10 Dr. Mac Culloch's Sketch of the 



a point without the aid of analysis, a considerable similarity of com- 

 position in the trap throughout the greatest part of the whole tract. 

 In almost every place where the rock approaches the surface, it is found 

 on cutting into it to be rotten to a considerable depth, often reduced 

 superficially to an absolute soil, and although appearing below 

 like a solid rock, capable of being cut without difficulty by the pick- 

 axe or spade. Occasionally it decomposes into a soil perhaps more 

 gravelly in some situations than in others, but in this case there 

 appear only time and a further continuance of the destroying 

 powers requisite for its complete change. Where the decomposition 

 is most perfect it forms a clayey loam of which the aspect at least is 

 favourable, and of which the fertility also is probably not so limited 

 as the appearance of the heath and grasses which it bears would at 

 first sight induce us to believe. The trap which I have mentioned 

 is remarkable for the enormous quantity of zeolites imbedded in it, 

 the mineralogical details of which I shall have occasion to speak 

 more largely of hereafter. In the decomposed soils these are frequently 

 found resisting change long after the rock is rotten and reduced to 

 clay. But in many other cases they also are decomposed together 

 with the soil, and in such quantity as to communicate their white 

 colour to it, and with that colour doubtless a degree of additional 

 fertility derived from the quantity of calcareous earth which they 

 contain. In many places such accumulated beds of decomposed 

 zeolites occur that they have been mistaken for marl, and have 

 when used produced similar effects ; although the narrow and sloven- 

 ly system of cultivation practised by these little highland farmer* 

 neither admits of a full trial nor of a fair result. 



We have seen that many tracts of this district are characterized 

 by a high degree of actual fertility, while neighbouring ones formed 

 of a soil apparently identical and under similar circumstances of cli- 



