MINERALOGY, 63 



considered as dimorphous. Now it is well known that there is no variety 

 of pyroxene of which there is not a corresponding variety of hornblende ; 

 and so it may be inferred that only difference in condition is necessary 

 to make one or the other species out of the same components. But 

 where we see, as in the case of our diorites, both minerals made in 

 a place where the conditions were necessarily the same, it is plain that 

 the chemical conditions have also influence, and that the species are not 

 strictly dimorphous, but different chemical compounds. This cannot be 

 proved in the case of our rocks, since they are fine in texture and the 

 materials inseparable; but as the point is of interest, I have looked about 

 for materials which can be substituted. At Edenville, N. Y., there is an 

 association of pyroxene and hornblende apparently analogous. At this 

 place cavities in the rocks are filled with crystalline masses of the two 

 species placed upon one another in all kinds of ways, — hornblende upon 

 pyroxene, and vice versa, sometimes in large crystals, and again in small. 

 The crystals take the commonest forms of the two species, and present 

 no peculiarities of note. A study of these specimens brings one to the 

 same conclusion as does the study of our rocks. The minerals being so 

 intimately associated, the conditions under which they were formed must 

 have been the same ; and hence it must be inferred that some chemical 

 differences have determined the crystallization. Material was carefully 

 selected, and analyses were made, to test this point, with the following 

 results : 



Hornblende. Pyroxene. 



Silica, 42.97 51-05 



Alumina 11.90 2.02 



Iron sesquioxide, ........ 3.08 1.30 



Iron protoxide, 13-84 12.18 



Manganese protoxide, ....... .48 .12 



Lime, 11.63 22.07 



Magnesia, 11 -49 10.02 



Potash, .88 



Soda, 2.73 



Ignition, .38 .34 



99.38 99.10 



These analyses show that there is a marked difference in the composition 

 of the associated pyroxene and hornblende; and indicate that the pres- 



