434 PROCEEDINGS OF THE AMERICAN ACADEMY. 



naked eye a marked flexure and even twisting of the prism-faces. I 

 consider that the curious distortion of the etch-tigures on (110) is due to 

 the warping and consequent molecular strain.* 



Several of the crystals are twinned parallel to (100). 



The different etching behavior of the crystal-face and of the surface 

 of cleavage has been explained as due to the zonal structure which is 

 a prominent characteristic of the mineral. Six oriented sections and 

 numerous cleavage pieces display the structure : it is illustrated in Fig- 

 ures 1, 2, and 3. 



Figure 1. Figure 2. Figure 3. 



Figures 1 and 2. — Two sections parallel to (010) of the same doubly termi- 

 nated crystal, showing zonal structure. In Figure I three zones, in Figure 2 four 

 zones, are indicated. The zone of deepest tint is the most closely shaded, that of 

 the lightest tint is left unsiiaded. The lines of shading run parallel to the cleavage 

 trace. The trace of the edge 010 : Oil slopes downward from right to left. 



Figure 3. — A section parallel to (110), showing three zones represented as in 

 Figures Ijand 2. The extinction of the lightest zone is 17°, that of the intermedi- 

 ate zone is 19°, and that of the darkest zone is 22° 30'. 



That this structure is rare in the amphiboles is clear from the recent 

 statement by Becke in his essay on the zonal structure of crystals in the 

 eruptive rocks.f Brogger describes one casein connection with his cato- 

 phorite series. He notes the fact that sometimes the core of a crystal 

 may consist of catophorite and the outer zone of arfvedsonite.J Tscher- 

 mak long ago noted another example in a Vesuvius hornblende. § 

 Palache has figured the structure in crossite. || 



The diagrams show that the bulk of each crystal is composed of 

 pretty uniform substance, in which a darker colored phase of the mineral 

 may be apparent, either without definite arrangement with respect to the 

 former or in the form of true hour-glass intergrowth with it. In the 



* See These Proceedings, Vol. XXXIV. page 400. 



t Min. und petrog. Mittheilungen, 1898, Bd. XVII. p. 101. 



t Die Gesteine der Grorudit-Tinguait-Serie, pp. 27 et seq. 



§ Min. und petrog. Mittheilungen, 1871, Heft I. p. 40. 



II Bulletin, Department of Geology, Univ. of California, Vol. I. p. 187. 



