PART I. CHAPTER VIL 



93 



Hornblende and Augite. 



once regarded as very distinct, although now some eminent 

 mineralogists are in doubt whether they are not one and the 

 same mineral, differing only as one crystalline form of native 

 sulphur differs from another. 



The history of the changes of opinion on this point is curious 

 and instructive. M^erner first distinguished augite from horn- 

 blende ; and his proposal to separate them obtained afterwards 

 the sanction of Haiiy, Mohs, and other celebrated mineralogists. 

 It was agreed that the form of the crystals of the two species 

 was different, and their structure, as shown by cleavage^ that is 

 to say, by breaking or cleaving the mineral with a chisel, or a 

 blow of the hammer, in the direction in which it yields most 

 readily. It was also found by analysis that augite usually con- 

 tained more lime, less alumina, and no fluoric acid ; which last, 

 though not always found in hornblende, often enters into its 

 composition, in minute quantity. In addition to these charac- 

 ters, it was remarked as a geological fact, that augite and horn- 

 blende are very rarely associated together in the same rock ; and 

 that when this happened, as in some lavas of modern date, the 

 hornblende occurs in the mass of the rock, where crystallization 

 may have taken place more slowly, while the augite merely lines 

 cavities where the crystals may have been produced rapidly. It 

 was also remarked, that in the crystalline slags of furnaces, 

 augitic forms were frequent, the hornblendic entirely absent; 

 hence it was conjectured that hornblende might be the result of 

 slow, and augite of rapid cooling. This view was confirmed 

 by the fact, that Mitscherlich and Berthier were able to make 

 augite artificially, but could never succeed in forming horn- 

 blende. Lastly, Gustavus Rose fused a mass of hornblende in 

 a porcelain fijrnace, and found that it did not, on cooling, assume 

 its previous shape, but invariably took that of augite. The same 

 mineralogist observed certain crystals in rocks from Siberia 

 which presented a hornblende cleavage, while they had the 

 external form of augite. 



If, from these data, it is inferred that the same substance may 

 assume the crystalline forms of hornblende or augite indiffe- 

 rently, according to the more or less rapid cooling of the melted 

 mass, it is nevertheless certain that the variety commonly called 

 augite, and recognized by a peculiar crystalline form, has usually 

 more lime in it, and less alumina, than that called hornblende, 

 although the quantities of these elements do not seem to be 

 always the same. Unquestionably the facts and experiments 

 above mentioned show the very near affinity of hornblende and 

 augite ; but even the convertibility of one into the other by melt- 

 ing and recrystallizing, does not perhaps demonstrate their abso- 



