MINERALOGY. 6$ 



Alkali (by difference), . . . 4.17 1.48 



Ignition, .71 1.17 



Here, again, the same results are evident, the preponderance of alumina 

 in the hornblende being the most striking difference. 



But in the case of the pyroxene at Edenville, and also in some of our 

 igneous and metamorphic rocks, the pyroxene has changed its cleavage 

 and optical properties, and become hornblende. The minerals have not 

 become hydrated, or the iron oxidized, as in ordinary decomposition ; but 

 a change has been effected without any alteration of composition, form- 

 ing the mineral which was called by Rose uralite, which is pyroxene 

 having the inner structure and optical properties of hornblende. In 

 this case it is evident that pyroxene and hornblende are dimorphous 

 forms of the same composition ; that is, the molecules of the original 

 pyroxene, under the subsequent influences, have rearranged themselves. 

 Tschermak has noticed that crystals of pyroxene with crumpled ends 

 are the ones most liable to this kind of alteration. This is the case with 

 the Edenville crystals, but whether it be a result or a cause of the alter- 

 ation is not plain. 



From these analyses, it appears that any given composition capable of 

 forming pyroxene, may, under different circumstances, form hornblende, 

 and hence the two minerals are dimorphous forms of the same material ; 

 but that under uniform conditions, chemical composition will determine 

 what species shall be formed, and alumina is an important agent in this 

 determination. 



Now, in the decomposition of rocks and the redeposition of sediments, 

 the lime that is held in their composition is most apt to be dissolved and 

 carried away in solution, and thus we obtain those immense beds of lime- 

 stones, whereby the sediments are left more aluminous ; and so in meta- 

 morphic rocks there is a much greater tendency to the formation of 

 hornblende ; and our stratified basic rocks are mostly diorites and amphib- 

 olites, while the amount of metamorphic pyroxenic rocks is small. As 

 a confirmation of this, see the following analysis of hornblende from the 

 diorite of Littleton: 



VOL. IV. 9 



