the Monoclinic Crystal System. 



119 



velopment are apparent on several of these crystals in the 

 Johns Hopkins University mineral collection, but I am in- 

 debted to the kindness of Professor Dana for the opportunity 

 of establishing it by an examination of a fine crystal from Ca- 

 naan, belonging to the Yale University cabinet. This crystal, 

 which is slightly over two inches in length, i represented in 

 fig. 8. Its planes are for the most part broken g 



or uneven from contact with the dolomite 

 and the substance of the crystal is superfi- 

 cially changed to fibrous white hornblende 

 (uralite), which renders measurement with a 

 reflecting goniometer impossible. The size 

 of the crystal, however, allowed of the satis- 

 factory determination of most of the planes 

 with a contact-goniometer. In this way the 

 following forms were identified : prismatic 

 zone : a, f, m, b. Upper extremity : c, large, 

 r, — JOP (552); o, K, + mPn (?) Lower ex- 

 tremity; c, small; h,— 4P(441); r,v,o,s. The 

 following approximate measurements of interfacial angle s 

 were obtained with a contact-goniometer, for comparison with 

 which the corresponding values as calculated by von Kok- 

 scharow are also given : 



iri 



125° 26' 



c : r back above 126° 



c : r front below 125-£° 



a : r back above 133° / 



a: r front below 133° + 



c:z " 132° ' 131° 24' 



133" 24' 



c: o front above 114° 114° 40' 



m : o back below 145^-° 144° 36' 



c:v front below 129i° 130° 5' 



h : m " 165° (ca) 163° 36' 



/: m " 1534-° 152° 52' 



The positive hemi-orthopyramid, K, allowed of no definite 

 determination as its planes were small and roughened by the 

 projecting ends of little hornblende needles. It seems, how- 

 ever, to lie in the zone o : o, and may be the form, +2P2 ( 2 11). 



It is a striking feature of this crystal that a small portion of 

 its upper half, represented on the left side of the figure, is of 

 quite normal habit, being terminated only by the positive 

 hemi-pyramid, o, as is the case below. 



The paragenesis of the Canaan crystals is identical with 

 those from Orange Co., ~N. Y., which are also superficially 

 changed to tremolite. This is a jjo-int of some interest, since 

 crystals from both localities show the same hemihedrism in 

 spite of a wide difference in habit. 



In a recent conversation with Professor C. W. Brogger, of 

 Stockholm, I also learned that he had not infrequently observed 

 a similar inclined-faced hemihedrism on aegirine crystals oc- 

 curring in the nepheline rocks of southern Norway. 



