CONTRIBUTIONS FROM THE MINERALOGIC LABORATORY 



SUMMARY OF MEASURED AND CALCULATED ANGLES 



aFound also on calcite from Rhisnes. 



bNew. 



The prevalence of the rare pyramid y (8.8.16.3) throughout the 

 varied types of calcite crystals occurring at Union Springs has led 

 the writer to compare these latter with the types presenting this 

 form, which have been noted at other localities. The pyramid y 

 (8.8.16.3) is found in the calcite at Rhisnes, about 4500 meters 

 northeast of Namurs in Belgium 1 , at Andreasberg in the Hartz 2 and 

 in the Bad Lands of South Dakota 3 . Regarding the crystals from 

 Rhisnes, Cesaro has noted not only the pyramid y (8.8.16.3) above 

 mentioned, but also the forms M (4041), y (0441), v (2131) and S'. 

 (19.10.29.6) as well as the occurrences of the prismatic zone [1120.- 

 1010]. Several of his types are identical in form and habit with 

 figures 21 and 23 as well as a twin crystal similar to figure 30. 

 Cesaro finds evidence that many of the crystals from Rhisnes of 

 the first generation have been formed around a parent crystal hav- 

 ing y (8.8.16.3) as the dominant form. 



He announces a theory of genesis of these crystals as follows: 



The examination of these crystals has led us to the conclusion 

 that they have been formed encircling a pre-existing second order 

 pyramid and were deposited by the action of three successive 

 mediums: the first producing pyramidal types, the second forming 

 around the first a combination the faces of which are truncations 

 of the lateral edges of y, the third depositing around the second 

 stage a crystal having for fundamental form scalenohedrons of the 

 zone [1011.1120], 



This sequence of generation appears to agree perfectly with that 

 already given on page 12 with reference to the Union Springs cal- 

 cites. The truncation of the lateral edges being produced in the 

 latter instance by the rhombohedron M (4041) as shown in 



1 Cesaro, G. Les Formes Cristallines de la Calcite de Rhisnes. Ann. de la Soc. Geol. de 

 Belorique 1889. 16:163. 



2 vqm Rath, G. PORK. Annalen 1867. 138:521. 



8 Penfield. S. L. & Ford. W. E. Siliceous Calcites from the Bad Lands, Washington County, 

 S. D. Am. Jour. Sci. 1900. 9:332. 



