44 CLASSIFICATION BASED ON MINERAL COMPOSITION. 



The Pyroxene- Amphihole Groups. 



In the enstatite-liypersthene-pja^oxene-amphihole group of minerals, a 

 similar relation seems to exist as in the feldspars, and a like variability. 



In these the distinctions are founded mainly on optical, crj^stallographic 

 and cleavage characters. May there not be a similar relation between the 

 orthorhombic and monoclinic pyroxenes as there is between the different 

 feldspars ? Specific distinctions between rocks have been based solely 

 on a difference in cleavage in minerals otherwise identical. This is the 

 case with augite and diallage, which thus become the means of sej)arating 

 diabase from gabbro. How valid this cleavage distinction is, may be 

 learned from the fxct, that the cleavages of augite and diallage are found 

 sometimes united in a single crj^stal of pyroxene, \yhile augite has been 

 regarded as distinctive of the augite-andesites, basalts, and diabases, more 

 recent observations show that it is not confined to any one species of rock, 

 but exists in every species, from the pallasites to the rhyolites. So, too, 

 it was regarded as entirely characteristic of modern or younger rocks, but 

 this is found not to be true. This belief in the occurrence of augite in 

 modern rocks has arisen mainly from its ready alteration to viridite, 

 chlorite, hornblende, biotite, etc., which would thus cause it nearly or 

 entirely to disappear in the older and more altered rocks. Again, the 

 probability that pyroxene in some of its varietal forms, like sahlite, is of 

 secondary origin, increases the difficulty of employing pyroxene as a species 

 character. 



In the case of hornblende but little distinction is made in nomencla- 

 ture, whether the mineral is foreign, original, or secondary ; but in all 

 these modes of occurrence it is given equal value in classification. As a 

 foreign product it occurs in the andesites, the augite apparently arising 

 from the crystallization of the dissolved hornblende material, while in the 

 older forms of the same andesites hornblende occurs as a secondary product, 

 after both the foreign hornblende and orig-inal auo;ite. But the same rock 

 is given three different names, according to the predominance of augite, 

 foreign hornblende, or secondary hornblende. This is done, however, uncon- 

 sciously by lithologists, since they do not practically make these mineralog- 

 ical distinctions. The writer has seen two sections taken from the same 

 hand specimen ; one of which pronounced the rock a diabase, the other a 

 diorite. 



