522 THE MARQUETTE IRON-BEARING DISTRICT. 



cores of altered and reddened plagioclase, surrounded by perfectly fresli 

 material. The magnetite is the most interesting component. It occurs in 

 the ophitic spaces between the plagioclases, and is apparently all, or nearly 

 all, a decomposition product of augite or of oli\'ine, remnants of which 

 have been left in its mass. No traces of the latter mineral have been 

 detected in the section, but from the shapes of some of the magnetite 

 areas it is thought very jjrobable that jjyroxene was once present in the 

 rock. The complete substitution of augite and oli-vine by magnetite is 

 unusual in rocks of this character. The jDrocesses to which the substitution 

 is due in the present instance were probably related to the jjrocesses that 

 gave rise to the ores. 



BASALTS. 



The rocks that have been refei-red to as basalts are rare. They consti- 

 tute well-defined dikes. In the hand specimen they are dark, dense rocks, 

 occasionally dotted here and there with small white spots consisting of 

 plagioclase. Under the microscope plagioclase laths, magnetite grains, and 

 small spicules of augite are detected in a glassy groundmass, which is 

 often filled with green alteration pi-oducts, and sometimes almost entirely 

 replaced by these substances. 



SUiaMARY. 



Tlie igneovis rocks occurring in the Marquette series are all basic, with 

 the composition of fresh or altered diabases of various kinds. They occur 

 as dikes, bosses, sheets, and tuff" beds. An association of sheets and tuflf 

 beds is found to constitute a well-defined horizon in the Upper Marquette 

 series. These rocks, called the Clarksburg formation, have consequently 

 been separated in the discussion from the other igneous rocks. The 

 remainder of the igneous rocks, which may be classed together under 

 the convenient and noncommittal name of greenstones, have been further 

 divided into two classes. In one of these are placed all the green- 

 stones associated only with rocks older than the Clarksburg formation, 

 and in the other those associated also with the beds younger than this 

 formation. 



Among the pre - Clarksburg greenstones dikes and boss masses are 

 common, while sheets exist to some extent, and tuffs are rare. The dikes 



