SKY. SOIL AND AGRICULTURE. 



in this respect in different situations where there was 

 nevertheless a considerable resemblance in the rocky 

 substratum. The different proportions of calcareous earth, 

 of alkali, and of carbon, which latter appears to be an 

 ingredient in some traps, are probably the circumstances 

 that constitute the chief differences in this case ; but with 

 this we must also recollect that the various rocks of this 

 class differ in composition in perhaps a greater degree 

 than any others with which we are acquainted, and that, 

 together with this essential variation in their composition, 

 they also possess infinite varieties in hardness, and in 

 the property of decomposing by the action of air and 

 water. Many considerations must therefore enter into 

 our views before we can decide on the fertility of the 

 resulting soil, unless that be actually exhibited in those 

 places where decomposition has already taken place. 



Although the mineralogist can often trace no difference 

 among the trap rocks which occupy many parts of this 

 island, yet the greatest differences in the produce of the 

 soil incumbent on them will be found to exist ; the one 

 displaying little but heath, rushes, and the other inha- 

 bitants of moor lands, while others are covered with a 

 dense mat of verdure. In many cases, doubtless, the 

 varieties of exposure, drainage, or declivity, will account 

 for these differences of effect, yet similar differences occur 

 even where these circumstances are in both cases identical. 

 It is evident, that, independently of variations in the 

 chemical composition of the subjacent rocks, the incum- 

 bent soils will differ according to the greater or less 

 facility with which they undergo decomposition. Where 

 that takes place to a considerable extent, the soil is com- 

 monly not only deep but fertile, and will often be found 

 covered with green pasture, as on the eastern side of 

 Trotternish and on the hills that surround Loch Eynort. 

 There are cases nevertheless where a deep decomposition 

 of the rock is to be seen, while the ground is still covered 

 with peat and productive of little else than heath. This 



