486 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



plainly on weathered surfaces, like the flattened pebbles in a squeezed con- 

 glomerate or the drawn-out parts of quartzose layers in a mashed bedded 

 rock. These bands, though not very well defined, run continuously for 

 long distances, and strike and dip conformably with the conglomerate beds 

 exposed 200 paces to the north. 



PETROGRAPHICAL DESCRIPTION. 



In the thin section the lighter-colored layers of these rocks are seen 

 to be composed of very irregularly outlined and rounded quartz grains, 

 cemented by a mass of finer quartzes and small grains of zoisite, little 

 clumps of chlorite, some decomposed feldspar, and particles of magnetite. 

 Occasionally a plate of yellowish epidote occurs in the midst of this aggre- 

 gate, and scattered here and there through it are large plates of green horn- 

 blende with the cellular stiiicture so common to secondary minerals. These 

 hornblendes lie irregularly in the slide, and include grains of all the other 

 components. The quartz grains are small and are independently oriented, 

 but frequently little groups of them, with the outlines of sand grains, are 

 met with. There is little evidence of schistosity in these layers, but they 

 exhibit a banding produced by the alternation of coarser and finer constit- 

 uents. In the darker la3^ers the proportion of hornblende is much greater 

 than it is in the lighter ones. Indeed, some bands consist almost exclu- 

 sively of large cellular plates and radial aggregates of plates of this mineral, 

 only the small interstitial spaces between the large amphiboles being filled 

 with an aggregate of quartz-zoisite, small hornblende needles, and magnet- 

 ite. In some sections biotite is also present. It occurs most abundantly 

 in the quartz-zoisite aggregate, filling the interstitial spaces between the 

 amphiboles, but is present also as inclusions in this latter mineral. Some 

 of the biotite in the hornblende appears to grade into its host, and certain 

 portions of the amphibole possesses the brown color of the mica, with the 

 optical properties of the hornblende. The large amphiboles are evidently 

 the youngest components in the rocks, though they were plainly produced 

 before the schistosity. In those layers in which the schistosity is strongly 

 marked this structure is produced mainly by tlie parallel arrangement of 

 the biotite and the small amphibole needles and plates in the quartzose 

 aggregate. The larger cellular hornblendes lie across the schistose planes, 

 and when they do so, the lines of biotite and of small amphiboles pass 



