512 THE MARQUETTE IKON BEARING DISTRICT. 



component of these schists is discovered to he kaohn instead of talc. Patton' 

 has ah-eady described one of these dike masses; so we quote his statements 

 concerning- it: 



Tlie iron ores in the region around Islipeining are frequently cut by dikes of 

 diabase so tliorouglily altered as to be no longer recognizable except by their dike 

 form. * * * 



Under tlie microscope this kaolin-like rock shows very well preserved the diabase 

 structure * » * , but in place of the plagioclase laths and the augite grains there 

 is a white, earthy substance with interspersed colorless quartz grains, as well as black 

 magnetite. The magnetite presents about the same appearance as in the unaltered 

 rock, and is evidently the only ingredient that has not been altered. A chemical 

 analysis of this rock, made by Mr. Fred. F. Sharpless, shows that it is composed of 

 about 79 per cent kaolin and 20 per cent free quartz (magnetite and impurities 

 amounting to 1 per cent). It appears that in the process of alteration Na, Ca, Mg, 

 and Fe, as well as SOj, have been removed and HjO has largely increased, 



Patton's description of this kaoUn rock apphes as well to some of our 

 specimens collected in the district. The rocks are evidently in the form 

 of dikes. They represent the extreme phase of alteration to which the 

 diabases in a few instances have been subjected 



It should be remarked, before leaving the discussion of the talc-bearing 

 and other light-colored schists, that there are certain other rocks, found 

 associated with the ores in some of the mines, that resemble very closely 

 the schists that are derived from diabases. These rocks are included 

 by the miners with the "soapstones" that occur as dikes. They are white 

 schists, often interbedded with quartzite and sometimes immediately over- 

 lying the hard-ore deposit at the base of the Upper Marquette series. In 

 the field these schists may occasionally be traced into well-characterized 

 quartzites. Under the microscope they are found to be fragmental in struc- 

 ture and to consist of altered and mashed feldspathic quartzites, not very 

 different from some of the finer-grained phases of the Palmer gneisses. 

 Many of them contain considerable quantities of sericite, so that in the hand 

 specimen they possess the same soapy feeling that is possessed by the talc- 

 schists. While very similar in appearance to the true soapstones occur- 

 ring in dikes, these rocks are not soapstones. They are sericite-schists, 



' Microscopic study of some Michigan rocks, by H. B. Patton : Report of the State Board of 

 •Geological Survey for 1891 and 1892, Lansing, 1893, pp. 185-186. 



