INTRODUCTION. XXVII 



limestone continued longer in the southeastern part of the Crystal Falls 

 district and in the Menominee district than in the remainder of the reg-ion. 

 The absence of the limestone and lower formations in the western two- 

 thirds of the Marquette district and the northeastern part of the Crystal 

 Falls district is explained by the fact that during early Algonkian time this 

 part of the region was not submerged. The Mesnard quartzite of the Mar- 

 quette district and the Sturgeon quartzite of the Crystal Falls and Menom- 

 inee districts stand opposite each other. 



From the foregoing it is apparent that the tlu-ee districts together pre- 

 sent a most interesting and complex structural problem. While there is 

 sufficient similarity in the foi-mations for one to feel considerable assurance 

 of their general equivalence in the different districts, it is certain that the 

 fonnations of similar kind did not begin and end at the same time. More- 

 over, there are remarkable lateral transitions in sedimentation, as a result of 

 the uneven surface of the Archean at the beginning of Algonkian time and 

 because of volcanic outbursts. As a result of the first of these conditions, it 

 is necessary to equate fragmental formations which occur in the central and 

 western parts of the Marquette district and the northeastern part of the 

 Crystal Falls district, with nonfragmental limestones in the area to the east 

 and south. Consequent upon the Upper Huronian volcanic outbursts in the 

 Marquette district, the Michigamme and Ishpeming formations are largely 

 replaced by the Clarksburg volcanies. Similar outbursts in the western part 

 of the Crystal Falls district in Lower Huronian time placed volcanic rocks 

 for this part of the district opposite the Mansfield slate and the Groveland 

 iron formation. 



The foregoing relations, combined with the great variety and complexity 

 of the sediments of the district, the presence of many forms of contempora- 

 neous volcanic deposits, the intrusion of the widest variety of igneous rocks 

 of various ages from Archean to later Algonkian time, and the complicated 

 folding and metamorphism to which the district has been subjected, will 

 readily convince one that the working out of the detail structiire of the 

 district by Messrs. Clements, Smyth, Bayley, MeiTiam, and others has not 

 been accomplished without most painstaking and laborious work, especially 

 as the region is covered by timber or brush and is overspread by a mantle 

 of glacial deposits. 



