SOLIFLUCTION 103 



abundant residuum of huge, hard quartzite blocks, almost unat- 

 tackable by weathering. Such a result of the combination of 

 solifiuction and river-action will be reached only with rocks com 

 posed of an alternation of soft and extremely unattackable material. 

 In other cases also the big blocks will gradually succumb to the 

 weathering, the river will be able to clear its course, and new mud- 

 streams will follow those already washed away by the running 

 water. 



Also in this respect a comparison with the conditions of Bear 

 Island is highly instructive. In the southern, mountainous part 

 of the island, where I have studied the solifiuction in its most varied 

 development, and where the ground is for the larger part composed 

 of rocks (slates, limestone, dolomite), which are through all its 

 mass almost uniformly attackable by weathering, I have found, 

 in the course of the small rivers, no accumulations of big, irregular 

 blocks like those of the stone-rivers. But as soon as one enters 

 the areas of the Ursa sandstone — a rock in parts much like the sand- 

 stone of the Falkland Islands, and consisting of alternating hard 

 quartzites and soft intercalations — the aspect of the ground is quite 

 altered. The vast Ursa sandstone area on the flat land in the north 

 of the island is all densely covered over with huge, edged blocks, 

 the whole forming a regular Gehenna for the wanderer — quite as 

 the stone-rivers. 



I feel sure that these immense block-fields of Bear Island are 

 formed in quite the same manner as the Falkland stone-runs: by 

 weathering of the rock; by solifiuction (though this is very slow 

 and hardly perceptible in the great plain of Bear Island); finally 

 also by the action of running water. The only differences between the 

 two occurrences are differences of topography and of age: in Bear 

 Island a great plain forming a stone-field, in the Falkland Islands 

 valleys filled at the bottom by stone-rivers — the former a recent, 

 the latter a fossil occurrence. 



According to the relation given above, the Falkland Islands 

 are characterized not so much by the extreme intensity of the soli- 

 fiuction in a past time as by an exceptional abundance of the resi- 

 duum left by the running water that has sifted the material of the 

 old mud-streams. Under other petrological conditions the result 



