472 THE MARQUETTE IKON 13EA1MNG DISTKICT. 



roun(le<l ones are observed that have been liuilt out by the addition of 

 quartz, and so have preserved the proof of their clastic origin. Other 

 grains resemble ordinary sand grains until they ai'e examined in polarized 

 light, when they break u}) into many interlocking areas, so that often a 

 section which in ordinary light has the typical clastic structure (PI. XXXII, 

 fig. 2) presents the appearance of a typical schist between crossed uicols. 

 The foliation of the rock is due principally to the an-angeraent of the 

 biotite in laminae that wind in and out between the larger quartz grains. 

 The biotite appears to have originated mainly in the sedimentary material 

 between the larger quartz grains. The large plates of amphibole that lie 

 in the groundmass, on the other hand, were formed after the rock became 

 schistose. They probably originated in greater part from the basic material 

 added to the sediments by the tuffs, either directly or through solutions 

 passing into the sediments from the tuif beds interstratified with them. 



The fine-grained material of the groundmass is composed principally 

 of quartz, chlorite, altered plagioclase, and magnetite dust. It has been 

 so completely recrystallized that all traces of its original structure have 

 disappeared. It was probably originally the finer-grained matrix between 

 the coarser components of a graywacke or slate. 



The most hornblendic of the fragmental rocks have already been 

 referred to as looking like massive crystalline rocks. Upon close examina- 

 tion, however, it is observed that even in the most crystalline of the beds 

 there is a distinct banding, which is emphasized by the different quantities 

 of biotite, magnetite, and hornblende in the different layers. Where the 

 hornblende is in great excess the bands look very much like an amphibolite. 

 Where the other components exceed the amphibole in quantity the bands 

 resemble more closely basic tuffs. A few bands are actinolitic. These 

 contain fairly large plates and groups of actinolite needles, identical with 

 the griinerite in the griinerite-slates of the Negaunee formation, scattered 

 throuo-h a groundmass of fragmental quartz grains and occasional garnets 

 embedded in a matrix of chlorite, hornblende, biotite, magnetite, and limon- 

 ite, closely aggregated and without any well-defined structure. 



Gradation varieties between sediments and tuffs. BetweCU tllC rOcks jUSt dcSCribed 



as consistino- largely of sedimentary material and others that are composed 



