PLANT SUCCESSIONS OF MT. ROBSON 235 



thicket. In these lanes the same shrubs occur but they are 

 scattered, not forming thickets. The ground cover is similar 

 but very sparse, there is little humus, and areas of bare ground 

 are frequent. The conclusion" is plain that these lanes are 

 deserted shallow stream channels, cut below the general surface, 

 and that the surfaces between them are therefore older. Natu- 

 rally the succession has progressed to a later stage upon the latter. 

 The interesting feature is in the comparison of the successions 

 upon shingle flat and moraine. In general character they are 

 much ahke, in stages and in species. It has been shown that 

 the moraine habitat is much younger than the shingle flat, since 

 it has been dumped upon the latter, the surface of which has 

 not been appreciably changed since that event. The vegetation 

 however has progressed to a much later stage upon the younger 

 habitat, the moraine. The trees, though somewhat smaller and 

 decidedly younger than those of the fiat, are many times more 

 numerous. The thicket growth is much denser, as is also the 

 ground cover. Upon the oldest moraine eight trees ranged in 

 age from 73 to 116 years, with an average of 95 (see table, p. 225) 

 Upon the shingle flat, close to the moraine front, three trees 

 ranged from 117 to 257 years, with an average of 183. Upon 

 the moraine, the rings of five out of six trees examined carefully 

 showed increasing growth rate with increasing age, indicating 

 rapidly improving conditions; while upon the flat the trees 

 examined showed no such change. Further, dead and rotten 

 trunks are common upon the flat, showing that the present 

 generation is not the first, while upon the moraine none were seen. 

 The conclusion is plainly that the succession upon the moraine 

 goes through its development much more rapidly — probably 

 in a small fraction of the time required by the almost identical 

 process upon the shingle flat. At least one plausible reason for 

 this difference is apparent. The shingle flat, being close to the 

 glacier, is composed almost entirely of coarse materials, the fine 

 silt having been carried to much greater distances by the swift 

 streams. The moraine on the other hand is composed of coarse 

 materials, embedded in large quantities of fine silt, upon which 

 swiftly flowing water has not as yet had an opportunity to act. 



