30 Minnesota Academy of Science 



with that made first in the gorge of the Mississippi river by Prof. 

 N. H. Winchell, as well as with the later similar measurements by 

 Gilbert and Spencer at Niagara Falls. 



For the student of the Archean there is a splendid field in Sweden. 

 The rocks are fresh, glaciated and exposed to view on sea coast, 

 along the new river courses and on the mountain sides. Every- 

 where the landscape reminds one of Minnesota and Ontario, and, 

 like Minnesota, Sweden has many great iron mines. In fact, there 

 is found a greater variety of workable ore deposits than has thus far 

 been discovered in our Archean territory. To the economic 

 geologist, Sweden is one of the most interesting countries on the face 

 of the round globe. Its great age as a center of mining, the magni- 

 tude of its deposits and their unusual types provide material for in- 

 numerable "kolossals" and "wunderbars" of the Teuton and chal- 

 lenge the utmost exaggeration of the Yankee to suggest their equal 

 in his own land. Historically, mining operations here are lost in 

 the mists of antiquity. Methods and machinery had established 

 vogues and patterns three hundred and fifty years ago in the days 

 when Agricola, the Nestor of mining, published his classic treatise. 

 Indeed, some of the machinery pictured by him in 1556 still finds 

 its counterpart in Swedish iron and copper mines, steadily doing duty 

 side by side with electric hoists or smelting plants of the most up- 

 to-date design. 



Although paleozoic and mesozoic strata are found in Sweden, 

 yet the larger portion is covered by the Archean rocks, gneisses, 

 schists and intruded rhyolite and porphyries. These eruptives have 

 been in many instances profoundly altered, and the ancient ore 

 deposits, which they were instrumental in producing, have under- 

 gone a varied and unusual history. 



The Swedish Archean consists of three petrographically and 

 geologically different groups of rocks. These have long been 

 named: The gneiss-group, the porhpyry-halleflintgneiss group, and 

 the granite group. Recently an alteration was made in this termin- 

 ology by the exchange of the term "halleflintgneiss" for leptite, the 

 latter having been proposed already in 1875 as a collective name for 

 the same rocks. 



The porphyry-leptite group, as the new designation also runs, 

 includes fine-grained gneisses, schists of many types, also green 

 schists, dense rocks, called in Sweden a long time ago "halleflinta," 

 limestones, dolomite and argillaceous schists, quartzite and conglom- 

 erates together with porphyries and porphyroid rocks to a large ex- 

 tent. Many of the rocks of this group bear evident traces of having 

 once been formed as real surface-products of the earth: lavas, 

 tuffs, tuffites or normal sediments, the latter, however, being only 

 subordinated* represented in the Archean. The leptites themselves, 

 which form by far the greatest part of this group, are closely re- 

 lated to the other rocks and seem to be metamorphosed rocks of 

 volcanic origin: lavas, tuffs or tuffites. Consequently the "porphyry- 

 leptite group" corresponds very well to the designation supercrustal 

 rocks, which has newly been proposed by the eminent explorer of the 

 Fennoscandian Archean, Dr. J. J. Sederholm. 



Supercrustal rocks also form a great part of the gneiss group, 



