THE GREAT SCANDINAVIAN OVERTHRUST 407 



Mount Areskutan he apparently accepts the suggestion of 

 Reusch, that its upper gneisses are altered eruptive rocks. 

 From the same memoir it appears that Schiotz, in 1902, 

 accepted the action of overthrusts to explain the structure of 

 Osterdal in Eastern Norway. 



In 1894 Herr Tornebohm's views received the powerful 

 support of Prof. Hogbom, now of the University of Upsala, 

 who demonstrated the truth of the overthrust theory by 

 careful mapping of the High Fells of Jemtland. His results 

 were published in a monograph, 1 and illustrated by a geolo- 

 gical map of the province on the scale of 1 to 500,000. 

 His survey proved that the overthrusting has taken place 

 on a scale even greater than in Scotland. Two years 

 later followed Dr. Tornebohm's important memoir, 2 which des- 

 cribed the geological structure of the main High Fells 

 both of Norway and Sweden, and extended the overthrust 

 explanation to the districts to the south-west of Jemtland. 



The surveys of Tornebohm and Hogbom leave no doubt 

 as to the occurrence of a great overthrust of pre-Cambrian 

 schists over Lower Palaeozoic sediments. The main overthrust 

 mass covers an area of some 5,200 square miles ; this vast 

 sheet of rock was thrust from west to east over the underlying 

 fossiliferous sediments. The evidence is clearest in the case 

 of some outliers of the schists, which have been cut off by 

 denudation from the main overthrust mass. The best known 

 of these outliers are at Offerdal, a locality to the north-west 

 of Ostersund and thirty miles east of Areskutan. Offerdal 

 is a valley running from north-west to south-east and containing 

 the upper arm of Lake Nalden. The floor of the valley is 

 occupied by the true Silurian beds, which are thrown into many 

 small folds, and are often isoclinals dipping to the west (fig. 5). 

 The valley is bounded to the north by a plateau, which is 

 cut through by the valley of Lake Landosjon ; the plateau 

 and the area to the north of that lake are formed by overthrust 

 mica schists and crushed Algonkian conglomerates and 



1 A. G. Hogbom, " Geologisk Beskrifning ofver Jemtlands Lan," Sver. Geo/. 

 Unders. Afhandl. Ser. C, No. 140 (1894), 107, p. 1, pi. and map. 



2 A. E. Tornebohm, " Grunddragen af det Centrala Skandinaviens Berg- 

 byggnad," Handl. K. Svens. Vet. Akad. vol. xxviii. No. 5 (1896), 210 pp. 4 pi. and 

 geological map of Central Scandinavia, on scale of 1 to 800,000 (with German 

 Summary pp. 179-97). 



