Eleventh International Geological Congre, 



ss. 



33 



When Gustav Vasa secularized the church lands, the mine re- 

 verted to the crown, and was first leased to Joachim Piper, a burgher 

 of Stralsund in 1532. Iron ore was still only a subordinate pro- 

 duct mentioned along with "sulphur, vitriol, antimony, lead, tin, 

 copper, silver and gold." By 1545 iron ore production had become 

 important, under a lease from the crown. Later, in the 17th cen- 

 tury, the works and mines passed into the possession of private 

 owners. 



Gunpowder for breaking the ore was first tried at Dannemora 

 in 1728; and in 1727 a wonderful "fire and air engine" was set up 

 for raising the water. In 1805 a steam engine of Watt's construc- 

 tion, the first in Sweden, was installed. 



The rocks at Dannemora are crystalline schists with intrusive 

 granites. There are also other intrusive dikes. The Dannemora ore 

 district is chiefly occupied by halleflinta, and the porphyritic halle- 

 flinta has the greatest extent. It almost encloses the limestone and 

 other varieties or phases of the halleflinta. This so-called porphy- 

 ritic halleflinta consists of a dark colored microcrystalline fine grain- 

 ed quartz feldspar rock with a rich admixture of a sericitic mineral 

 of secondary origin. Chemically, it is an acid quartz porphyry. 

 Many varieties in color, texture and geological relations occur in 

 this field; and north of lake Grufsjon it passes into granulite or 

 leptite. 



The ores are enclosed in a limestone mass, which is in turn 

 surrounded by the halleflinta. This limestone has a length of about 

 three thousand meters and a minimum width of about four hundred 

 and fifty meters. Composed exclusively of magnetite, the ore has 

 a characteristically fine-grained structure, often as compact as steel. 

 It occurs associated with skarn or gangue (called "bracka") and 

 more or less mingled with limestone. The percentage of iron varies 

 from 20% to 05%. 



The more important ore deposits belong to a number of com- 

 plexes, some of which contain several parallel ore layers. In each 

 complex there occur several independent ore-stocks, separated by 

 limestone or "pracka." Such ore stocks may be connected with 

 other ore stocks in the direction of the pitch or toward the depth. 

 They are usually nearly vertical. Their horizontal extension in the 

 line of strike may reach two hundred to three hundred meters and 

 their thickness thirty to forty meters. 



The mines of the central field have been opened from the sur- 

 face on several ore-bodies of great thickness, partly separated by 

 gangues and branching toward the north. On the south this exten- 

 sive ore-formation is almost entirely cut off by a system of parallel 

 chlorite leaders, which form the southern wall of the Hjulvind mine, 

 and somewhat resemble a fault. These chlorite bands or skols were 

 probably originally diorite dikes which acted as a dam in CUttting 

 off the ore injection or the solutions by which it was formed. 



Quantities of sulphides, chiefly zinc blende, have been intro- 

 duced metasomatically at a later period, impregnating both the ore 

 and the later granite intrusions. 



The mines of Langban have produced small amounts of argenti- 

 ferous galena and sphalerite and considerable quantities of man- 



