180 THE MARQUETTE IllOX-BEAKIXd DISTRICT. 



Occasioiiallv tin* structure of the diabases is porj)livritic rather than 

 ophitic. In the SW. \ see. 23, T. 48 N., R 25 W. (Atk.s Sheet XXXVIII), 

 for instance, is a fine-gTained dike that may l)e called a diabase-porphyrite. 

 It contains phenocrysts of plagioclase and augite in a g-roundinass com- 

 posed of a plexus of small plag-ioclase laths, round aug-ite grains, and cr^■stals 

 of magnetite, with the intersertal structure as defined ])y Kosenbusch. A 

 second diabase-porphyrite differs from the type just mentioned in tlie 

 absence of pyroxene phenocrysts and in the presence of magnetite in large 

 quantities. The latter mineral is found not only in the little grains between 

 the constituents of the groundmass, but also in large, irregular masses scat- 

 tered throngh the rock and in skeleton crystals resembling the microlites in 

 basic glasses (fig. fi). These microlitic growths form long, slender, straight 

 rods cutting indiscriminately through the grains of the 

 groundmass and through phenocrysts. 



i\Iost of the basic dikes in the Northern Complex are 

 epidiorites, that is, they are rocks showing unmistakably 

 the diabasic structure, but in which the augite has been 

 entirely replaced by uralite or some other green horn- 



Fio. 6.-Mngnetite in fine bleude. Tlicy are always more or less schistose, and there- 

 grained diabase or basait iii i ij^iti ml 



fore are much older than the fresh diabases, liiey vary 

 from one another mainly in the form of their honiblendic constituent and in 

 the freshness of their plagioclase. In some of them the augite has been 

 pseudomorphed by green hornblende. In others the hornblende is in 

 isolated or in grouped acicular crystals,' the ends of which often extend far 

 out into the altered plagioclase surrounding the areas originally occupied 

 In' ophitic augite. In the least altered epidiorites the plagioclase is fresh, 

 and in these varieties augite cores often remain as nuclei within the araphi- 

 bole areas. As alteration progresses the plagioclase becomes more and 

 more clouded by secondary products until finally it becomes an aggTegate 

 of epidote, chlorite, amphibole, and calcite. . 



Fine examples of leucoxene are seen in many specimens. The min- 

 eral occui-s as little cloudy masses around titanic-iron grains, and also as 



'The green.stone-schist areas of the Menominee and Marquette regions of Michigan, by G. H. 

 Williams: Bull U. S. Geol. Survey No. 62, 1890, fig, 2 of PI. XII. 



