428 PROCEEDINGS OP THE AMERICAN ACADEMY. 



interesting to know if other studies of etcli-hills will confirm this 

 hypothesis. 



(5) A point of considerable theoretical interest is raised by the be- 

 havior of the clinopinacoid of actinolite when etched by hydrofluoric 

 acid. It is another of those puzzling cases of the existence, side by 

 side, on the same face, of two quite different kinds of figures, an asso- 

 ciation for which valid explanation has not yet been vouchsafed. 



(6) The amphibole family merits particular notice from the student of 

 etch-figures because it forms a test case for the theory that isomorphic 

 crystalline bodies must have similar etch-figures on corresponding faces. 

 If actinolite and common hornblende, for example, be isomorphous, 

 then their etch-jiits must be " similar " in a sense very different from that 

 adopted in the foregoing paper. By our usage, they can only be said to 

 be " analogous." In view of these facts, if this theory of the association 

 of isomorphism and etchings be of universal apj^lication, it will be neces- 

 sary to define more closely than has yet been done by an advocate of the 

 theory, the degree of variability that may occur in the etch-figures of 

 any isomorplious series. 



(7) The amphiboles are throughout holohedral. 



(8) A further proof of the extraordinary similaiity between the pyrox- 

 ene substance and the amphibole substance is aff"orded by the study of 

 the pits on crystals showing (010), (100), and (110). This is especially 

 true of the phenomena which can be observed when the clinopinacoid is, 

 in each case, etched with hydrofluoric acid. 



(9) It is incontestable that, in spite of their different crystalline de- 

 velopment and angles of cleavage, pyroxene and amphibole are so 

 closely and so instructively allied that the standard orientation for both 

 should bring out as conveniently as possible their points of resemblance. 

 Taking etchings as particularly significant of what there be of genuine 

 likeness in the two kinds of substance, there can be no doubt that the 

 orientation proposed by Tschermak should be universally adojjted in pref- 

 erence to the older orientation. 



(10) It is further possible to make etch-figures on amphibole of prac- 

 tical value by using them as a means of orienting cleavage flakes and 

 other crystal fragments. This use is parallel to that proposed by "Wiilfing 

 for the pyroxenes. 



(11) Lastly, so far as etch-figures may be trusted to show relation- 

 ships, we have the following results of our survey as to the systematic 

 classification of the amphiboles. 



The patent pronounced separation between non-aluminous and alumi- 



