Eleventh International Geological Congress. 31 



but the high-grade of metamorphism which designates the gneisses, 

 mostly conceals their primary petrographic character and geolog- 

 ical relations so as to make their origin in many cases doubtful. 

 The gneiss group includes also gneiss-granites. By this term Swed- 

 ish petrographers understand granites of strongly regional meta- 

 morphic character, i. e. crushed, foliated or granulated granites, of- 

 ten with a clearly marked secondary parallel structure. 



The third group is the granite group. This embraces all the 

 numerous types of granites, in which the Swedish Archean is so 

 very rich. Great areas of Sweden consist of these rocks. Together 

 with the gneiss-granites they certainly make up much more than 50 

 per cent, of the whole Archean system. The contacts show that the 

 granite-magmas have cut all the other rocks and they must, there- 

 fore, be considered younger than these rocks. Their properties are 

 those of real plutonic eruptives. Together with the said gneiss- 

 granites and the gabbros and diorites, which appear in smaller 

 quantities, the granites may be said to form the infracrustal rocks 

 of the Archean, in accordance with the nomenclature of Sederholm. 



In the coast-regions, east and south of Stockholm, gneissose 

 rocks predominate; yet the granites and the porphyry-leptite-group 

 are also very well represented. 



Of the gneisses there are found both supercrustal and infracrustal 

 types. The origin of some gneisses cannot yet, however, be stated 

 with certainty. 



What is known as the porphyry leptite group of rocks is asso- 

 ciated with iron ores in central Sweden and also in the coast regions 

 near Stockholm, extending along the coast to the islands Uto, Orno, 

 Xamdo and Runmaro. The rocks are halleflintas, leptites, mica 

 schists, porphyry, epidote-or amphibole-bearing green schists, cal- 

 careous schists, limestones and iron ores. They are all bedded, of- 

 ten regularly and with alternations which make them closely re- 

 semble stratigraphical complexes. Their structures are, however, 

 wholly crystalline and the bedding planes are now always nearly ver- 

 tical. 



The iron ore at Uto is the type known as "randig blodsten," 

 i. e. quartz-banded hematite similar to our jaspilite. The quartz is 

 gray or reddish, and the iron ore is hematite with more or less mag- 

 netite. Beds of amphibolitic rocks accompany the ore and thin 

 green layers alternate with the iron ore strata. The mines at Uto 

 were worked as early as the beginning of the 17th century, and 

 operations continued down to 1S79. From 1711-1878 the output was 

 2,070,900 tons of iron ore. The total production is estimated at :\- 

 500,000 tons. The ore was not high grade, containing before con- 

 centration but little over 40% of iron. The Nykdping mine frafl 

 worked to a depth of about G50 feet, and the Finn mine about 500 

 feet. 



Two famous lithia pegmatite dikes cut across the folded iron 

 ore body of Nykdping. Here the element lithium was first detected 

 by Arvedson, a pupil of B'erzelius, in 1818. These dikes are among 

 the most important known natural resources of lithia. The dikes 

 consist mainly of petalite, quartz, lepidolite and coarse green ortho- 

 clase. They also contain spodumene, blue and red tourmaline (indi- 



