248 THE CRYSTAL FALLS IRON BEARING DISTRICT. 



after tlie occurreiK-e of the mountain-building movements to which the 

 intruded rocks have been subjected. It is not believed that the mass 

 referred to in this description is an exception to the general rule, but that 

 its metamorphosed condition is referable to local causes, such as have pro- 

 duced the shearing planes to which allusion has been made (p. 234). It may 

 perhaps occur near or along some minor fault plane, though no indications 

 of a fault have been observed. 



The outcrop is composed in part of massive gabbro and in part of the 

 metamorphosed kind. The gabbro in its typical massive form (fig. A, 

 PI. XLIII) is a medium to coarse grained graiuilar rock, composed essentially 

 of plagioclase and dirty brown-green compact hornblende, the latter being 

 quite full of the inclusions mentioned above as commonly occurring in the 

 hornblende of the rocks in this region. In the metamorphosed rock the 

 grain is much finer, the rock possesses imperfect schistosity, and the color 

 is a lighter green than in the original. The component minerals are chiefly 

 hornblende and feldspar, with some quartz, chlorite, epidote, calcite, and 

 rutile. The hornblende has a light-green color. It occurs mainly in aggre- 

 gates of small irregular grains. In some cases these surround an angular 

 nucleus of dirty brown-green original hornblende containing the same 

 inclusions and in every way similar to that of the coarse-grained uncrushed 

 rock. The light-green hornblende contains none of these minute inclusions 

 and only here and there grains of rutile, which, if the diagnosis of the 

 interpositions as titanium minerals is correct, may possibly be considered as 

 having been derived from them. This hornblende is believed to be sec- 

 ondary. It is produced by a process of mechanical breaking down of the 

 original hornblende, accompanied by recrystallization. The process is 

 somewhat similar to the crushing of ordinary plagioclase and the produc- 

 tion of a more acidic variety. As may be seen from the photomicrograph 

 (fig. B, PI. XLIII) of a section, the feldspar exhibits signs of intense crushing. 

 The twinning lamellse arc strongly bent, and the pieces possess wavy 

 extinction. Many, in fact most, of the feldspars are fractured. Wlierever 

 these fractures occur the feldspar along the edges of the fractured portions 

 has been altered, producing secondary epidote, muscovite, plagioclase feld- 

 spar, and quartz; the last, however, in small quantity. The biotite has 

 been crushed. This has partly altered to chlorite, and the latter contains 

 many grains of epidote. Some granular calcite and considerable iron 



