378 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the collimator, the metal casing of the lamp was pierced so as to allow 

 of a source of transmitted light for getting extinctions in cleavage plates. 

 Orientation could thus be eifected by means of extinction (when the 

 amphibole was known), as well as by using terminal planes. 



For reasons explained further on, Tschermak's orientation is adhered 

 to throughout this paper (/3 = 73° 58'). 



The photographic illustrations I owe to the skill of M. Monpillard of 

 Paris. The difficulties in reproducing anything like the sharpness of the 

 etch-figures on amphiboles, especially on faces other than that of tlie 

 fresh cleavage flake, are very great and fully explain any lack of defi- 

 niteness that may be observed in the micro-photographs. The diagram- 

 matic figures were drawn by means of a camera lucida and bring out 

 more clearly than the photographs, the points of essential resemblance 

 and dissimilarity which need emphasis. 



Both in the diagrammatic figures and the wash-drawings of Plate I. 

 the cleavage trace on each etched surface is represented by a straight 

 line, which the reader will immediately recognize. This line is replaced 

 in the photogravures by the longer edge of the page. The top of the 

 crystal will as usual be directed toward the top of the page in the case of 

 planes in the vertical zone ; the front of the crystal toward the bottom 

 of the page for terminal planes. 



Materials of Study. 



The work the results of which are embodied in the following pages was 

 begun in the laboratory of Professor Rosenbusch at Heidelberg, where 

 the initial experiments were carried on with crystals of Vesuvius horn- 

 blende obtained from the collection of the Mineralogical Institute, and 

 with Zillerthal actinolite, St. Gothard (?) tremolite, and Bohemian horn- 

 blendes from the private collection of Professor V. Goldschmidt of 

 Heidelberg. To this material were added 41 specimens from the Im- 

 perial Museum, Vienna, through the kindness of Professor Berwerth, 

 some 20 others from the collection at the Jardin des Plantes, Paris, due 

 to the liberality of Professor Lacroix, six fine crystals of aeuigmatite and 

 arfvedsonite from Professor Ussing, Copenhagen, classic glaucophane 

 from Professor Barrois, Lille, and much American material from Pro- 

 fessor Hobbs, Madison. To these gentlemen I should like to express 

 my hearty thanks for the privilege of securing so many specimens with 

 little trouble to myself, — material which in many cases is classic, and of 

 considerable value from the mere monetary point of view. Without the 

 use of so many representatives of the group, I should not have felt 



