508 THE MARQUETTE IKON BEARING DISTRICT. 



PETROGRArHKAI. CIIAHAC TKIt. 



Many ot" the dike rocks are iso similar ti) tlie altered forms of diabase 

 characteriziny the knobs of the eastern portion of the Marquette district 

 that detailed descriptions of them are unnecessary. The extremely schistose 

 varieties of the greenstones are especially prominent in the smaller dikes, 

 including- as they do the chlorite-schists and talc-schists, the "soapstones" 

 and the "jiaint-rocks" of the miners. The former occur throughout the 

 entire district, but are of course much more common in those areas where 

 there has been a great amount of mashing. 



The great majority of the less schistose dikes contain large quantities 

 of fibrous ami)hibole and the remnants of altered plagioclase Sometimes 

 the feldspar is represented by saussurite, quartz, or by these minerals, caleite, 

 and chlorite; at other times by a sericitic mineral and quartz; and again, 

 very frequently, by a mosaic of quartz and fresh feldspar Biotite is also 

 present in some of the schists, especially those containing large quantities of 

 the quartz-feldspar mosaic. The principal bisilicate constituent is always 

 either anq)liibole or amphibole and its alteration product, chlorite. Occa- 

 sionally, even in the schistose rocks, these minerals occur in ophitic areas, 

 but usually the mashing has been so great that the darker-colored com- 

 ponents of the dike rocks, as well as the lighter ones, are in narrow lenticular 

 bodies. The cause of the foliation of the dike masses, like that of the boss 

 masses, is clearly seen to be dynamic. Not only are the components nearly 

 alwavs in the lenticular forms referred to, but very frequently the altered 

 plagioclases are crushed and broken and their fragments moved apart in the 

 direction of the foliation. Moreover, on their edges the larger grains are 

 not infretjuently granulated. 



The petrographical varieties of these dike rocks are very numer- 

 ous, but their features are in general not different from those of the boss 

 masses. A few varieties, however, should be mentioned as peculiar. In 

 one or two instances (as in Specimen 16180, from a dike 8 feet wide in 

 cue of the pits in the NW. 4 sec. 12, T. 47 N., R. 29 W., and in Specimen 

 17505, from a large ledge in the NW. ^ sec. 30, T. 47 N., R. 30 W.) the 

 rocks were originall}' porphyrites, or, perhaps, V)asaltic phases of diabase. 

 The first of these rocks now shows plagioclase crystals, that are more or 



