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PROCEEDINGS OF THE AMERICAN ACADEMY. 



capable of fairly accurate measurement. The crystals belonged to H. 30, 

 V. 35, and V. 54. The first yielded figures after 30 seconds' boiling 

 in cone. HF, the second on one minute's exposure on the water bath, 

 and the third after two minutes' immersion in a dilute (10%) solution of 

 HF. The plane (TOl) was in every case strongly affected, and penta- 

 gonal pits of corrosion were visible (Fig. 9, and Plate I. Fig. 30). 



Figure 9. 



Figure 10. 



The measurements on the crystal of V. 35 were as follows : — ■ 

 ZBAE 84°. ABCD 96°. /.ABC 132°. 



The outline is symmetrical to the plane of symmetry of the crystal. 

 The acute angle BAB points toward the front (in Tschermak's orien- 

 tation). Each edge corresponds to a plane figure-face indicated in the 

 drawing, and there is a diminishing bottom-face parallel to (101). The 

 twinned character of V. 54 was clearly manifested in the attitude of 

 the pits on the two adjacent dome-faces intersecting in the twinning- 

 plane (100). 



An actinolite (V. 16 at two minutes' exposure) and richterite (V. 20 

 at one minute) furnished the type of (TOl) pits characteristic of the non- 

 aluminous amphiboles (Plate I. Fig. 31, and Fig. 10). The figures on 

 the former were extremely sharp, but a photograph could not be pre- 



