OUTLINE OF THIS MONOGRAPH. XXY 



ashes, and breccias, which locally .are interleaved with or grade into graywacke, slate, or conglomerate. 

 Much of the material has lieen profoundly metamorphosed, and schist-conglomerates, mica-schists, 

 and hornblende-schists have resulted. All of the foregoing rocks are cut by dikes and nuissos of 

 greenstone. The formation is confined to the south-central part of the district. It grades into the 

 Ishpeming formation or the Michigamme formation below and into the Michigamme I'ormatiou above. 

 No accurate estimate of the thickness of the formation can be given. The volcanic material was 

 poured out from a number of vents, the more importantones which have been recognized being located 

 near Clarksburg, Greenwood, and Champion. 



Chapter V treats of the igneous rocks of the Marquette series not belonging to the Clarksburg 

 formation. These rocks are all basic, having the composition of altered diabases. As to age, they are 

 divided into pre-Clarksburg greenstones and post-Clarksburg gTeenstones. The older rocks occur 

 as dikes, bosses, sheets, and tuft" beds, although the latter two are subordinate. They vary from 

 rather fresh diabases to schistose rocks which are micaceous hornblende-schists, chlorite-schists, or 

 talc-schists. The more metamorphosed forms are most abundant on the peripheries of the masses, 

 and especially close to the rocks of the Negaunee formation, and such rocks arc often heavily ferrugi- 

 nous. The post-Clarksburg greenstones comprise only dikes and bosses. They are much fresher than 

 the older greenstones, being mainly nonfoliated. Their alterations are chiefly metasomatic. They com- 

 prise oli vine-diabases, quartz-diabases, porphyrites, and basalts. It is conjectured that these rocks 

 are correlative with the eruptives of the Keweeuawan series. 



Chai'TER VI treats of the Republic trough. This is an isoclinal syncline, extending southeast 

 from the western end of the district. The rocks of the Republic area belong to the Archean, Lower 

 Mar(iuette, and Upper Marquette series. The Archeau rocks comprise granites, gneisses, and crystal- 

 line schists. The schistose structure is especially developed adjacent to the Algonkian rocks. The 

 Lower Marquette series includes the Ajiblk quartzite and the Negaunee formation. The Ajibik 

 quartzite, probably not exceeding 100 feet In thickness, rests nnconformably upon the Archean. At 

 one place a coarse basal conglomerate is found in direct contact with the rocks of the underlying 

 series. For the most jiart the formation has been transformed to a micaceous vitreous quartzite or 

 into a mica-schist. The Negaunee formation consists of two horizons, a griinerite-magnetite-schist 

 below, and a specular jasper .above. The Upper Marquette series consists of the Goodrich quartzite 

 and the Michigamme mica-schist. The Goodrich rock has been transformed to a quartz-schist or to 

 a micaceous quartz-schist. In the southeastern part of the trough, at the bottom of the Goodrich 

 quartzite, is a great conglomerate, the detritus of which is derived mainly from the underlying Negau- 

 nee formation ; also there is a diiference in dip between the Goodrich quartzite and the Negaunee 

 formation. From these facts it is certain that the two are unconformable. The Michigamme mica- 

 schist occupies the center of the tongue. This grades down into the Goodrich quartzite. Basic 

 intrusives occur in both the Upper and the Lower Marquette series, the same as in the remainder of 

 the district. A fault with hade nearly but not quite parallel to the bedding occurs on the eastern 

 side of the trough near Republic. The iron-ore deposits are at the contact of the Ishpeming and 

 Negaunee formations or within the Negaunee formation. The important deposits are at the end of the 

 trough, and especially at the bottom of subordinate plunging synclines. They are secondary concen- 

 trations, produced by downward-percolating waters. 



Chapter VII treats of the general geology, and involves a consideration of the Basement Com- 

 plex, the Lower Marquette, and the Upper Marquette series. The Lower Marquette has a possible 

 maximum thickness of 6,120 feet, but it is not probable that any single section will give as 

 great a thickness as 5,000 feet. The Upper Marquette series, excluding the volcanics, is probably 

 less than 5,000 feet thick; including the volcanics it is probably more than 5,000 feet thick. 

 Before the beginning of the deposition of the Lower Marquette series the Basement Complex had 

 been deeply eroded. The transgression Imrizon of the Lower Marquette series is a conglomerate, 



