E. S. Dana— Choiidrodite from the IWij- Foster Iron Mhic. 7.3 



as 7 : |: fl or 28 : ;^0 : 27. Tlie method adopted by Seacclii and vom 

 liatli lias the advaiitao-o of exi)ressing the sinij>lest })Ossil)le iminer- 

 ical ratios between tlie three tyi)es. It is the view of tlie aiitlior 

 above referred to, however, tliat the variation in the anuU' ot 

 ^^1-7(01]) is to be regarded in tlie same light as the variation in the 

 vertical axes of the rhoiubohedral carbonates, or in the orthorhombic 

 sulphates ; or in other Avords, the three types form an isoniorjihous 

 series, and the variation observed is no greater than is constantly 

 seen in analogous isomorphous groups. This view seems to find con- 

 firmation in the crystallographic relations of humite and chrysolite, a 

 subject already discussed by Scacchi, Rammelsberg and vom Kath, 

 Taking the fundamental form, as in Dana's " Mineralogy" (here i-sz^T" 

 of other authors), the lateral axes are nearly identical with those of 

 humite, while the vertical axis (1 •25928 Kokscharow) has exactly the 

 ratio of -^jj to that of humite, type II, and f to that of humite, type 

 III ; in other words, we have the ratios : 



Humite. 



Chrysolite. III. I. II. 



24 : 27 : 28 : 30. 



If we adopt the vertical axes of Scacchi and quadruple that of chry- 

 solite, we obtain 



II. I. Chrysolite. III. 



5:7:8:9 



These relations were in effect lirought out by Scacchi when he showed 

 that what he called the common fundamental form of humite, ob- 

 tained by dividing the vertical axes by 7 (I), 5 (II), and 9 (III) re- 

 spectively, was nearly identical with that accepted by him for chry- 

 solite. This fact seemed to Rammelsberg of so much importance that 

 he proposed to refer all the planes of humite to this common funda- 

 mental form ; and in this he has been folloAved by Kokscharow. The 

 result of this will be seen in the following table, which gives the 

 symbols thus obtained for the two common macrodomes in each tyj)e. 



Tr.\ns. Conn. Acad., Vol. III. 10 July, 1875. 



