404 PROCEEDINGS OP THE AMERICAN ACADEMY. 



as in the case before us. The tendency of fluosilicate of magnesium to 

 crystallize out in the form of skeletal groupings is noted and figured by 

 Boricky in his classic work on microchemical methods.* He also de- 

 scribes the actual determination of these various fluosilicates on (010) of 

 an amphibole,t etched with hydrofluosilicic acid. 



The Glaucophane Type, (110). 



Glaucophane furnishes a new tyi^e of pit on (110). It is more elon- 

 gated than the Hornblende type, is characterized by a more pronounced 

 straightness of edges, and is unique by reason of the parallelism between 

 its longest edge (corresponding to A C \n Figure 2), and the trace of the 

 cleavage. (See Plate I. Figs. 15 and 16.) It is likewise triangular in 

 outline, possesses three figure-faces on the sides and a migrating bottom 

 face. Gastaldite from the Champ de Praz (P. 59) afforded pits in no 

 respect to be distinguished from those on the He de Groix glaucophane. 

 On the other hand, crossite gave figures decidedly differently and more 

 closely allied to the Hornblende type (Plate I. Fig. 17). 



The Rieheckite Type, (110). 



Figures of corrosion were obtained on riebeckite only with much dif- 

 ficulty, apparently due to its extreme attackability in concentrated acid. 

 They were alwaj's excessively small, often with imperfect development; 

 the upper end of the pit was the first to become clearly evident, in the 

 process of maturing. The figure has many points in common with the 

 sub-type noted above on Edenite ; it is usually quadrilateral, though 

 sometimes three-sided and analogous to the Wolfsberg sub-type. Put it 

 differs from both in its being much darker than they in vertically incident 

 light: — the figure-faces are steeper than in the same (110) pit on com- 

 mon hornblendes (Plate I. Figs. 18 and 19). 



The Arfvedsonite Type, (110). 



Quite an exceptional category of etch-figures is represented in the pits 

 generated on the prism-face of arfvedsonite by the use of hydrofluoric 

 acid. (Plate I. Figs. 21 and 22, and Photograph 11.) Their peculiar- 

 ities are so salient as to enforce the belief that, in the matter of cohesion 

 on this particular face, arfvedsonite is at least as far removed from the 



* Archiv d. naturw. Landesforschung von Bohmen, III. 5, Prague, 1877, Plate I. 

 Fig. 12. Translated by Winchell, 19th Ann. Rep. Minnesota Geological Survey, 

 t Op. cit., Plate II. Fig. 7. 



