446 CANNA. GENERAL DESCRIPTION. 



flats in the same situation supply peat for the consumption 

 of the inhabitants ; but the quantity diminishes so rapidly 

 as to threaten at no very distant time the total anni- 

 hilation of this necessary article. This island is one 

 of the few instances now remaining in the Highlands 

 of the system of Tacks, being held by one principal 

 tenant, and subset to the miserable population by which 

 it is crowded. The pasture is applied to the rearing of 

 cattle, and the land in cultivation is so limited that the 

 inhabitants subsist in a great degree on fish. It is one 

 of the islands in which the growth of grain is rapidly 

 giving way, for obvious reasons, to the cultivation of 

 potatoes. 



Its fertility depends chiefly on the quality of the soil, 

 the result of the decomposed trap whence it is generated. 

 But the observations already made in treating of Sky, 

 and similar facts occurring in other places where the 

 same rocks exist, prove, that this circumstance is not 

 in itself sufficient to ensure the fertility of the resulting 

 soil. In a general sense the fertile quality of trap soils 

 has long been known, but mineralogists have not suffi- 

 ciently inquired, either into the circumstances which 

 produce this very important effect, or into the causes 

 of the numerous exceptions which occur to the rule. 



The neighbouring islands of Rum and Egg afford ex- 

 amples of a mixed nature, in which the grassy surface 

 is often far overpow r ered by the quantity of heathy and 

 useless covering they contain. The top of Glamich in 

 Sky, exhibits at the elevation of nearly 3000 feet, a lawn 

 as verdant and luxuriant as those which are found in the 

 ornamental parks of England. But a large proportion 

 of the surface of that island, founded on trap rocks, wears 

 an aspect so brown and dreary as to be nearly prover- 

 bial; although their general character in those parts 

 scarcely differs from that of the rocks of Canna. Such 

 also is the case in Mull, while Staffa is, like Canna, 

 covered with grass. It is probable that these varieties 



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