wet │ │ │ │ NOTE.—_Vide_ Chapter II. │ │ ───────────────────────────────────────┴────────────┴────────────────── Friction is the chief cause of stability in granular soils and those readily affected by moisture which have for practical purposes no immutable cohesion. In few earths are both cohesion and friction of considerable and reliable value, one or the other quickly becoming impaired or destroyed. Movement is caused by such various means that each earth must be separately considered, and also the circumstances under which it is placed. The particles of the earth may be dissolved by water and become in a muddy state, or they may be considered insoluble as in clean sand and gravel, although in compact sand or gravel the cementing material may crack and weather. Provided it was certain any earth would always remain as originally formed, a condition which it is impossible to guarantee in work, the cohesion and frictional resistance being known, the correct slope could be mathematically determined, but as all earths are subject to varying deteriorating influences, such a deduction is only valuable as a guide for reasonable inference. Cohesion may be more quickly impaired by certain action than friction, and _vice versâ_. Probably, of all soils, that to be most distrusted is one that expands and contracts, such as clay earth, which although possessing considerable cohesion may become upon drying a mere congregation of disconnected lumps ready to move upon the return of wet weather, thus its considerable power of cohesion may practically be one of the chief causes of a slip. On the contrary, clean sand, although devoid of cohesion, will not crack or have a greasy surface, but the particles may be washed away.