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The Rock-Formations of the District

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The Rock-Formations of the District THE GEOLOGY OF THE BERGEN DISTRICT, NORWAY. 3 and often torrential; lakes of varied size, and often of great beauty, abound, but the great charm of the country arises from the fjords or inlets of the sea, which, running far inland, form a series of waterways throughout the district. The Hardanger Fjord has more than one opening to the North Sea a little to the south of Bergen, and penetrates into the country for a distance of over 90 miles. There is a network of fjords in the neighbourhood of Bergen itself, and one of them extends so far inland that the station of Bolstad, 48 miles from Bergen by rail, stands on an inlet of the sea. The Sogne Fjord lies to the north of Bergen, and its innermost port, Skjolden, is as much as J 30 miles from the open sea. Nordfjord is a district with a fjord about half the length of the Sogne Fjord, the different parts of which bear different names. All these fjords are deepest in the inner parts and have a shallow area dividing them more or less distinctly from the Norwegian Submarine Channel, and this is at least partly due to the deposit of moraine-material in the outer parts of the fjords at a time when the inner parts were filled with ice, and thus protected from deposition. No doubt some or all of them may be rock-basins to a certain extent, and there may be more than one rock-basin in the same fjord. Many of the lakes, too, may be rock-basins, but it is often difficult to say whether there may not be an old channel which is now hidden and filled with moraine-material; and in some cases the lakes are obviously formed by dams which are terminal moraines of retreated glaciers. n.-THE ROCK-FORMATIONS OF THE DISTRICT. By H. W. MONCKTON. r. The Archcean Rocks.-A large part of the Bergen district is mapped" Grund/ie/d," or fundamental rock. It extends from Nordfjord to Sogne Fjord, including the whole area of the snowfield jostedalsbne ; it forms both sides of the Sogne Fjord from near Leikanger and Fejos to the sea; there are patches at Lserdal, Aurland, and other places in the eastern part of the district. It extends southward from the Sogne Fjord into the Voss district, forms several mountains near the city of Bergen and many of the islands off the coast. There are also large tracts of Gnmdjjeld around the head of the Hardanger Fjord and in other parts of South Bergenhus. The Grundjjeld, which is practically equivalent to the Archsean, consists of- r. Granite, more or less gneissic. 2. The Telemark Formation. 3. Gneiss, with which are associated various more or less gneissic igneous rocks, both acid and basic. 4 CARL FRED. KOLDERUP AND H. W. MONCKTON ON Upper Telemark is an extensive district in southern Norway forming the northern part of the Bratsberg Amt, and a thick series of quartzites and conglomerates, associated with various igneous rocks, is developed there and is believed to be of pre-Cambrian age and to correspond to the Huronian of Canada. This series is known as the Telemark Formation. (Werenskiold, "Om pst Telemarken," Norges Geol. Unders., vol, liii, 1910, part 2.) Mr. Kaldhol, in 1903 (Norges Geol. Unders., vol, xxxvi, part 3), described the Telemark Formation in the Suldal-Roldal tract on the southern side of South Bergenhus, dividing the Archeean as follows: I. The older granite (newest part of the Archrean). 2. The Telemark Formation. 3. The old Gneiss. Mr. Rekstad, in the same year, speaking of the mountain-tract which forms the eastern part of South Bergenhus, says (Norges Geol. Unders., vol. xxxvi, part 4): "Quartzite and gneiss-like rocks with the character of the Telemark Formation occur in several places below the Cambro-Silurian Series, and are traversed by numerous veins of the granite which occurs in the Archeean, thus showing that they are older than the granite which forms, as has been said, the newest part of the Archrean." Subsequently (No. 94) he has traced the Telemark Formation to the northern end of the peninsula, upon which lies the great snowfield Folgefond. The formation there contains a conglomerate of quartz pebbles varying in size from that of a hazel to that of the fist. The conglomerate is underlain by a thick bed of quartzite, the two together forming the Telemark Formation. The Archrean granite of this peninsula is described by Mr. Rekstad as being usually porphyritic, with large crystals of red or grey orthoclase, there being at times small flakes of albite and oligoclase, which form a microperthitic intergrowth with the orthoclase. Owing to pressure, the felspar crystals have often been broken up, and in places the rock has been converted into an eyed gneiss. Passing to the north of the Hardanger Fjord we find Archsean Gneiss associated with considerable areas of more or less gneissic igneous rock. There is, for instance, a rather extensive mass of diorite at Evanger, on the Bergen-Voss railway, and near Bergen itself there are several patches of granite, which in some places is very little altered, whilst in others it has been changed into an eyed gneiss. The islands off the Bergen coast consist mainly of gneiss and the associated older granite. (See the maps by Reusch and Kolderup, No. 72 and No. 79, and post, p. 18.) There are many beds of quartzite in the district north of the Hardanger Fjord, some of which may belong to the Telemark THE GEOLOGY OF THE BERGEN DISTRICT, NORWAY. 