76 Reviews — The Nonvegian Geological Survey — 



The Silurian. — The Bine-grey Quartzite. 



On a great part of the Hardanger Vidde the black Cambrian 

 schists are overlain by thickly bedded, blue-grey quartzite, in some 

 places associated with a little calcareous sandstone. It is traversed 

 by numerous veins of white quartz, often with cavities containing 

 good rock crystals. The tliickness of this quartzite varies fi'om 

 a few yards up to 180 feet. In some parts of the district it is 

 absent, and in the Suldals Fjeld it appears to be represented by 

 a greyish quartzite witb schistose layers on the bedding-planes. 

 A very similar blue quartzite is found in Valdres, Gausdal, and 

 other districts of Norway, and Professor Brogger suggests that it 

 possibly belongs to the lower part of Etage 3 of the Cambro-Silurian. 



Above the blue-grey quartzite there is frequently a bed of grey 

 or yellowish, rather fine-grained, crystalline limestone, containing 

 flakes of mica and chlorite ; these flakes become more and more 

 numerous in the upper part of the limestone, until the limestone 

 becomes a calcareous schist and finally passes into the overlying schists. 



Professor Brogger suggests that this limestone may correspond 

 to the Orthoceras Limestone which occurs at the top of Etage 3 

 in the Christiania district. This limestone in Hardanger Vidde 

 attains a maximum thickness of about 30 feet. Apart from this 

 limestone there is above the blue-grey quartzite a considerable 

 thickness of schists of a very variable character, as a rule becoming 

 coarser-grained in the upper beds. If the limestone above the 

 blue-grey quartzite corresponds to the Orthoceras Limestone, then 

 these schists will fall into Etage 4 of the Cambro-Silurian System. 



The Great Thrusts. 



The summits of the district are occasionally formed of the blue- 

 grey quartzite, since owing to its hardness it has withstood denudation 

 better than the schists, but the tops of the mountains and the high 

 ground much more frequently consist of older and still harder rocks 

 which have been thrust over the Cambro-Silurian. 



We have mentioned that the Cambro - Silurian rests upon a 

 remarkably even platform of granite and gneiss, but they themselves 

 have been greatly crushed and folded, and it is probable that this 

 crushing and folding were mainly caused at the time of the thrust 

 movements, the soft schists and associated beds having been com- 

 pressed between the old rock platform and the overthrust mass of 

 hard granite and gneiss. In consequence it is found that, though 

 the bottom of the schists is so level a plane, their top is often most 

 irregular, being puckered and contorted with the overthrust mass. 

 The most accessible place at which to study the relation of the 

 overthrust mass to the other beds is the pass between Seljestad 

 and Breifond, above the celebrated winding road known as Horre- 

 braekkene. The succession is as follows : — 



1. Overthrust mass of gueiss, granite, etc., forming the top of the mountain 



Horrehei. 



2. Silurian schists, etc. 



3. The black schists of the Cambrian. 



4. Granite, gneiss, etc. 



