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, ‘ , - : ‘ a 3 3 : II.90 2.02 
Iron sesquioxide, . 5 3 P . . i i 3.08 1.30 
Iron protoxide, . e ‘ 4 i F : ‘ 13.84 12.18 
Manganese protoxide, . : : , - 3 5 48 12 
Lime, “ ‘i 11.63 22.07 
Magnesia, 2 e a 3 é 3 : . 11.49 10.02 
Potash, . 2 E ‘ é . : : 5 . .88 sine 
Soda, . 2.73 oie apay 
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- 
