228 THE CRYSTAL FALLS IRON-BEARINa DISTRICT. 



These knobs are cut by a number of small dikes from a fraction of au 

 incli to 3 inches in width. In all of these dikes the rock shows the same 

 characters. It is very light gray to pink in color, and aphanitic. 



An examination under the microscope enables the separation of each 

 dike into a A^ery compact fine-grained saalbaiid and a somewhat coarser- 

 grained porphyritic central portion. In the central part of the dike pheno- 

 crvsts of quartz, feldspar, and biotite lie in a very fine groundmass of 

 quartz and feldspar. The texture of this g-roundmass is microgranitic. 

 The saalband is composed of the microgranitic groundmass without the 

 phenocrysts. The quartz phenocrysts sliow the usual characters. The 

 feldspar phenocrysts are in most cases so completely replaced by a musco- 

 A^te aggregate as to preclude any exact determination of their original 

 character. In some cases indistinct remains of polysynthetic twimiing are 

 seen. Even when the main mass of the feldspar phenocrysts is entirely 

 altered, there is a narrow zone of very fresh feldspar material surrounding 

 it. Twinning- in the center is also continuous through this zone. More- 

 over, this zone itself shows a very noticeable zonal structure by the change 

 in extinction angle observed in passing from the inner to the outer portion. 

 This less altered zone of feldspar contains immerous inclusions of quartz 

 from the groundmass. The character of the feldspar phenocrysts could not 

 be determined, but the presumption is that they are of the same character 

 as the feldspar in the coarser main mass — that is, andesine — with a more 

 acid feldspar, possibly oligoclase, surrounding them. The further presump- 

 tion is then warranted that the feldspar of the groundmass agrees with this 

 outer feldspar zone in character — that it is also oligoclase, or at least is 

 more acid than the phenocrysts. Automorphic biotite plates are now repre- 

 sented by chlorite pseudomorphs, with here and there some secondary epidote. 



The groundmass consists chiefly of quartz and feldspar, but contains 

 disseminated through it inany minute plates of white mica and a few 

 crystals of zircon. The feldspar of the groundmass is too small to permit 

 of its accurate determination. A plagioclase feldspar in sections indicating 

 an approach to automorphism was observed. Its character as oligoclase (f), 

 or at least a feldspar of a more acid character than that of the centers of 

 the j)henocrysts, is surmised for reasons given above. Microcline in sec- 

 tions showing characteristic twinning and in more or less rectangular out- 

 lines was observed in considerable quantity. An untwinned feldspar was 



