234 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



mica (both nuiscovite and l)iotite), epidote-zoisite, and calcite. The plagio- 

 clase shows A'erv heantifully the effects of dynamic action in local granu- 

 lation of the peripheries of the individual. Such lines of granulated feldspar 

 can be followed through the sections, probably indicating shearing planes. 

 Inclusions are common. Some stout rutile crystals were observed in the 

 feldspar. In some cases minute hair-like needles, which iu a few instances 

 were of a size sufficient to admit of their ready determination as rutile, 

 wei-e also found penetrating the plagioclase. Crystals of apatite and iron 

 ores are also commonly included in it. There have also been found iu a 

 few cases minute hexagonal plates, which are translucent, with brown color, 

 and are presumably micaceous ilmenite. 



Mention of the presence of orthoclase in these rocks is made with con- 

 siderable doubt. Here and there a few plates of imtwinned feldspar, 

 showing a character somewhat different from that of the plagioclase plates, 

 were observed. As will be seen, the possibility of its presence is indicated 

 by the potash shown in the analysis. 



Biotite. — The biotite in the coarsely granular rock is in irregular plates. 

 They are frequently included in and attached to the outside of the horn- 

 blende. Its period of crystallization thus overlaps that of the hornblende, 

 though on the whole being contemporaneous with it. In the fine-grained 

 rocks biotite is better developed than the hornblende, and is apparently 

 for the most part older than it. In color it varies from a rich reddish brown 

 for rays vibrating parallel to the cleavage to a light yellow for those per- 

 pendicular thereto. It includes crystals of zircon and apatite, which are 

 surrounded by pleochroic halos. 



Hornblende. — lu most of the scctions hornblende is the most striking com- 

 ponent. It is present in the gabbros in three different varieties. The most 

 prominent kind is a reddish-brown hornblende, which has a dirty green 

 hornblende commonly associated with it and frequently intergrown with it 

 zonally. This hornblende occurs without the green, but the green is 

 invariably associated Avith the brown. The two are optically continuous in 

 the intergrowths. It is possible, though not susceptible of proof, that the 

 green is the result of the incipient alteration of the brown. The second 

 kind is compact, strongly pleochroic, common green hornblende, and the 

 third is a noncompact, reedy variety of light-green hornblende. The first 



