HISTuRLCAL REVIEW OF THE GEOLOGY OF MICHIGAN. 187 



jiiass of material regarding methods, miues and electrical relations has 

 historical value. 



I'umpelly with Credner," who has been for years one of the most dis- 

 tinguished of the German Geologists, furnished much of the material for 

 the description of the Menominee Iron Region, but it would be well before 

 taking uj) the work in other fields to consider the progress of knowledge 

 in the Marquette Kange. This field is the earliest opened and most ex- 

 tensively studied, and has naturally been the Mecca for numerous geolo- 

 gists who intended to apply information collected here to other regions, 

 and so we have references to the facts upon this range and inferences 

 founded upon them, which depend upon first hand information and actual 

 observation, very widely scattered in geological literature.'' Our brethren 

 in AVisconsin, INIinnesota and Canada have certainly similar rocks and 

 similar problems, a successful attack on one of which in one place will 

 be likely to modify conceptions all along the line. 



Among the rocks of the Archean we find coarse grained plutonic rocks, 

 granites, syenites and peridotites which are mainly attributed to the 

 lowest part of the formation, — the Laurentian of the earlier writers, 

 a term introduced from Canada which I think Ave should still keep. In 

 this lower group there are also gneiss and schists and possibly narrow 

 bands of rocks like those above. The stratigraphic relations are of ineffa- 

 ble complexity and we have not begun to decipher them. Van Hise 

 called them the Easement Complex and suggested that if there is such a 

 thing visible, they nmy be part of the original crust of the earth. Above 

 this Laurentian comes the iron bearing series proper, called the Huronian. 

 Beside the familiar types of rock such as black slate, dolomite and quart- 

 zite passing into conglomerate, there are two important types which 

 require especial attention. There are at first a lot of dark colored rocks 

 composed of hornblende with more or less lime soda feld spar, chlorite 

 and numerous other minor constituents. Instead of the hornblende 

 there is often augite. The hornblende is in fact in most cases a product 

 of alteration from the augite, though various authors have overlooked 

 the augite entirely, and others have considered the hornblende, which 

 some consider as secondary', as i^rimary. There is also a question about 

 this grou]) of rocks as to whether they are stratified members or a series 

 of sediments or igneous intrusives, and about the points just mentioned 

 along and at times acrimonious difference of opinion has developed, be- 

 ginning with Brooks and his assistants Julien and Wright and continu- 

 ing through i)apers by Wadsworth, G. H. Williams and Van Hise, 

 Smyth and Clements down to the present day. Personally I should 

 summarize the matter as in the following paragraph and the U.. S. Geolo- 

 gists would now, I believe, agree with me in the general conclusions, 

 though we may still differ in specific applications. 



There is a nund)er of different types of the group of rocks above 

 mentioned, most of which are undoubtedly igneous, — flows, intrusions 

 or dykes in the iron bearing series and originally, mainly augitic. In 

 the first place, it is relatively easy to cut out a group of dykes which 

 cut through all members of the iron bearing series and whose augite is 

 usually very little altered. They are the old feeders to the series of 

 Copper-bearing Rocks and are subsequent to the formation and much of 

 the distortion of the iron bearing series. Secondly, we have dykes which 

 are similar, but cut by the former set, of a different prevailing strike, 



