of the Trimetric System. 63 



Comparing the species in Table IV. A, with those of Table 

 IV. we observe the following affiliations : — 



Marcasite and Aurotellurite are near section I. (Sternbergite). 



From Valentinite to Sulphur (from Table II.) are near sec- 

 tion II. (Aragonite section). 



From Chrysolite to Bournonite (from Table L), with also 

 Manganite, are not coincident with either section of Table IV., 

 but they have approximately the ratio to the Aragonite section 

 of 4 : 3. This is the ratio between Chrysolite and Chrysoberyl. 

 If 119° 12' in Chrysolite be considered as corresponding to 119° 

 46' in Chrysoberyl, then the macrodome of 64° 24' in Chrysolite, 

 if referred to the form of Chrysoberyl, would be i I. 



From Calamine to Cotunnite, the vertical axis is to that of the 

 Barytes series nearly as 5 : 3. 



In reviewing the Groups of Trimetric forms, the most promi- 

 nent fact observed is the prevalent approximation in the values 

 of the angles of the unit prisms, to the three monometric angles, 

 90° 109° 28', and 120°, or their supplements, 70° 32', and 60° ; 

 abo've all, the angles approaching 109° 28' and 70° 32' much 

 predominate. When the vertical prism is near 90°, domes near 

 109° 28' and 70° 32', characterize very many of the species ; 

 while domes near 120° belong to the rest of the species. And 

 in the second great group, macrodomes near 70° 82', and 109° 

 28', and brachydomes near 60° and 120°, determine the vertical 

 angle of the prism, which approaches 101° 36'. Another large 

 group has 120° and 60° as approximately the angles of the ver- 

 tical prism. _ _ 



The fact that the axial ratios 1 : V2 and 1 : V3 are typical of 

 certain groups has been mentioned. It is easy to make out, in 

 many cases, simple ratios between the axes, or the sum of two 

 of the axes and the third; but the importance that should be 

 attached to such ratios is questionable. The following are a few 

 examples : — 



