182 



GEORGE B. RIGG 



only one case shows any conspicuous evidence of the effect of wind. 

 It seems difficult to accept as the single common cause an influ- 

 ence that shows any considerable effect in onl}^ one case. 



All the islands of the San Juan group are glaciated and Bretz 

 has suggested the agency of the glacier in bringing about the soil 

 conditions found on the two slopes of Cattle Point Hill and South 

 Hill. Of the former he says: "The gravelley southern face of 

 this great hill of Vashon drift is interpreted as having been origi- 

 nally a steep alluvial fan built against the ice front at the debou- 

 chure of a glacial stream The till on the northern 



slope records the ice wall against which the deposit was banked." 

 Of the latter he says: "It is considered to record a briefer pause of 

 the retreating edge of the ice." 



To what extent the relative water-retaining capacity of the 

 two soils mentioned is responsible for the forested condition of the 

 one and the barren condition of the other, as well as the question 

 of their relative fitness for the growth of mycorrhiza and micro- 

 organisms which may possibly be determining factors in this for- 

 est distribution are subjects worthy of investigation. 



It seems quite probable that several factors are concerned in 

 the question of forest distribution in the San Juan Islands. It 

 is evident, however, that in a determination of the causes of the 

 distribution large consideration will have to be given to edaphic 

 factors. 



