212 THORODDSEN 



and Antrim upon chalk; in Iceland, where the basalt formation has 

 a thickness of at least 3000 metres, the underlying rock has not 

 vet been found. 



\s 



The principal rocks of which Iceland is composed are two, 

 basalt and palagonite breccia; more than one-half of the surface 

 and the rock-foundation of the country consists of basalt, but the 

 palagonite formation, which is composed of breccias, tuffs and 

 conglomerates of different age and which, taken as a whole, is 

 younger than the basalt formation, forms an irregular band across 

 the country, occupying an area somewhat smaller than that occu- 

 pied by the basalt. Compared with these two formations, all other 

 rocks and formations have quite an unimportant distribution. The 

 basalt mountains, the precipitous walls of which often rise from 

 the sea to a height of 600 - 1000 metres, are composed of layers of 

 varying thickness, wedged in between each other: the thickness of 

 the individual layers often decreases rapidly along a short distance 

 until the layer disappears and gives place to another. In the basalt 

 formation, beds of tuff and breccia sometimes occur between the 

 basalt layers, but their amount is inconsiderable compared with that 

 of the basalt. Dykes are frequent; the majority of them pierce down 

 through the entire series of layers. Seen from a distance, the basalt- 

 mountains with their steep, terraced walls, have a monotonous and 

 gloomy appearance, but on closer inspection exhibit rather great 

 variation. Some of the basalt layers are compact, others are coarsely 

 crystalline, doleritic, porphyritic, amygdaloidal (with more or less 

 completely filled vesicular cavities), slaggy, banded, etc. In some 

 districts the basalt is cleft into beautiful columns; in others into 

 more or less irregular, angular blocks; in others it has an almost 

 slaty appearance. In the vesicular cavities of the basalt zeolites, 

 quartz, chalcedony and calc-spar are often found. As a general rule 

 the basalt layers have a slight inclination (3 5) from the coast 

 inwards towards the tuff and breccia formations, which appear to 

 fill a flat, saucer-like depression in the underlying basalt plateau; 

 but many local deviations occur owing to dislocations and sub- 

 sidences of larger or smaller areas of the underlying rock. 



In the middle of the basalt-formation in Iceland (as also in 

 the Fasroes and in Ireland) rather considerable clay deposits are 

 found with the impressions and remains of plants of Tertiary times; 

 also lignite and compressed tree-trunks, all called in Icelandic u sur- 

 tarbrandur." This plant-bearing formation attains its greatest thick- 



