2(50 THORODDSEN 



cracks, the circumstances are somewhat different. During spring the 

 ground is partially thawed ; it freezes in the night and thaws during 

 the day. The sub-surface ice forms a downward limit which does 

 not permit the water to drain away, and uniform circulation and 

 evaporation at the surface are prevented in a "rudemark" by ice- 

 formation in the many cracks, originally full of water, which to- 

 gether with the sub-surface ice as a base form a vascular- or cell- 

 system over the entire flat, and this system lasts at least for some 

 time. The water from the melting snow and ice on the surface col- 

 lects mainly in the cracks and depressions, where it freezes during 

 the night; this is best observed on a knolly flat, which during the 

 thaw of spring produces an entire network of small water-canals. 

 The water cannot penetrate downwards on account of the ice in 

 the cracks or, if they should be free from ice, it w r ill vet remain 



/ *^ / 



for a long time in them, for as the water-layer is thicker there than 

 upon the polygonal cakes, the evaporation is slower. The heating 

 of the sun and consequent evaporation of water is therefore greater 

 on the cakes, so that the wet from below, from the slowly thawing 

 parts of the ground and the ice of the subsoil is drawn up into the 

 centre of the cakes. A clayey soil with particles of a certain size 

 has great capillary power and can absorb water and draw it up in 

 great quantities. 1 The power of absorption of the clay-soil is greatly 

 increased when it is covered by soil, humus and plant remains. 

 During spring, frost and thaw alternate constantly and daily. The 

 u rudemark" freezes in the night, at least partially, and thaws in the 

 morning; in the course of the day the water rises in the individual 

 clay-prisms owing to the rapid evaporation from the surface, but 

 in the night it freezes, expands and raises the central part of the 

 cake. This occasions a constant wandering of the particles of the 

 clay soil upwards into each clay-prism, and by the constant pres- 

 sure exerted by the frozen soil throughout a long period, the heavier 

 particles are sorted out, and as they are less mobile, they are left 

 behind or pushed to the sides. This sorting-out of the coarser ma- 

 terial is the most characteristic feature of the "rudemark." The 

 enormous pressure due to the freezing process is well known. As 

 will be mentioned again later on, in several places in the Iceland 

 mountain-bogs there are opportunities of observing how the frozen 



The air contained in sandy clays in a dry condition may amount to 40 /o 

 of their volume, and by infiltration, as large a volume of water may replace the 

 air. See A. G. Hogbom in Geol. Forhandl., Stockholm, 1905, XXVII. p. 22. 



