48 proceedings: geological society 



in such rocks rapidly disintegrate and crumble until they reach an angle 

 on which the products of disintegration will form a protective mantle. 

 Vegetation greatly assists in this protection. While the failure of the 

 canal slopes from this cause will not be a serious matter, it will involve 

 the removal of a large amount of material in the aggregate and necessi- 

 tate constant attention for a long time. Under certain conditions this 

 surface disintegration becomes a more serious matter. For example, 

 the contact between the intrusive andesite of Contractor Hill and the 

 sedimentary clays, dips away from the canal at an angle of 55°. The 

 crumbling of the clays below the contact leaves the andesite mass over- 

 hanging, and the overhang will increase as the canal is deepened and as 

 more clay is exposed. With a rock so fractured as the andesite this con- 

 dition is unsafe and the exposed surface of the underlying clay should 

 be protected from farther disintegration by a layer of concrete. 



Robert Anderson, Secretary. 



