Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/17

 arts. From the identity of rocks in different tracts a conclusion may be drawn with regard to the identity of the resulting soils, and the analogies discovered in them may perhaps with less trouble and as great certainty as any chemical analyses, lead agriculturists to hazard those experiments in improvement which seem justifiable from similarity of composition no less than similarity of climate and exposure. I shall therefore I trust be excused for dwelling a little longer on a subject intimately connected with geological science, and in itself among the most practically interesting that can occupy our attention.

It is already well known that many of the rocks of the trap family, like some lavas, afford on decomposition one of the most fertile soils with which we are acquainted. On former occasions I have mentioned this fact, and noticed at the same time the great differences which appeared 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 which constitute the chief differences in this case; but with this we must also consider 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 it be actually exhibited in those places. where decomposition has already taken place.

In the district of Sky which I am new considering, there appears, as far as an accurate mineralogical investigation can determine such