THE CLAEKSBUEG FORMATION. 483 



inclosed in a plainly fragmental jTreenstone resembling a tuff, and, on 

 the other hand, equally as large fragments of tuff are found in the schist. 



The impression made on the mind by the confused association of these 

 different rocks is not cleared up until the nature of the rocks is revealed 

 through the microscope. It then seems plain that we have here a series of 

 tuffs, sediments, and lavas. The tuffs contain a great deal of sedimentary 

 material, and the sediments much tuffaceous material. The two are inter- 

 bedded and grade into each other. The dark lustrous rock is an almost 

 pure tuff in some cases, and in others a lava. A lava flow caught up frag- 

 ments froni the tuffs and sediment. Later this lava contributed fragments 

 to subsequent tuffs and sediments. 



The little group of hills east of Champion, in the SE. ^ sec. 32, T. 48 N., 

 R. 29 W. (Atlas Sheet XII), affords other excellent exposm-es of the conglom- 

 erates. Here the black, lustrous, schistose groundmass is usually full of 

 little garnets. Large bowlders and sharp-edged fragments of griinerite- 

 schist, granite, and quartzite are crowded into the schist in great numbers. 

 The conglomerate appears also to be interbedded with narrow seams of a 

 fine-grained quartzite. The rocks are all very much contorted, the bedding 

 planes of the black schist being marked by rows of garnets. 



The hills north of the railroad track, in sec. 13, T. 47 N., R. 28 W. 

 (Atlas Sheet XXII), present a somewhat different aspect. They are built 

 up of alternating conglomeratic and nonconglomeratic beds striking about 

 N. 20° W. and dipping 50° NW. The conglomeratic beds are composed 

 principally of tuffaceous material, and the nonconglomeratic ones mainly 

 of sedimentary substance. All the rocks are schistose, with the foliation 

 inclined to the bedding, dipping in the same direction as the latter, but at 

 a smaller angle. At the base of the hill, on the south side, are true slates 

 or fine-grained graywackes interbedded with the tuffs, and a little to the 

 northwest, across a north-south valley, are some small sheets of the tuffs 

 interbedded with massive crystalline greenstones. 



In all the instances described the major portion of the volcanic part of 

 the Clarksburg rocks consists of tuffs and lavas, the former predominating. 

 Occasionally dikes and small knobs of massive greenstone are associated 

 with these, but they are rare. In sees. 7, 17, and 18, T. 47 N., R. 28 W. 



