g6 ESMARK : MOLLUSCA OF NORWAY. 



coast. Jsedderen is, however, covered with glacial gravel, to- 

 gether with erratic blocks of all sizes, and is very infertile. 

 Violent tempests from the sea rage there horribly in the 

 winter time — it is also mostly snowless — and the coldish sum- 

 mer makes it difficult for trees and bushes to grow. It is, there- 

 fore, the more interesting to observe that moUusks can thrive 

 there. 



Higher up than the lake Mjosen are great quantities of 

 sparagmit, sandstone, mica-shist, and quartzite, forming QEster- 

 dalen, Gudbrandsdalen, and Dovre ; the eruptives in other 

 places forms great masses of finest granite in the southern 

 interior parts, and gabbro in the grand mountain district of 

 great renown, called ' Jotunheim.' In the diocese of Thrond- 

 hjem, we have ' Throndhjem-schists,' that to a great extent con- 

 sists of mica-schist, whilst also Silurian limestone strata are 

 found, but they have, however, no significance as a substratum 

 for the moUusks living on them. 



In Nordland and Finmarken we have schists, sandstone, 

 limestone, and quartzite of the primitive rocks ; the limestone, 

 as well in greater quantities as in lesser strata intermixed in 

 mica-shist, as in the islands of Groto and Gaaso in Nordland. 



The Silurian formation is chiefly to be found in the valley 

 of Christiania and neighbouring parishes going towards Dram- 

 men, particularly as limestone and argillaceous slates. It ex- 

 tends to Modum and Ringerige, and along the eastern side of 

 Tyrifjorden as upper silurian limestone. We have also upper 

 Silurian strata on the eastern side of Skiensfjorden. The Lake 

 Mjosen is also surrounded by silurian rocks. 



How far the substratum has any influence on the appear- 

 ance of moUusks is yet impossible to state, as the investigations 

 have been too few. It is, however, a fact that the greatest 

 number of species are found and collected on silurian rocks (in 

 the valley of Christiania and Skiensfjord), but at the same time 

 ■ it must be stated that most of the species also appear in places 

 very poorly provided with limestone, and then even in great 



J.C, v., July, 1886. 



