484 THE MAKQUETTE lliO.V liKAlMN(r DISTEICT. 



(Atlas Sheet XIX), southeast of the viUage of Clarksburg-, however, the 

 case is different. Here coarse gi-eenstones, in the form of knobs, are 

 the predominant rocks. These are associated with a few amygdaloids and 

 occasionally with tuffs. Sedimentary beds are present in small quantity in 

 the interior of the area, and when present the sediments are freely inter- 

 mingled with tuffaceous materials. Toward the northern and southern 

 borders of the belt sediments are more abundant, but the transition from 

 well-marked tuffs into typical sedimentary rocks is more sudden here than 

 elsewhere along the belt. 



Because of the great abimdance of coarse greenstones at this place, 

 the location of the principal vents for the volcanic portions of the formation 

 are supposed to have been here. Tlie knobs are taken to be volcanic plugs 

 or portions of thick flows that have escaped erosion. The amygdaloids are 

 lava flows. Tuffs may have been present in large quantity in the valleys 

 between tlie knobs, but if so they have been almost entirely removed by 

 denuding agencies. 



The Clarksljurg formation is a set of interbedded tuffs, lavas, sedi- 

 mentary and volcanic conglomerates and breccias, and other sediments, cut 

 throuo-h and through by dikes and bosses of an altered diabase or basalt that 

 is similar in composition to the older greenstones intrusive in the pre-Clarks- 

 burg beds of the Marquette series. The eruptive materials are basic. They 

 are in all probability the surface facies of the greenstones above mentioned 

 as intrusive in the Marquette series. 



From its relations to the Goodrich quartzites and the Michigamme 

 slates it is learned that the period of deposition of the volcanic series 

 embraced the closing stages of Ishpeming time and the opening stages of 

 Michigamme time. 



All the rocks of the Clarksburg series except the greenstones and the 

 lavas are banded and bedded. Most of them are foliated, and nearly all are 

 more or less completely recrystallized. Altliough originally approximately 

 horizontal, the beds are now contorted and folded so intricately that no 

 accm-ate estimate of the thickness of the formation can be made. 



