6 PROCEEDINGS OF MADISON MEETING. 



that any are sedimentary, although it is not asserted that such rocks do not exist 

 within the Basement Complex. 



Following the Basement Complex unconformably is the Lower Marquette series. 

 At the base of this series is a conglomerate and quartzite formation, usually thin, 

 and upon this rests an iron-bearing formation, so called because within it occur 

 ore-bodies. The change from the quartzite to the iron-bearing formation is usually 

 gradual, and before the typical phases of the formation appear there are in places 

 several alternations of fragmental and non-fragmental material. The iron-bearing 

 formation comprises many varieties of rocks, actinolite-magnetite-schists, ferrugi- 

 nous chert and jasper, however, being the most prevalent, but ferro-dolomite is 

 also present. 



Resting unconformably on the Lower Marquette series or upon the Basement 

 Complex is the Upper Marquette series. Looked at broadly, this is a great shale, 

 mica-slate and mica-schist formation. However, at its base at many places are 

 quartzites and conglomerates, the character of which depends upon the immedi- 

 ately subjacent rocks. A¥here the inferior rock chances to be the iron-bearing 

 formation of the Lower Marquette, the detritus forms a recomposed iron- formation, 

 and when secondary concentration has also occurred, this may carry ore-bodies. 

 The Upper Marquette slate contains another iron-bearing horizon, which is situated 

 several hundred to one thousand feet from its base. The phases of rock consti- 

 tuting this formation are similar to those of the iron-bearing formation in the Lower 

 Marquette, although jasper is not so plentiful, and cherty ferro-dolomite is more 

 abundant. 



At the east end of the Marquette district appears an area of cherty quartzites, 

 slates and cherty limestones, to which the term Mesnard series has been applied 

 by the Michigan state survey. This series is lithologically very different from the 

 Lower Marquette or Upper Marquette series as developed in the iron-bearing district 

 to the westward. Its position has not yet been positively determined. It is pos- 

 sibly a downward continuation of the Upper Marquette series, and also possibly 

 occupies a position unconformably between the Lower Marquette and Upper Mar- 

 quette series as they exist to the west. The succession in the Mesnardarea in its 

 most typical development comprises in ascending order (1) a lower quartzite, at the 

 bottom of which is frequently a basal conglomerate ; (2) a formation consisting of 

 dolomite interstratifled with slates and quartzites, often cherty, and (3) an upper 

 quartzite often similar to the belts of this rock interstratifled with the dolomite. 



Included within the Marquette series are great intrusive dikes and bosses of 

 altered diabase. Their intrusion is in many places the cause of the minor folding. 

 Also in the Upper Marquette district is an extensive area of contemporaneous vol- 

 canics, largely tuffs, running from north of the Saginaw and Goodrich mines to 

 Champion. The greatest width of this volcanic belt and the locus of the ancient 

 volcano was southeast of Clarksburg. In passing east or west from this center 

 more and more of water-deposited sediments appear mingled with the volcanic 

 debris, until the rocks pass into the ordinary sediments of the region. 



To the above succession and the separating unconformities both the United 

 States and the Michigan geological surveys are agreed. The latter survey has, 

 however, recently announced that the Lower Marquette and the Upper Marquette 

 are each divisible into two series by minor unconformities ; the first into Republic 

 and Mesnard, the second into Holyoke and Negaunee. This order places the 

 Mesnard in the second of the two possible places above suggested. The evidence 

 upon which these new subdivisions are based has as yet not been published. 



