318 THE MAKQUETTE IRON-BE AEING DISTRICT. 



feiTUgiiious chert which are identical with the similar rocks of the iron 

 formation. In color the nonferrug-inous varieties of the rocks are usually 

 dark-gray or greenish-gray, but some of the coarser kinds are light-gray. 

 In these the naked eye distinctly sees the well-rounded grains of quartz 

 and feldspar. Also, in many of them there appeal- to be large fragmental 

 grains of mica. In general, the iron oxide staining the slates is hematite, 

 but in some cases it is magnetite. The fine and coarse varieties of the 

 rock are interlaminated at many places, a layer of coarse graywacke being 

 between two fine-grained, slaty layers, and these bands being composed of 

 still finer bands of different degrees of coarseness. 



Microscopical. — Tlic least altered and coarsest graywackes are composed 

 mainly of large, well-rounded grains of quartz, a few of them finely com- 

 plex and cherty-looking, and of grains of feldspar, between which is a, 

 sparse matrix consisting of chlorite, biotite, muscovite, finely crystalline 

 quartz, and more or less ferrite. Usually the chlorite is predominant, but 

 in some cases the biotite and muscovite are equally abundant. Frequently 

 the quartz grains are distinctly enlarged. In most cases they show pressure 

 effects by undulatory extinction and fracturing, the latter sometimes being 

 in a rectangular sj-stem. The feldspar grains comprise orthoclase, micro- 

 cline, and plagioclase. They show beautifully their metasomatic change 

 into chlorite and quartz, biotite and (juartz, or musco^^te and quartz. In any 

 one case the alteration of individual grains may result in only one of the 

 micaceous minerals, more often chlorite than any other; very frequently 

 the alteration is into chlorite and biotite, or into biotite and sericite, 

 although chlorite may also be a simultaneous product. All stages of the 

 change may be seen, from those cases where the outer borders of the feld- 

 spar grains are surrounded by a film of the chlorite and mica, through 

 those in which the grains are interlocking masses of the chlorite, mica 

 quartz, and feldspar, to those where the feldspar grains have entirely dis- 

 appeared, their places being taken by a roundisli, complex mass of the 

 secondary materials. This alteration of the large feldspar grains is so 

 general that it strongly suggests that the most of tlie chlorite, biotite, and 

 sericite in the matrix developed from a feldspathic background. 



