218 THORODDSEN 



In the Pliocene period Iceland was fissured transversely by nume- 

 rous lines of fracture which caused violent volcanic action, bv which 



7 / 



the tuffs and breccias of the palagonite formation were produced. 

 Along the same lines the volcanoes of the Glacial period and of 

 the present day appeared. From the end of the Pliocene period to 

 the present time the coast-line has been subject to considerable 

 changes the boundary-values of \vhich appear to be a positive dis- 

 placement of 150 metres and a negative displacement of 250 metres 

 of the position of the sea-surface relative to the land. At the end 

 of the Pliocene, or during the earliest part of the Glacial period, 

 the coast-line sank about 250 metres below the present level, and 

 in the broad coastal platform which thus became dry land, erosion- 

 grooves were formed leading off from each main valley. Now, each 

 fjord and bay is continued out to the edge of this submarine plat- 

 form by submarine fjords, as has been proved by the soundings 

 taken of late years by the Danish Marine Department. 



The marks left by a negative displacement of the coast-line in 

 post-Glacial times occur around the whole coast, but are especially 

 well-developed round the north-western peninsula. Everywhere along 

 the rocky coast are found marine terraces of gravel, coast-lines and 

 surf-terraces marked on the solid rock; in several places remains 

 of shells are also found and sometimes bones of whale and wal- 

 ruses, also old drift-wood ; sometimes far from the present line of 

 coast. Round the north-w r estern peninsula occur distinct and well- 

 developed coast-lines and surf-terraces at two levels (70 80 metres 

 and 3040 metres above sea-level); such are also found on other 

 parts of the coast, especially the lower line; the upper line is rarely 

 as distinct as the lower one; in some places on the main land there 

 appear to be indistinct marks of a water-level up to 100 150 metres. 

 In South Iceland caves, hollowed out by the surf during the time 

 of a higher water-level, are rather common, and marine clay-forma- 

 tions occur upon all the low land, the latter having been submerged 

 during the final part of the Glacial period. In the clay-deposits in 

 south-western Iceland, Yoldia arctica and other High Arctic molluscs 

 are found at a level which corresponds with the higher coast-line 

 (70 80 metres); other shell-mounds, with a fauna which resembles 

 the present one, correspond with the level of the lower coast-line 

 (30 40 metres), Saxicava is especially characteristic of these shell- 

 mounds. Since the Glacial period the coast-line has retreated, but 

 with some oscillations, and even now several indications may be 



