H E. 8. Dana — (Jhoiidrodite from the Tilly-Foster Iron Mine. 



It will be seen from these few examples that the plan proposed in- 

 troduces a set of common molecular axes at the expense of all crys- 

 tallographic simplicity in the relations of the several planes of each 

 type. Moreover, the view of Rammelsberg loses some of its plausi- 

 bility, if, as shown by vom Rath, the vertical axes do not stand in 

 direct relation to the amount of fluorine present. The view of Prof. 

 Dana here advocated seems to have the advantage of presenting all 

 the relations in their most natural light. 



It may be added, as completing the history of the subject, that 

 DesCloizeanx refers all the humite planes to one, and that the second, 

 type. 



In regard to the letters employed, it seemed to ofler the simplest 

 solution of an obvious difficulty to retain all the letters of Scacchi for 

 the second type, and for the third to use simply the corresponding 

 Greek letters in the same order, and for the first type to use the cor- 

 responding capital letters. It was not deemed advisable to use the 

 same letter for two planes, on different types, which bore no imme- 

 diate relation to each other. 



Table II. 



A= O(OOl). 



Table III. 



C= w(OlO). 



