PETEOGRAPHICAL CHARACTER OF THE SIAMO SLATE. 321 



pai't iiK'luded in the enlavg-ed borders of the quartz grains. These ferruo-i- 

 uous slates have intedaminated layers of material which in all respects, 

 except that an occasional fragmental grain of quartz may be seen, are like 

 the ferruginous and sideritic slates and cherts and griinerite-magnetite- 

 schists of the Negaunee iron fonnation. These are subsequently described 

 in connection A^ith that formation. In many instances the ferraginous chert 

 belts cut across the layers in a minor way, and thus show that they are 

 certainly a secondary product which formed by the alteration or replacement 

 of some original constituent. In other instances they are along cracks 

 which formed as a consequence of movement. Many of these belts are 

 probably replacements of original sideritic layers, which were interlaminated 

 with the fragmental sediments at the basal and topmost horizons. From 

 the siderite the other minerals developed, just as in the Negaunee formation. 

 In other cases the ferruginous and cherty materials which till the cracks are 

 probably from an extraneous source. 



RELATIONS TO ADJACENT FORMATIONS. 



It has already been said that the Ajibik quartzite grades upward into 

 the Siamo slate. The transition rocks are usually feldspathic quartzites and 

 graywackes. The best locality at which to observe all the phases of this 

 gradation is east of Teal Lake, in sec. 32, T. 48 N., R. 26 W. (Atlas Sheet 

 XXX). For the most part along the contact there are no conspicuous 

 exposures which show the exact manner of transition. 



Above, the slate is overlain conformably by the Negavinee iron forma- 

 tion. In many places the transition is gradual; in others, rather abrupt. 

 Near the center of sec. 20, T. 47 N., R. 26 W. (Atlas Sheet XXXII), the 

 formation is a coarse graywacke, and even approaches a quartzite, which 

 grades upward into the iron-bearing formation. There are various inter- 

 laminations of fragmental and nonfragmeutal material, imtil finally the 

 latter becomes predominant. Within this gradation zone the slate contains 

 more or less of nonfragmeutal material, and after the iron formation 

 becomes practically continuous it includes some fragmental material. The 

 interlaminated beds were closely infolded, and consequently brecciation and 



MON XXVIII 21 



