Plate XXXY. — From the Eastern Area. 



jTiG. 1. Ferruginous chert iiud quartzite. .Specimen 12937, slide 6547. From near tile center of 

 Sec. 23, T. 47 N., R. 44 \V., Michigan. In polarized light, x 60. The section shows the 

 sharp transitiiiii whiili IVeijuently occurs between nonfragmental and fragmental sediments 

 in the irou-liearing belt of the Eastern area. I'jion one side of the figure is seen a small 

 part of a band ol'piiri' nonfragmental chert. This abrn)itly changes into fragmental quartz 

 set ill a matrix consisting of ncinfragiiiciital chert and iron oxide. A jiortion of tlic frag 

 meuts of (jnartz are sim]do and two grains are ]daiiily enlarged. (Jtber grains are of cliert. 

 In tile change from nonfragnieiital lo Ihiginental sedimentation the underlying nonfrag- 

 meni.'il material lias liecn lirolicn up to some extent and has yieldeil fragments which are 

 associated witli tlic simple grains of ([iitirtz. (See )). 363.) 



Fig. 2. Greenstone-conglomerate. Si>e< inien !I313, slide 4929. From the SE. ^ of Sec. 14, T. 47 N., K. 

 44 W., Michigan. In ordinary light, x 25. The figure shows the extraordinary struc- 

 ture so constant in tlie greenstone-conglomerates. It is from one of the phases in which 

 chlorite and quartz are the jireponderating constituents. The anastomosing parts wliicli 

 contain the irregular areas are largely composed of i|uartz, while the irregular areas are 

 chierty luatTe up of quartz, clil(ni(e, and ipidotf. I'liis represents a pha.se of rock in which 

 the extreme of .alteration lias t.ikcii place. The pi'oducts are minerals whiih, with the 

 exception of ililorite, are ultimatcs in the series of Iransformations of rocks. (See pji. 3><4- 

 385.) 



Fig. 3. Greenstone-iimglomerate. Sjiecinieii 9369, slide 3036. From the SW. } of Sec. 15, T. 47 N'., K. 

 44 \V., Michigan. In ordinary light, x 25. The figure shows another |diase of the 

 remarkalde structure characteristic of these rocks. It contains apparent fragments which 

 are exciediugly angular and which arc set in an anastomosing matrix, consisting largely 

 of finely ciystalliiie quaitz. The fragments in this secticm are gray amor]dnius material, 

 in which arc numerous minute tabular crystals of plagioclase, (See pp. 3><l-382.) 



Fig. 4. The same, in ]iolarized light, X 25. 'I'liis lignre shows the reticulating quartzose background. 

 The almost completely amor|dioiis character of the major portion of the fragments is 

 sharply brought out. Within them, as in the ]ircvioiis figure, the tabular crystals of 

 jilagioclase stand out. (See pp. 381-382.; 

 518 