5 Formation, but they are for the most part apparently of newer date. Two small patches of Telemark Formation are mapped near Hallingskarvet (see the map in No. 101). In the Lrerdal district the Archrean consists of gneiss with associated granite, and as the granite contains inclusions of gneiss it is clearly the newer of the two. The granite, which covers a large area, has been described by Mr. Rekstad (No. 86) as mostly porphyritic, the large felspar crystals consisting in part of orthoclase and in part of microcline. They include not in­ frequently smaller parts of a lamellar plagioclase (albite), and many examples contain also some oligoclase. The quartz usually shows undulating extinction under the microscope; the biotite occurs in bow-shaped crystals with often an unravelled appearance at the ends. Besides the biotite there is also some potash-mica, which seems as a rule to have followed on the decomposition of the felspar, This granite has also occasionally some hornblende. Accessory minerals are apatite, titanite, pyrites and magnetite. Zircon is not uncommon, and there is often epidote as a change­ product after plagioclase. The rock often shows fractured crystals of felspar and other signs of pressure. The Archrean between the Sognefjord and Nordfjord does not appear to have been much studied. It consists chiefly of grey and red gneisses, sometimes alternating with mica-schists. These gneisses are older than the gneiss-granites and porphyritic­ granites in this district. In the inner part of Nordfjord (Gloppen and Hyen) there is a formation of old quartzite. 2. The Cambro-Silurian Series.-The rocks which overlie the Archrean in Norway have been divided into a number of "etages," or stages, of which etages Ib to 9 represent the Cambrian and Silurian Series. These etages are not well developed in the Bergen district, and it is consequently only necessary to give a list of them here, and to refer for further details to Sir Archibald Geikie's "Text Book of Geology" (jrd Edition, 1903, vol. ii, pp. 924 and 969), and to the "Norwegian Geology" by Dr. Reusch (No. II2, p. 31). CAMBRIAN SERIES. Etage lb.-The Dlene/lus-beds. Etage IC, td, and Etage 2.-Black shale known as the Alum-shale. SILURIAN SERIES. Etage 3.-The Asaphus-beds, which include the Ceratof>yge- and Orthoceras-limestones and the Phyllograptus.shale. Etage 4.-The Chasmops-beds. Etage 5.-Consists of 5bthe Calcareous Sandstone-beds, and Sa the Gastero/Jod-limestone. Etage 6.-The Lower Llandovery with StricRlandia lens. Etage 7.-The Upper Llandovery with Pentamerus ob/ongus. Etage S.-The Wenlock Series with Spiriftrplicatlllus. Etage g.-The Ludlow Series, Spirifer e/evatus. 6 CARL FRED. KOLDERUP AND H. W. MONCKTON ON In the Bergen district these divisions can very rarely be recognised, and the whole Cambro-Silurian is included in one great formation which is mapped Fyllit or Ltrglimmtrskijtr, and is of Cambrian and Silurian age. A rather narrow band runs across the district in a north-easterly and south-westerly direction, forming much of the shore of the Lyster Fjord, crossing the Sogne Fjord at Fejes, and running into the Voss district, where it has a considerable extension. To the east of the two Amts or Provinces of Bergenhus there is a large area of this formation around the lake Vangsmjosen, It crosses our boundary near Maristuen, and is seen in the valley of the River Lsera and at many places between that valley and the Aurlands Fjord. It occurs as patches on the high ground over which the Christiania­ Bergen railway runs, and is found on Hardangervidden. It forms much of the shore of the Hardanger Fjord and is also developed around Bergen, where it has been known under the name Bergeus Skifernt (see p. :22). In 1865, Dr. Tellef Dahll found Dictyonema in the dark phyllite of this formation at Hulberget, on Hardangervidden, and some years later Dr. Reusch found fossils near the village of Os, some 1 S miles south of Bergen, at two places in Sondhordland, on Froyen in the Sondfjord district, and in Samnanger.
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