38 Transactions. 



covered with glaciers ; but, in spite of this, the area exposed to 

 frost action would be more extensive, and therefore the supply 

 of waste would be in excess. A very large amount of erosion 

 due to glaciers, as estimated by the proportion of sediment in 

 the rivers flowing from their terminal faces, is due primarily to 

 the action of frost on the hillsides above the glaciers. 



The supply of waste in this case would be increased during 

 elevation, owing to the previous loosening action of the plants 

 on the rocks rendering them subject to other weathering 

 agencies ; again, if this were also attended with a general desicca- 

 tion of the climate on the mountains fronting the east, the 

 supply of waste would be further increased owing to the dis- 

 appearance of the protective plant-covering. 



From a general survey of the country in the upper basins 

 of our rivers I am of opinion that the period of maximum 

 weathering has passed. The old and mature shingle- slips are far 

 larger than those now existing. Vegetation in many cases has 

 got the better of the moving shingle, and in some cases the old 

 fans are completely covered with forest. Our shingle- slips at 

 the present time are diminishing in extent, and they will con- 

 tinue to do so unless the plant-covering is destroyed either by 

 nature herself or by man. 



The excess of waste during a period of elevation accounts 

 for the present form of the Canterbury Plains. They have been 

 formed by the overlapping fans of great glacier streams, as can 

 be conclusively proved by carefully contouring their surface. 

 The contour lines show them to have been formed in exactly 

 the same way as an ordinary shingle fan, except that their grade 

 is more gentle. They were built up to their present height 

 when the rivers were overloaded with sediment, during a time 

 of high land, severe glaciation, and acute frost action. On the 

 land being depressed, the supply of waste would fall off, and the 

 rivers would begin to terrace their old deposits in a manner 

 analogous to that in which a stream terraces its fan during a 

 falling flood. This action was certain to occur unless the 

 volume of the river fell off in a relatively greater proportion. I 

 believe that such would not occur in Canterbury, owing to the 

 excessive amount of waste which would be poured into the rivers 

 falling off in a greater ratio than the decrease in snow or rain. 



It will be noted in every case that the grade of the rivers is 

 less than that of the plains ; the rivers are therefore able to do 

 their work on a gentler slope than formerly. This can only be 

 due to — (1.) Elevation of the interior of the country since the 

 plains were formed. (2.) Rivers having a greater volume, and 

 therefore power to move their load on a gentler grade : this is 

 extremely unlikely. (3.) A diminution in the supply of waste : 



