PHYSICAL GEOGRAPHY 251 



formed by the rivers, and converted into terraces, which form the 

 substratum of bogs and grassland. The glacial moraine-gravel often 

 extends far up the mountain-sides and forms here a substratum 

 for soil and plant-growth. In other places in the valleys are steep 

 rock-faces, stony slopes, heaps of large fragments of rock (urd), 

 and the conical heaps of finer and coarser gravel brought down 

 by the mountain-streams, which all help to give variety to the 

 plant-formations. While basalt -mountains are slightly and slowly 

 disintegrated, tuff-mountains are extremely liable to disintegration, 

 hence the products of the latter, combined with the action of wind, 

 glaciers and rivers, play a more important part. The contribution 

 of the basalt towards soil-formation dates mainly from the Glacial 

 period. As we have seen from the above, the substrata which 

 support plant-growth are (1) firm ground, having a rocky base 

 (basalt, liparite, breccia and lava); (2) loose soil, consisting of mo- 

 raines, river-gravel, sand, clay, blown sand, volcanic ashes and tuft- 

 dust (mohella); and (3) the products of the plants themselves: boggy 

 soil composed of peat and humus. 



The character of the subsoil below the humus-layer and the 

 plant-covering is consequently in close relation to the chemical and 

 mineralogical composition of the underlying rock. Over the greater 

 part of Iceland the inorganic soil consists of decomposed basaltic 

 rocks, the main mineralogical constituents of which are plagioclase 

 (especially lime-felspars) and augite, but magnetite and olivine also 

 occur, often in great quantities, and apatite and a small quantity 

 of titanic iron. The chemical composition of the Icelandic basalts 

 is rather uniform. On an average they contain 43 53 % of silica, 

 1118 /o of alumina, 1122 % of iron (Fe and Fe 2 () 3 ), 813 % 

 of lime, 29% of magnesia, 0.22% of potash and 14% of 

 soda. Because anorthite, of the plagioclases, is very largely distri- 

 buted in the Icelandic rocks, not only in the basalt, but also in 

 the recent lavas and tuffs, these Icelandic rocks often contain a 

 comparatively small amount of silica and a very considerable amount 

 of lime and also alumina. The reason why the Icelandic soil is 

 nevertheless poor in carbonate of lime may be found in the fact 

 that the lime can only with difficulty be separated from its siliceous 

 compounds, and because in the whole of the island, no sedimentary 

 calcareous rocks are found, though such are of common occurrence 

 in other countries. In districts where sulphurous acids sent out 

 from fumaroles have affected the rocks, as is common in tuff-districts, 



The Botany of Iceland. I. 17 



