Chap. 10] ELECTRICAL METHODS 705 



bodies averaged 6 to 7 m in width and contained from 70 to 80 per cent 

 ore. 



The field at Bjurfors has had a similar history. At first much trenching 

 was done because of the abundance of ore boulders in the area, but no 

 ore was found. The equipotential-line survey, started in 1918, was at 

 first unsuccessful. The work was taken up again in 1922 when a party 

 consisting of both geologists and geophysicists made a detailed survey of 

 the area and located some strong equipotential-line distortions. The indi- 

 cations were confirmed by diamond drilling, and commercial mineraliza- 

 tion was encountered in the eastern and central portions of the field. In 

 the eastern part the ores were about 5 to 9 m deep while in the western 

 portion the ores ran from 16 to 20 m in depth. 



East of Bjurfors an extended zone more than 3 km long and over 1 km 

 wide was surveyed in order to locate more ore bodies in the direction of 

 strike. Numerous very distinct indications were obtained, some of them 

 accompanied by magnetic effects. The presence of wet gravels in this 

 area made trenching difficult, and exploration by drilling proved these 

 indications to be largely due to graphitic and pyritic slates and noncom- 

 mercial mineral impregnations. This survey is an example of extensive 

 and distinct indications which are not due to commercial ore. In cases 

 of this kind only repeat measurements with other electrical and magnetic 

 methods, accompanied by as much geological mapping as applicable, will 

 lead to correct interpretations. 



In the glaciated areas of the northern United States and particularly in 

 Canada, where conditions are very similar to those in Sweden, the equi- 

 potential-line methods have been applied successfully. In Newfoundland 

 an extensive equipotential survey was made covering an area about 2 miles 

 long and 1 mile wide, following the location of lead-zinc ore at the Buchans 

 mine in the center of the area. The ore, consisting of lead-zinc copper 

 sulfides in a baryte gangue, occurs in lenticular masses in bedded tuffs 

 and porphyritic lava flows of Archean age. The thickness of the glacial 

 overburden varies from a very few to 60 feet. East of the Buchans mine 

 large indications were found at Oriental; two of these were drilled and 

 found to be high grade lead-zinc ore. West of the Buchans mine a large 

 area of indications was encountered and was confirmed by a number of 

 trenches. Subsequent drilling and underground work has indicated more 

 than 3 million tons of high-grade ore. A number of equipotential surveys 

 for ore exploration were made by Edge and Laby^* in the eastern part of 

 Australia (Queensland, New South Wales, Victoria, and Tasmania). 



Op. cit., pp. 75, 78, 83, 85, 86, 92, 122, 123, 124, 129, 132. 



