CANNA. GENERAL DESCRIPTION. 447 



may arise from essential differences in the composition 

 of the rocks, as I formerly suggested, and possibly from 

 different proportions of calcareous earth ; but there is 

 another cause of an incidental nature to which these 

 various degrees of fertility may perhaps with more justice 

 be attributed. This is the state of drainage, which, in 

 Canna is complete, from the uniform declivity of the 

 whole island. Wherever indeed, as on the higher flat 

 parts of the land, this effect does not take place, there 

 the ground becomes mossy, and peat accumulates. The 

 island may in fact be considered as in a constant state 

 of natural irrigation much resembling the effects of the 

 artificial process, in which the alternation of moisture 

 and drainage is one of the most essential circumstances. 

 In this respect, as well as in the appearance of its soil 

 and surface, Canna bears a great resemblance to St. Kilda, 

 where the same rocks accompanied with similar circum- 

 stances of declivity are attended by the same results. 



The vicinity and relative position of Rum, ensures to 

 Canna a rainy climate, from which, if situated alone, like 

 Coll or Tirey, it would have been exempted ; its own 

 altitude being insufficient to affect the greater part of 

 the clouds as they arrive from the western sea. It thus 

 gives rise to different small streams, which, originating in 

 those springs that occur in the rocks of this class, become 

 more permanent than those which in the Long island are 

 produced from ten times the extent of surface. But their 

 courses are necessarily short, and unaccompanied by any 

 of those deposits which in countries of greater extent 

 so frequently attend them. The principal waste of rock 

 is on the northern side of the island. Here, the frag- 

 ments which are constantly falling from the cliffs accu- 

 mulate in high slopes, increasing its superficial extent, 

 and laying the foundation of a radical change of feature 

 with accessions of new pasture lands. 



