LABORATORY II 



sity and were described by Messrs Penfield and Ford 1 . The writer' 

 has found by a careful study of the bulk of the material collected 

 by Dr Clarke (some 70 specimens) some points of additional interest; 

 not shown in the comparatively small amount of material avail- 

 able for the above article. 



The calcite crystals under consideration occur in vein material 

 in the Onondaga limestone associated with saddle-shaped aggre- 

 gates of dolomite and more rarely with crystallized quartz. They 

 represent two generations, separated by a period in which dolomite 

 was deposited, of which the older consists of brilliant individuals 

 of extremely varied habit which are for the most part small, vary- 

 ing from' 3 to 10 millimeters in length. One type of these crystals 

 of the first generation is represented in figure 1; of the article by 

 Penfield and Ford, above cited. 



The crystals of the second or younger generation are generally 

 larger in size than those of older deposition and are largely of scale- 

 nohedral type, showing a marked tendency to twinning according 

 to several laws. They are frequently of a dull surface and black 

 or dark gray in color as the result of bituminous inclusions. It is 

 these latter which have been described at length by Penfield and 

 Ford. 



The small brilliant crystals of the first generation contain frequent 

 inclusions of pyrite chalcopyrite and marcasite in microscopic in- 

 dividuals, the latter mineral in beautiful doubly terminated twin 

 crystals, specially prevalent in forms of the rhombohedral type. 

 Frequent zones of deposition of these inclusions occur which ren- 

 ders their aspect almost that of a phantom within the crystal. 



Pyramidal type. The second order pyramid, y (8.8.16.3) is a 

 peculiarly dominant form in crystals from this locality, particu- 

 larly so with crystals of the first generation. In the type shown 

 in figure 21 which occurs in the lining of a thin seam, the form 

 occurs developed to the exclusion of all modifications except those 

 of the terminal rhombohedrons r (1011) and e (01 12). These 

 crystals are exceedingly small, the largest not exceeding 4 milli- 

 meters in length and quite brilliant, giving very satisfactory reflec- 

 tions when measured. 



The pyramidal type as shown in figure 21 is, in some cases, 

 modified by narrow faces in the prismatic zone, the type gradually 

 merging into the combination shown in figure 23 which may be 

 regarded as a transitional type between the pyramidal [fig. 21] and 



1 Penfield, S. L. & Ford, W. C. Some Interesting Developments of Calcite Crystals. Am. 

 Jour. Sci. ipoo. 10:237—41. 



