Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/18

 a point without the aid of analysis, a considerable similarity of composition 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 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 slovenly system of cultivation practised by these little highland farmers 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 climate