MINERALOGY AND PETROLOGY. 957 



Hornblende.] 



rare rock called "hypersthene gabbro" (Nos. 1037, 692, 1710, 178E). It here exists 

 in all stages of development, from the initial "globular" form (Nos. 1042, 1037, 1710, 

 1784) to crystals of large size (Nos. 692, 1042, 1037, 1362, 1364, 1712, 2197, 397(a)H), 

 which frequently embrace all the other minerals poikilitically (No. 618). In 

 the globular state it is found entirely surrounded by biotite in No, 1042, by 

 hypersthene in No. 1037, by magnetite in No. 1040, by feldspar in No. 1784. 



Enstatite and bronzite. The former has been noted in muscovadyte (Nos. 705, 

 983), but the latter has only doubtfully been identified. 



Bastite, which is perhaps an alteration product of enstatite or bronzite, at least a 

 secondary mineral, was once noted in a hypersthene gabbro (No. 1710). This is at 

 or near a muscovadyte horizon, noted for the novelty and the multiplicity of its 

 mineral associations, and hence it is reasonable to suppose that it is another of the 

 curious creations of this zone of metamorphism, rather than an alteration product 

 after another mineral. 



Hornblende. Among the colored minerals hornblende to a notable extent plays 

 the role of quartz among the white ones. It is easily formed, either under ordinary 

 decay of some of the other earlier minerals, or under metamorphism. It is hence 

 present, in some of its varieties, in nearly all rocks that have been examined. The 

 green color of all the greenstones is almost wholly due to ordinary hornblende. It 

 is abundant in all the crystalline schists, even in the mica schists. There is scarcely 

 a granite that is free from it, nor a syenyte, while in all the diorytes it is mainly 

 hornblende that gives the dark, or spotted-dark, color to the outward aspect, sharing 

 it only with some epidote or a monoclinic pyroxene, which latter is usually diopside. 

 It passes through all the stages of development from the globular to the idiomorphic 

 and porphyritic. It is necessary here to mention only some of the most important 

 of the features it presents in Minnesota and to enumerate the varieties that have 

 been identified. 



Globular stage of hornblende. Beginning with the earliest recognizable condition 

 in which hornblende has been seen, the globular forms which are mentioned in Nos. 

 1345, 2104, 2264, 19H, are fair illustrations. It here occurs in incipient granite, re- 

 sulting from the metamorphism of a clastic debris, in a sphene crystal, likewise 

 formed in the regeneration of a more basic debris and in a muscovadyte which has 

 an alliance with dioryte in the abundance of hornblende which it contains. In the 



last the globular composite structure of some of the larger hornblendes is still 







evident in the patchy coloration between crossed nicols, and in the varied shades of 

 green which fleck them when rotated over a single nicol. In the main hornblende 

 has so strong a crystalline independence that the initial globular grains are perfectly 

 arranged in uniform orientation, thus building up the prismatic columns into which 



