464 THE MARQUETTE IKON-BEARIXG DISTRICT. 



east and west of this place. Around these, lava flows and a few tuff" beds 

 accumulated. As one jjasses away from the center along the belt compact 

 sheets of lava become less and less noticeable, while well-bedded tufts and 

 eruptive conglomerates and breccias become more abundant, and sedimen- 

 tary layers are interleaved with them. Toward the edges of the belt the 

 latter rocks increase in importance and the Avell-characterized tufts diminish 

 in quantity. The volcanic activity continued from the later portion of 

 Ishpeming time into the earlier portion of Michigamme time, beginning 

 and ending gradually. The volcanoes were evidently submarine, or at any 

 rate their products were deposited in water, even if the apices of some of 

 them were above the water's surface for a part of the time. The submarine 

 character of the volcanoes explains not only the interbedding of tuffs and 

 sediments and the formation of true conglomerates containing pebbles of 

 the underlying rocks, but it also exj^lains the existence of bedded breccias 

 composed of fragments of volcanic origin in a tuffaceous base and the 

 presence of conglomerates formed of volcanic fragments in a sedimentary 

 groundmass. 



The sediments, tuffs, conglomerates, breccias, lavas, and coarse green- 

 stones have all suffered a great amount of alteration, but in mauA' cases 

 their original nature can still be made out. The recognition of the true 

 character of the tuffs depends mainly upon their field relations and field 

 habits, but a pyroclastic structure can be detected in many of their sections. 

 Almost all the rocks, except the best-jireserved sediments, are now partly 

 or wholly crystalline. This condition has been brought about mainly by 

 the development within them of hornblende, biotite, and quartz. The 

 change from the fragmental to the crystalline texture is most nearly 

 complete in the groundmass of some of the ciinglomerates. This ground- 

 mass is a biotite-schist, not very unlike the biotite-schists of the Southern 

 Complex. 



THE MASSIVE GREENSTONES. 



The coarse crystalline greenstones that occur so frequently as 

 knobs in the area southeast of Clarksburg and farther east have the same 

 composition as the "diorites" of the Negaunee and other pre-Clarks- 

 bm'g fonnations, but a somewhat difterent structure. A few sj^ecimens 



