266 THE HTPERTHENE SOILS OF SKYE. 



draining can remove. They apply also to other conntries where trap 

 rocks abound — the only fertile tracts of Abyssinia, for instance, being 

 found in vallies and on mountain slopes, where the soil is com nosed of 

 the detritus of trappean rocks (Dr. Ruppell., 



Yet there are exceptions to this general rule. 



Where the felspar is largely predominant, the soil formed from the 

 rock will partake more or less of the cold and barren character of the 

 stiffer granitic soils. Such appears to be the case with some of the traps 

 which occur in the border counties of England and Wales (Murchison). 



In the Isle of Skye, again, a local peculiarity of a different kind ob- 

 tains, the effect of which upon the soil is also to render it poor and un- 

 productive. In that island the singularly beautiful ridge^f the Cuchul- 

 len Hills consists of a variety of trap in which the augite so far predomi- 

 nates as to form nearly the whole of the mountain masses, But the 

 augite in this case is a variety to which the name of hypersthene has 

 been given, and which contains much magnesia and oxide of iron, but 

 scarcely a trace of either lime or alumina. The rock is very hard, and 

 decays with extreme slowness; yet however rapid its decay might be, 

 it could never produce a fertile soil. We have seen that the serpentine 

 and granite soils are essentially deficient in lime, but a hypersthene soil 

 is in want both of lime and of clay. It would be still more difficult, 

 therefore, to render the latter productive — even supposing, as in the case 

 of the serpentine soils, that the magnesia of the hypersthene* were most- 

 ly washed away by the rains. 



Thus we perceive how eactly the study of the composition of the dif- 

 ferent varieties of the trap rocks explains the observed differences in the 

 quality of the soils derived from them. When the minerals they contain 

 abound in lime, the soils they yield are fertile — when those minerals 

 predominate in which lime is wanting, the soils are inferior, sometimes 

 scarcely capable of cultivation. Again, the granites abound in potash; 

 but except in the syenites they rarely contain lime, and their soils are 

 generally poor. Let them be mixed with the trap soils, and they are 

 enriched. This would seem fairly and clearly to imply that the fertility 

 of the one is mainly due to the presence of lime, and the barrenness of 

 the other to the absence of this earth. 



On this subject I will only further add, that the more modern volcanic 

 lavas which overspread It-aly, Sicily, parts of France, Spain, and Ger- 

 many, are closely related to the trap rocks in their general composition 

 — aad the fertility which overspreads thousands of square miles of de- 

 composed lava streams and ejections of volcanic ashes in Italy and Si- 

 cily, is too well known to require any detailed description. 



§5.0/" superficial accumulations of foreign materials^ and of the means 

 hy which they have been transported. 

 Abundant proof, I think, has now been advanced that a close relation 



' The hypersthene of Skye has been found to consist of— 



Silica 51-35 1 Protoxide of iron 33-92 



Lime 1-84 I Water 0-50 



Magnesia 1109 I 



I 98-70 

 The composition probably varies in different parts of the rock, some containing more mag* 

 nesia and less iron than is here represented. 



