234 CONSOLIDATION OF UNSTABLE SLOPES. 



(i) Steep gradients. 



(ii) Broken surface of the ground and sudden changes of gradient, 

 causing water to rush down with great velocity at some points, 

 slowly at others, instead of with a smooth even flow everywhere. 



(iii) Schistose or otherwise friable nature of the rocks. Portions 

 of such rocks are liable to break away by their own weight alone. 

 This tendency is increased by the mechanical pressure of the 

 water that sinks into and fills up the fissures and interstices, and 

 by its solvent action, which is assisted by its constant motion down- 

 wards. 



(iv) Free nature of the soil. 



(v) Clayey nature of the rock or soil, which, absorbing large 

 quantities of moisture, increases enormously in weight, swells up 

 and becomes soft, and thus either breaks away in masses under 

 the force of gravitation alone or is easily scoured and furrowed 

 and deeply cut into by the surface drainage. Sometimes the clay 

 may absorb so much water that it may be converted into liquid 

 mud. This denuding action of water is still further increased if 

 ferruginous elements are present in the soil, those elements being 

 dissolved out of the soil, which then becomes porous and less 

 resisting. 



(vi) Alternation of rocks of different densities and absorbing 

 water with different degrees of energy. With changes of weather 

 very unequal expansion or contraction may thus take place 

 throughout the entire mass of the rocks, resulting in the breaking 

 up and loosening of the entire hillside. The effect is greatest 

 when a band of clay underlies porous rocks above ; the clay swells 

 and lifts up and breaks these rocks, or, becoming dissolved and 

 diminishing in bulk, is no longer able to support them. 



(vii) Earthquakes. 



(viii) Alternation of wet and drought. The wet causes the soil 

 and rocks to swell up, when, drought ensuing, unequal contraction 

 takes place, cracks are produced and the solidity of the hillside 

 becomes permanently impaired. 



(ix) Alternation of frost and thaw. Under the action of frost 

 the water in the soil and in the fissures and crevices of the rocks 

 suddenly expands, enlarging all existing cracks and producing new 

 ones. As long as the frost lasts, the frozen water acts as a cement 

 on the loosened fragments and masses ; but as soon as the ice has 

 melted, there is nothing left to hold them together. 



(x) Pelting or abundant rain. The impact of the innumerable 

 drops following one another in quick succession disturbs very ap- 



