THE IRON-BEAKING MEMBEK. 231 



often produced after a certain amount of alteration had been effected, inasmueb as 

 tlie fragments themselves have evidently been in part altered before being torn apart. 

 The process continuing, these disrupted pieces, so far as they were still pretty pure 

 carbonate, had developed in them by oxidation the concretionary structure, and 

 tiually were often replaced more or less completely by siliceous material. Into many 

 of the fragmental areas ramifying veinlets of the siliceous groundmass extend, so 

 that the various appearances liiesented by the section seem all explictable by a contin- 

 uing process of solution, oxidation, and silicitication, accompanied by a certain 

 amount of dyuamic movement. 



While the above process explains a part of the fragmental character of the rock, 

 it seems probable that it has been to some extent actually shattered by erosion. 

 The layers here contain some fragmental quartz; they are at a high horizon, 

 which represents probably the beginning of the change from nonclastic to clastic 

 sedimentation. The nonclastic sediments immediately after deposition were jterhaps 

 broken to a greater or less extent, forming detritus, which was mingled with the 

 same kind of sediments which continued to form at favorable times. (PI. xxii. 

 Figs. 3 and 4; PI. xxvji, Figs. 2 and 3.) 



From the section on the Mount Hope property. 



35. Heoatitic flint, from a middle horizon. Specimen 7G10 (slide 2306) ; from 

 1150 N., W., Sec. 23, T. -17 N., R. 47 W., Michigan. 



The thin section shows seams composed entirely of a very minutely crystiilline 

 to quite amorphous silica, interbanded with others in which the red and brown iron 

 oxides are mingled with more or less of the silica. There is no brecciation apparent 

 in the section. Here and there are minute patches of iron carbonate from whose 

 oxidation it may be supposed the iron oxides came. 



From the section on the Puritan property. 



36. Ferruginous cherts or flints, from lower and middle upper horizons. Speci- 

 mens 7G04 (slide 2304); from 70 N., 1000 W.; 7605 (slides 2011. 2000); from l.W N.. 

 1050 W.; 7000 (slide 2305); from 276 N., 1175 W. All from Sec. 1«, T. 47 N., R. 46 

 W., Michigan. 



The rocks are grayish to whitish cherts, banded by reddish seams. 



The sections of these rocks are made up of minutely crystalline chalcedonic 

 and amorphous silica, mingled with bands and irregular patches of brown and red 

 iron oxides. Occasional renmants of iron carbonate may be detected, and at tiu)es 

 the background presents vaguely the concretionary appearance described in foregoing 

 sections. 



37. Hematitic flint or chert, from a low horizon. Specimen 7601 (slide 2302); 

 from 275 N., W., Sec. 18, T. 47 N., R. 46 W., Michigan. 



The prevailing siliceous gronndnniss is in large proportion of exceedingly finely 

 crystalline spotty (juartz mingled with amorphous material. In irregular areas and 



