REVIEWS 403 



genetically related. With the Negaunee are masses of basic igneous 

 rock, both extrusive and intrusive, the latter being the more common. 

 There are three classes of ore deposits, one at the bottom, one within, 

 and one at the top of the iron-bearing formation. The first two are 

 generally of soft hematite, while the third, which is associated with 

 the uppermost part (jaspilite) of the iron -bearing formation and passes 

 up into the lower part of the Upper Marquette, is of hard hematite. 

 In origin the ore is the same as in the Penokee-Gogebic range, i. e., it 

 is derived from an original sideritic chert, and has been concentrated 

 in certain positions by percolating waters, and the silicious portions of 

 the rock have been removed from these areas of ore concentration. 

 The ore bodies lie in pitching troughs formed of comparatively 

 impervious rock, as slate or basic dike rock, or both. 



The Upper Marquette, resting unconformably on the Lower Mar- 

 quette, is divided into the Ishpeming, the Michigamme and the Clarks- 

 burg formations. The first is composed of the Goodrich quartzite and 

 the Bijiki schist, the latter of which is a griinerite-magnetite schist. 

 Small ore bodies occur near the top of the schist and also in the 

 Michigamme formation. The Clarksburg is composed of volcanic 

 material, and the volcanic activity is thought to have begun during the 

 time of deposition of the Ishpeming formation, and this igneous mate- 

 rial is regarded as replacing, where it occurs, the upper part of that 

 formation and the lower part of the Michigamme. 



The Republic trough is a syncline with the axis nearly horizontal 

 and running in a northwest-southeast direction. The three uncon- 

 formable series described in the Marquette area are here represented, 

 but of the Lower Marquette only two upper members are present, 

 and the Clarksburg formation of the Upper Marquette is lacking. 

 The iron ore deposits, which are of magnetite and hematite, are in 

 the upper part of the Lower and the lower part of the Upper Mar- 

 quette. 



In structure the Marquette district is complicated, the rocks having 

 been subjected to severe folding. The general structure is a broad 

 complex synclinorium, in which the folds have an east and west direc- 

 tion. At the edges the folds are sharply overturned, and on going 

 toward the center higher and higher beds are encountered. The struc- 

 ture is similar to the composed fan folds of the Alps, except that the 

 whole has sagged down, forming a synclinorium rather than an anti- 

 clinorium ; to this the name of Marquette type of fold is applied. In 



