Trees — Rock Formation. 47 



strangely enough, the spirit tincture extracts the tonic bitter with 

 but very little of the peppery principle. 



The summits of the low hills, which are usually bare of trees 

 or brushwood, are covered with a sort of swamp formed of 

 astelias, gaimardeas, and calthas, whose interlacing roots form 

 a more or less compact sod, which, as one walks on it, shakes 

 from the fluctuation of the bog water beneath. 



The rock of the district is a cross-grained syenite, intersected 

 with dykes of greenstone, of very variable thickness. This is the 

 prevalent rock ; but about Port Rosario, on the north side of 

 " IVIadre dc Dios" island, there is an outcrop of limestone. The 

 latter is of a pale-blue colour, in some cases assuming the character 

 of marble ; and when much exposed to the weather, presents a 

 curious honey-cornbcd appearance, due to the solvent action of 

 the rain. This rock is unfossiliferous. The disintegration of the 

 syenite from the usual atmospheric agencies is rapid enough ; 

 but the resulting detritus does not contribute to form a good 

 clay. 



If an artificial section be made of the soilcap, or if advantage 

 be taken of a landslip to examine it carefully, it will be seen to 

 be composed of a dense network of interlacing roots, containing 

 in its interstices a small quantity of black mould, the latter 

 increasing in proportion as the basement rock is reached. This 

 spongy mass of tangled vegetation, ever saturated with moisture, 

 is the soil on which the trees clothing the hillsides take root. 

 On the little plateaus about the hill-tops, however, it only con- 

 tains the roots of the marsh plants above mentioned, and those 

 of an odd stunted bush. On first coming to this region, I was 

 much struck on seeing that the forest approaches so close to the 

 water's edge, and that the banks overhang so much that fre- 

 quently the branches of the trees dip into the salt water ; and 

 in some places a black snag projecting above the surface of the 

 inshore water tells the fate of a tree that had perished from 

 immersion. These phenomena, among others to be hereafter 



