THE CLARKSBURG FORMATION. 461 



similar knol)s are found, and these are taken t(j indicate tliat there were a 

 number of vents from which the lavas were extruded. From these vents 

 as centers the lavas and tuffs were sent out over the surrounding countr}", 

 but not to great distances, for the coarser materials did not span the '\\'iilth 

 of the basin in which the Michigamme slates were deposited. The sup})0- 

 sitiou of the existence of a number of vents situated along a line parallel to 

 the axis of greatest folding of the Marquette series, but to the south of It, 

 together with the north-south compression to which the Clarksburg beds 

 were subjected, in company with all of the other Algonkian formations of 

 the region, Avill explain satisfactorily the occurrence of the beds as a belt on 

 one side only of the Marquette synclinoi'ium. 



The topography of the area underlain by the Clarksburg rocks is 

 not essentially different from that of other portions of the Marquette range 

 where greenstones are prominent. It is characterized by the occurrence of 

 numerous small and large rounded knobs and long narrow ridges, often 

 bare at their summits, and sepai-ated from one another by stretches of swamp 

 laud or by sand plains. Where the bedded rocks are in excess of the intru- 

 sive ones the hills often possess precipitous southern exposures, but this 

 feature of the topography is not sufficiently striking to be characteristic. 



RELATIONS TO ADJACENT FORMATIONS. 



The relations of the Clarksburg rocks to the surrounding foiTnations are 

 often difticult to interpret. In a few cases where contacts are plainly visible 

 the interpretation is clear. On the south the volcanic series is bcnmded 

 by gr-inerite-magnetite-schists of the Negaunee formation and by Goodrich 

 quartzites and Michigamme slates. Southeast of Champion the volcanics 

 appear to rest for a short distance upon griinerite-schists. Although actual 

 contacts of the two formations have not been seen, well-characterized ledges 

 of the schists and of the volcanics are met with, sejiarated by covered inter- 

 vals of but a few feet in width. The schists appear to strike directly into 

 hills composed of the Clarksburg rocks, and, what is more significant, great 

 bowlders of the schists, some sharped-edged and others rounded, are found 

 thickly strewn through the lower beds of the volcanic series. 



With respect to the relations with the Goodrich quartzite and the 

 Michigamme slate there is somewhat greater obscurity. In the NW. ^ 



