PHYSICAL GEOGRAPHY 211 



lies 84 metres below the level of the sea. Lagoon-lakes, situated 

 close to the coast, occur also, especially in North Iceland; the 

 largest of them are Hop, H6f5avatn and Miklavatn. According to 

 the condition of the bars and the outlets the quality of the water 

 of the lakes may often change quickly; sometimes they contain 

 fresh water, sometimes brackish water, sometimes salt water. The 

 fauna in these lakes is also subject to periodical changes. Some- 

 times marine animals immigrate through the outlets; at other times 

 they disappear, and a freshwater fauna becomes dominant. 



Fishing in the Iceland lakes is of great economic importance 

 to the inhabitants. In the larger rivers and in many smaller streams, 

 salmon is caught, and now the fishing is often rented out to Eng- 

 lish sportsmen, especially in south-west Iceland. Many rivers, and 

 most of the lakes, abound in trout and salmon-trout (Salmo alpinus 

 and 5. tratta) which play a great role as a means of support of the 

 inhabitants. The quantity of living organisms varies greatly according 

 to circumstances; there appears to be most life in the shallow lakes, 

 especially in those with lava-bottoms, in which warm or luke-warm 

 springs are also sometimes found at the lake-bottom, around which 

 plant- and animal-life collect. Some Phytoplankton (Diatoms) occurs 

 in the lakes in South Iceland, but on the plateau and in North Ice- 

 land only Zooplankton (Daphnias) is found, and this is abundant. 

 In Thingvallavatn, Chara and Nitella grow at a depth of 15 30 

 metres; in Myvatn there is a quantity of Nostoc which is found 

 thrown up in great masses along the shores. In the lakes, there 

 occur in addition, several species of Limmea and Pisidium, and 

 Lepidiirns is frequently found in great quantities; there is, also, an 

 abundance of gnat-larvae and other larvae which serve as food for 

 the trout. 



Geology. Iceland is almost entirely built up of volcanic rocks, 

 none of which appears however to be older than Tertiary times. 

 The foundation of the island is a depressed and broken basalt- 

 plateau similar to the other Tertiary basalt-plateaus on both sides 

 of the Atlantic Ocean, in East Greenland, the Faeroes, Antrim in 

 Ireland and on the islands of Mull and Skye. It is therefore as- 

 sumed that in Tertiary times a continuous basalt-land extended 

 across the Atlantic Ocean, that it subsided in Mid-Tertiary times 

 and that Iceland and the Faeroes are the remains of this land. In 

 Greenland and on Skve the basalt rests on Jurassic strata, in Mull 



