THE IGNEOUS ROCKS. 507 



schistose greenstones associated with tlie iron-bearing- ro(!ks, so far as known, 

 are of fragmental origin. All of them are metamorphosed igneous rocks. 



In some cases the smaller dikes are fresh, dark olivine-diabase or Ijasalt. 

 These appear to be independent bodies that are younger than the schistose 

 dikes and the bosses, for they traverse the latter as well as the fragmental 

 beds. One of them may be seen on the side of the cliff overlooking the 

 lake shaft of the Cleveland mine. Most of these dikes are the quartz- 

 diabase of Lane, and are identical in nature with the fresh dialiases cutting 

 the Archean rocks below the Lower Marquette series, and with those occur- 

 ring in the upper portion of the Upper Marquette series. They are therefore 

 of })Ost-Clarksburg age, and are discussed with the younger dikes. 



The small greenstone dikes, like the boss greenstones, are found to cut 

 all the rocks below the top of the Clarksburg formation, but also, like the 

 boss masses, they are confined principally to the iron-bearing formation. 

 They are very frequently met with in traversing the country underlain by 

 this formation, but are even more numerous than they appear to be. Many 

 of their exposures are small and l:)adly decomposed, so that they often 

 escape notice even whei-e not completely covered by loose material. In the 

 mine workings, however, their lower extensions are brought to light, when 

 their abundance is better appreciated. In Chapter III, Section VI, they 

 are shown to be closely connected with the accumulation of ore bodies. In 

 those areas where the alteration of the dikes and the inclosing schists was 

 not carried so far, as, for instance, at Mount Humboldt (Atlas Sheet XVI), 

 where the iron formation is chiefly rein-esented by griineritic schists rather 

 than by jaspers, ores, and feri-uginous schists, the dikes may easily be recog- 

 nized on the surface, and their number is fully realized. The sketch map 

 (PI. XXXIII) of the W. J of sec. 12 and the E. J of sec. 11, T. 47 N., R. 29 

 W., shows approximately the number of dikes met with in traversing at 

 intervals of one-eighth mile a square mile of territory. 



It is possible that some of the greenstones that are considered as dikes 

 may be interleaved sheets, since some of the latter are chlorite-schi.sts that 

 are identical with the chlorite-schists of undoubted dikes. The number of 

 the sheets, however, is probal)ly not great, and it is not important to distin- 

 guish between them and the dikes, especiall}' since many of the former are 

 undoubtedly intrusive, just as are the dikes themselves. 



